Diffusing alpha-emitter radiation therapy for colon cancer

FIELD OF THE INVENTION

The present invention relates generally to radiotherapy and particularly to apparatus and methods for providing tumor-specific radiation dosages in radiotherapy treatment.

BACKGROUND OF THE INVENTION

Ionizing radiation is commonly used in the treatment of certain types of tumors, including malignant cancerous tumors, to destroy their cells. Ionizing radiation, however, can also damage healthy cells of a patient, and therefore care is taken to minimize the radiation dose delivered to healthy tissue outside of the tumor, while maximizing the dose to the tumor.

Ionizing radiation destroys cells by creating damage to their DNA. The biological effectiveness of different types of radiation in killing cells is determined by the type and severity of the DNA lesions they create. Alpha particles are a powerful means for radiotherapy since they induce clustered double-strand breaks on the DNA, which cells cannot repair. Unlike conventional types of radiation, the destructive effect of alpha particles is also largely unaffected by low cellular oxygen levels, making them equally effective against hypoxic cells, whose presence in tumors is a leading cause of failure in conventional radiotherapy based on photons or electrons. In addition, the short range of alpha particles in tissue (less than 100 micrometers) ensures that if the atoms which emit them are confined to the tumor volume, surrounding healthy tissue will be spared. On the other hand, the short range of alpha radiation has so far limited their use in cancer therapy, as there was no practical way to deploy alpha emitting atoms in sufficient concentrations throughout the entire tumor volume

Diffusing alpha-emitters radiation therapy (DaRT), described for example in U.S. Pat. No. 8,834,837 to Kelson, extends the therapeutic range of alpha radiation, by using radium-223 or radium-224 atoms, which generate chains of several radioactive decays with a governing half-life of 3.6 days for radium-224 and 11.4 days for radium-223. In DaRT, the radium atoms are attached to a source (also referred to as a “seed”) implanted in the tumor with sufficient strength such that they do not leave the source in a manner that they go to waste (by being cleared away from the tumor through the blood), but a substantial percentage of their daughter radionuclides (radon-220 in the case of radium-224 and radium-219 in the case of radium-223) leave the source into the tumor, upon radium decay. These radionuclides, and their own radioactive daughter atoms, spread around the source by diffusion up to a radial distance of a few millimeters before they decay by alpha emission. Thus, the range of destruction in the tumor is increased relative to radionuclides which remain with their daughters on the source.

In order for the treatment of a tumor to be effective, DaRT seeds employed in the treatment should release a sufficient number of radon atoms to destroy the tumor with a high probability. If an insufficient amount of radiation is employed, too many cancerous cells will remain in the tumor, and these cells may reproduce to reform the malignant tumor. On the other hand, the seeds should not release too many radon atoms, as some of their daughters are cleared from the tumor through the blood and could therefore damage distant healthy tissue, including organs such as bone marrow, kidneys and/or ovaries of a patient.

The amount of radium atoms on the DaRT source is quantified in terms of the activity, i.e., the rate of radium decays. The DaRT source activity is measured in units of micro-Curie (μCi) or kilo-Becquerel (kBq), where 1 μCi=37 kBq=37,000 decays per second. When using DaRT, the radiation dose delivered to the tumor cells depends not only on the radium activity of the source, but also on the probability that the daughter radon atoms will leave the source into the tumor, upon radium's alpha decay. This probability is referred to herein as the “desorption probability”. Thus, instead of referring to the activity of the source, one can use the “radon release rate”, which is defined herein as the product of activity on the source and the desorption probability of radon from the source, as a measure of the DaRT related activity of a source. Like the activity, the radon release rate is given in μCi or kBq. The activity and radon release rate values given herein are, unless stated otherwise, of the source at the time of implantation of the source in the tumor

The above mentioned U.S. Pat. No. 8,834,837 to Kelson suggests using an activity “from about 10 nanoCurie to about 10 microCurie, more preferably from about 10 nanoCurie to about 1 microCurie.”

SUMMARY OF THE INVENTION

Embodiments of the present invention relate to providing accurately tailored amounts of radiation to a tumor in a radiotherapy treatment. The embodiments include radiotherapy sources designed to provide suitable amounts of radiation, and kits including a suitable number of sources for tumors of specific sizes. Further embodiments relate to methods of preparing kits of radiotherapy sources for a specific tumor and methods of treatment of a tumor.

There is therefore provided in accordance with an embodiment of the present invention, a method for treating a colon cancer tumor, comprising identifying a tumor as a colon cancer tumor, and implanting in the tumor identified as a colon cancer tumor, as least one diffusing alpha-emitter radiation therapy (DaRT) source with a suitable radon release rate and for a given duration, such that the source provides during the given duration a cumulated activity of released radon between 3.5 Mega becquerel (MBq) hour and 8.4 MBq hour, per centimeter length.

Optionally, implanting the as least one radiotherapy source comprises implanting an array of sources, each source separated from its neighboring sources in the array by not more than 4 millimeters. In some embodiments, implanting the as least one radiotherapy source comprises implanting an array of sources in a hexagonal arrangement, each source separated from its neighboring sources in the array by not more than 4 millimeters. Optionally, the as least one radiotherapy source has a radon release rate of between 0.8 and 1.9 microcurie per centimeter length. Optionally, the as least one radiotherapy source has a radon release rate of between 1 and 1.6 microcurie per centimeter length. Optionally, the method comprises selecting the given duration before implanting the at least one DaRT source in the tumor, and removing the sources from the tumor after the given duration from the implanting of the sources passed.

There is further provided in accordance with an embodiment of the present invention, a method of preparing a radiotherapy treatment, comprising identifying a tumor as a colon cancer tumor, receiving an image of the colon cancer tumor; and providing a layout of diffusing alpha-emitter radiation therapy (DaRT) sources for the colon cancer tumor, wherein the sources have a radon release rate of between 0.8 and 1.9 microcurie per centimeter length. Optionally, providing the layout comprises providing a layout in which a spacing between sources in the tumor is 4 millimeters or less. Optionally, the sources have a radon release rate of between 1 and 1.6 microcurie per centimeter length.

There is further provided in accordance with an embodiment of the present invention, an apparatus for preparing a radiotherapy treatment, comprising an input interface for receiving information on a tumor, a processor configured to determine that the tumor is a colon cancer tumor and to generate a layout of diffusing alpha-emitter radiation therapy (DaRT) sources for the tumor, wherein the sources in the layout have a radon release rate of between 0.8 and 1.9 microcurie per centimeter length and the sources in the layout are arranged in a regular pattern having a distance between adjacent sources of not more than 5 millimeters; and an output interface for displaying the layout to a human operator.

There is further provided in accordance with an embodiment of the present invention, a method of preparing a radiotherapy treatment, comprising receiving a request for diffusing alpha-emitter radiation therapy (DaRT) sources for a colon cancer tumor, determining a number of radiotherapy sources required for the colon cancer tumor, and providing a sterile kit including the determined number of radiotherapy sources, wherein the sources have a radon release rate of between 0.8 and 1.9 microcurie per centimeter length.

Optionally, determining the number of required radiotherapy sources comprises determining a number of sources required such that the area of the tumor is covered by sources with a spacing between the sources which is not greater than 4 millimeters. Optionally, the sources have a radon release rate of between 1 and 1.6 microcurie per centimeter length.

There is further provided in accordance with an embodiment of the present invention, a diffusing alpha-emitter radiation therapy (DaRT) source for implantation in a colon cancer tumor, wherein the DaRT source has a radon release rate of between 0.8 and 1.9 microcurie per centimeter length. Optionally, the radon release rate is between 1 and 1.6 microcurie per centimeter length.

There is further provided in accordance with an embodiment of the present invention, a kit of diffusing alpha-emitter radiation therapy (DaRT) source for implantation in a colon cancer tumor, comprising a sterile package; and a plurality of DaRT sources placed in the sterile package, the sources having a radon release rate of between 0.8 and 1.9 microcurie per centimeter length. Optionally, the radon release rate of the sources is between 1 and 1.6 microcurie per centimeter length.

There is further provided in accordance with an embodiment of the present invention, a method for treating a tumor, comprising identifying a tumor as a colon cancer tumor, and implanting in the tumor identified as a colon cancer tumor, an array of diffusing alpha-emitter radiation therapy (DaRT) sources, in a regular arrangement having a spacing between each two adjacent sources of between 3.5 and 4.5 millimeters. Optionally, implanting the array of sources comprises implanting in a hexagonal arrangement, each source separated from its neighboring sources in the array by not more than 4 millimeters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a system for planning a radiotherapy treatment, in accordance with an embodiment of the present invention;

FIG. 2 is a flowchart of acts performed in preparing a radiotherapy treatment of a tumor, in accordance with an embodiment of the invention;

FIG. 3 is a schematic illustration of a regular arrangement of sources in a hexagonal arrangement, in accordance with an embodiment of the invention;

FIG. 4 is a schematic illustration of a kit of DaRT sources, in accordance embodiments of the present invention;

FIG. 5 is a schematic illustration of a radiotherapy source, in accordance with an embodiment of the present invention;

FIGS. 6A-6D are graphs showing the value of the radon-220 release rate required for assuring a minimal alpha particle dose of 10 Gy for different seed spacings, lead-212 leakage probabilities and radon-220 and lead-212 diffusion lengths;

FIGS. 7A and 7B are inset graphs of FIGS. 6C and 6A, respectively, used in calculating a required radon-220 release rate of sources for specific tumor types, in accordance with embodiments of the invention;

FIG. 8A shows a spatial distribution of the photo-stimulated luminescence (PSL) signal in a histological section of a 4T1 tumor with denoted region of sampled data in white (0-4 mm) and the fit region with magenta dashed line (0.5-3 mm);

FIG. 8B is a graph of the radial activity distribution for the sampled data of FIG. 8A, fitted by a theoretical model to extract the diffusion length of lead-212;

FIG. 8C shows a PSL spatial distribution in another histological section from the same tumor where the seed position is determined automatically by calculating the intensity center-of-gravity;

FIG. 8D is a graph of the radial activity distribution for the sampled data of FIG. 8C, fitted by a theoretical model to extract the diffusion length of lead-212;

FIGS. 9-14 show the obtained lead-212 (²¹²Pb) diffusion lengths for different tumor types as a function of the tumor mass;

FIG. 15 shows Four histological sections of a DaRT treated tumor, acquired using a digital autoradiography system, for measurement of the radon-220 diffusion length;

FIG. 16A shows a single histological section used for measurement of the radon-220 diffusion length; and

FIG. 16B shows calculated average counts as a function of distance from a seed location, with a fitted model used for measurement of the radon-220 diffusion length.

DETAILED DESCRIPTION OF EMBODIMENTS

An aspect of some embodiments of the invention relates to setting radon release rates of DaRT sources used in treating different types of tumors according to characteristics of the tumors. Applicant has identified that important factors in optimizing the activity levels of DaRT sources that are to be implanted in a tumor are the diffusion length of lead-212 in the tumor, the diffusion length of radon-220 in the tumor and the leakage probability of lead-212. The diffusion length represents the typical distance from the point an atom was created in the decay of its parent radionuclide to the point where it decays. It determines the spatial distribution of the diffusing atoms around the seed; when the radial distance from the seed increases by one diffusion length, the alpha particle dose drops by approximately a factor of 3. For seeds with radon release rates considered here, the diameter of the region receiving an alpha particle dose of 10 Gy around the seed is roughly 10 times larger than the diffusion length. Methods of calculating the diffusion lengths of lead-212 and radon-220 are described in the appendices. The leakage probability of lead-212 represents the chances that a lead-212 atom released from the source will leave a tumor through the blood system before it decays.

The diffusion lengths of radon-220 and lead-212 and the leakage probability of lead-212 have different values in different types of cancer tumors. Generally, the shorter the diffusion length of lead-212, the more activity is required to achieve similar results. Applicant has measured the diffusion length of lead-212 in various types of tumors and accordingly has determined radon release rates of sources to be used in treating the tumor types.

FIG. 1 is a schematic illustration of a system 100 for planning a radiotherapy treatment, in accordance with an embodiment of the present invention. The treatment generally includes implantation of a plurality of sources in a tumor which is to be destroyed. System 100 comprises an imaging camera 102 which acquires images of tumors requiring radiotherapy. In addition, system 100 includes an input interface 104, such as a keyboard and/or mouse, for receiving input from a human operator, such as a physician. Alternatively or additionally, system 100 comprises a communication interface 106 for receiving instructions and/or data from a remote computer or human operator. System 100 further comprises a processor 108 configured to generate a layout plan of radiotherapy sources in the tumor and accordingly to provide through an output interface 110, details of respective kits of radiotherapy sources for treatment of the tumors. Output interface 110 may be connected to a display and/or to a communication network. Processor 108 optionally comprises a general purpose hardware processor configured to run software to execute its tasks described hereinbelow. Alternatively or additionally, processor 108 comprises a dedicated processor, such as a signal processing processor, a digital signal processor (DSP) or a vector processor, configured with suitable software for performing its tasks described herein. In other embodiments, processor 108 comprises a dedicated hardware processor configured in hardware, such as an FPGA or ASIC to perform its tasks.

In some embodiments, processor 108 is further configured to estimate the radiation dose expected to reach each of the points in the tumor, for example as described in PCT application PCT/IB2021/050034, filed Jan. 5, 2021, and titled “Treatment Planning for Alpha Particle Radiotherapy”, the disclosure of which is incorporated herein by reference.

FIG. 2 is a flowchart of acts performed in preparing a radiotherapy treatment of a tumor, in accordance with an embodiment of the invention. The method of FIG. 2 generally begins with system 100 receiving (202) input on the tumor such as an image of the tumor and/or a type of the tumor. A spacing between the sources to be inserted to the tumor is selected (204) for the tumor and accordingly a number of sources to be included in a treatment kit for the tumor is determined (206). In addition, a duration of the treatment is selected (208). The radon release rate of the sources is also selected (210). In some embodiments, instructions on a layout of the sources in the tumor are also prepared (212). Thereafter, a kit including the number of sources of the selected parameters is prepared (214) and packaged in a suitable, possibly sterile, package. In some embodiments, the method further includes the treatment procedure. In those embodiments, the method includes implanting (216) the sources from the kit into the tumor, for example in accordance with the prepared (212) layout. In some embodiments, the method includes removing (218) the sources after the selected (208) duration. In other embodiments, the sources are not removed and remain in the patient.

In some embodiments, the type of the tumor is determined based on clinical and/or histopathological observations, such as an analysis of a portion of the tumor taken in a biopsy and/or an amount and/or density of blood vessels in the tumor as determined from an image of the tumor. The type of the tumor is selected, for example, from a list including squamous cell carcinoma, basal cell carcinoma, glioblastoma, sarcoma, pancreatic cancer, lung cancer, prostate cancer, breast cancer and colon cancer.

In some embodiments, the sources are arranged in the layout in a regular geometrical pattern which achieves a relatively low distance between each point in the tumor and at least one of the sources.

FIG. 3 is a schematic illustration of a regular arrangement of sources in a hexagonal arrangement 160, in accordance with an embodiment of the invention. In hexagonal arrangement 160, a surface through which sources are entered into a tumor to be treated is divided into hexagons 164 and the center 162 of each hexagon is designated for insertion of a source. The centers 162 for insertion of the sources are located at the vertices of equilateral triangles distance 166 between each two sources is referred to herein as the spacing of the layout. The hexagons 164 are formed by bisectors to the lines connecting the centers 162 to their six nearest neighboring centers 162. The smallest dose of radiation from the sources is at the center of gravity of the triangles, which are at the hexagon vertices. The spacing between the sources is optionally selected (204) responsively to the type of the tumor, because radon-220 and lead-212 have different diffusion lengths in different tumor types, and therefore DaRT sources have different effective ranges in different tumor types. Alternatively or additionally, in selecting the spacing, the accessibility of the location of the tumor within the patient's body is taken into consideration. For example, for tumors in internal organs which need to be accessed by a catheter or an endoscope, a larger spacing is used than for similar tumors which are easily accessed. In some embodiments, the spacing is selected (204) according to a type of treatment of the tumor. One type of treatment is directed to complete destruction of the cells of the tumor. Another type of treatment is directed to reduction of the mass of the tumor to a size that is not visible by a naked eye, or to a size that will make the tumor resectable. Complete destruction generally requires a higher activity level of the sources and/or a smaller spacing between the sources. In some embodiments, the spacing between the sources is selected while taking into consideration the time and complexity of implantation of the sources. The smaller the spacing, the more sources are required and accordingly the time of implantation of the sources increases. Therefore, in accordance with some embodiments of the invention, the largest spacing that would still allow for destruction of the tumor, is used.

Optionally, for reasons discussed below, the spacing for colon cancer is not greater than 4.75 millimeters, not greater than 4.5 millimeters, not greater than 4.2 millimeters or even not greater than 4 millimeters. On the other hand, to avoid unduly increasing the number of sources required, the spacing is optionally greater than 3.4 millimeters, greater than 3.6 millimeters, greater than 3.8 millimeters or even not smaller than 4 millimeters.

FIG. 4 is a schematic illustration of a kit 700 of DaRT sources 21 in accordance embodiments of the present invention. Kit 700 comprises a sterile package 702 including a plurality of alpha-emitter radiotherapy sources 21, for insertion into a tumor.

Optionally, the sources 21 are provided within a vial or other casing 706 which prevents radiation from exiting the casing. In some embodiments, the casing is filled with a viscous liquid, such as glycerine, which prevents radon atoms from escaping the casing 706, such as described in PCT application PCT/IB2019/051834, titled “Radiotherapy Seeds and Applicators”, the disclosure of which is incorporated herein by reference. In some embodiments, kit 700 further includes a seed applicator 708, which is used to insert sources 21 into the patient, as described in PCT application PCT/IB2019/051834. Optionally, applicator 708 is provided preloaded with one or more sources 21 therein. In accordance with this option, separate sources 21 in casings 706 are supplied for cases in which more than the number of preloaded sources is required. Alternatively, sources 21 in casings 706 are not provided in kit 700 and only sources within applicator 708 are included in the kit 700.

The number of sources to be included in a treatment kit 700 for the tumor is determined (206) according to the selected spacing and source layout, in order to cover the entire tumor. In some embodiments, an extra 10-20% sources are provided in the treatment kit.

The duration of the treatment is optionally selected by the operator, according to a desired treatment. In some embodiments, the duration of the treatment is selected (208) in advance based on parameters of the tumor such as its location in the patient's body and the availability of the patient for removal of the sources. Alternatively, the duration of the treatment is selected (208) during the treatment, based on the progress of the treatment.

The activity of the sources and their desorption probability are optionally selected (210) responsive to the selected spacing, the treatment duration and the tumor type. In some embodiments, the activity of the sources and their desorption probability are further selected responsive to a type of treatment of the tumor. If, for example, an operator indicates that a complete destruction of the cells of the tumor is to be aimed for, a higher activity and/or desorption probability is used than for an indication that a removal of the tumor from naked eye surveillance, or a reduction of the tumor size to make it resectable, is required. Optionally, the activity and source probability are selected with an aim to achieve at least a specific radiation dose at each point throughout the tumor (or at least at above a threshold percentage of points throughout the tumor). The specific radiation dose is optionally at least 5 Gray, at least 9 Gray, at least 14 Gray or even not less than 20 Gray. To avoid an overdose, in some embodiments, the specific radiation dose is not greater than 30 Gray, is not greater than 20 Gray or even is not greater than 15 Gray.

It is noted that while the risk of an overdose of radiation for a single small tumor is low, when treating large tumors and/or multiple tumors, the treatment may include implantation of several hundred sources. In such cases, it is important to accurately adjust the activity of the sources to prevent administering an overdose of radiation to the patient.

The activity is measured herein in units of microcurie per centimeter length of the source. As the radiation dose reaching most of the tumor is dominated by radionuclides that leave the source, a measure of “radon release rate” is defined herein as the product of activity on the source and desorption probability. For example, a source with 2 microcurie activity per centimeter length and a 40% desorption probability has a radon release rate of 0.8 microcurie per centimeter length.

The total amount of radiation released by a source in a tumor, referred to herein as “cumulated activity of released radon”, depends on the radon release rate of the source and the time for which the source remains in the tumor. If the source is left in the tumor for a long period, for example more than a month for a radium-224 source, the cumulated activity of released radon reaches the product of radon release rate of the source multiplied by the mean life time of radium-224, which is 3.63 days or 87.12 hours, divided by ln 2, which is about 0.693. For example, a radium-224 source having a radon release rate of 1 microCurie (μCi)=37,000 becquerel (Bq), has a cumulated activity of released radon of about 4.651 Mega becquerel (MBq) hour. It is noted that the same amount of cumulated activity of released radon may be achieved by implanting a source with a higher radon release rate for a shorter period. For such a shorter period, the cumulated activity is given by:

$cumulated activity = S (0) * τ * (1 - e^{- \frac{t}{t}})$

where S(0) is the radon release rate of the source when it is inserted into the tumor, τ is the mean radium-224 lifetime and t is the treatment duration in hours. For example, a two-week treatment provides a cumulated activity of:

$cumulated activity (14 days) = 0.037 MBq * 125.7 h * (1 - e^{- 14 * \frac{24}{125.7}}) = 4.33 MBqh$

It is noted that the acts of FIG. 2 are not necessarily performed in the order in which they are presented. For example, in cases in which the activity of the sources is not selected (210) responsive to the treatment duration, the activity of the sources may be selected (210) before, or in parallel to, selecting (208) the treatment duration. As another example, the preparation of the layout and the preparation of the kit may be performed concurrently or in any desired order.

FIG. 5 is a schematic illustration of a radiotherapy source 21, in accordance with an embodiment of the present invention. Radiotherapy source 21 comprises a support 22, which is configured for insertion into a body of a subject. Radiotherapy source 21 further comprises on an outer surface 24 of support 22, radionuclide atoms 26 of radium-224 as described, for example, in U.S. Pat. No. 8,894,969, which is incorporated herein by reference. It is noted that for ease of illustration, atoms 26 as well as the other components of radiotherapy source 21, are drawn disproportionately large. In some embodiments, a coating 33 covers support 22 and atoms 26, in a manner which controls a rate of release of the radionuclide atoms 26 and/or of daughter radionuclides of atoms 26, upon radioactive decay. In some embodiments, as shown in FIG. 5, in addition to coating 33, an inner coating 30 of a thickness T1 is placed on support 22 and the radionuclide atoms 26 are attached to inner coating 30. It is noted, however, that not all embodiments include inner coating 30 and instead the radionuclide atoms 26 are attached directly to the source 21.

Support 22 comprises, in some embodiments, a seed for complete implant within a tumor of a patient, and may have any suitable shape, such as a rod or plate. Alternatively to being fully implanted, support 22 is only partially implanted within a patient and is part of a needle, a wire, a tip of an endoscope, a tip of a laparoscope, or any other suitable probe.

In some embodiments, support 22 is cylindrical and has a length of 5-60 mm (millimeters). Support 22 optionally has a diameter of 0.7-1 mm, although in some cases, sources of larger or smaller diameters are used. Particularly, for treatment layouts of small spacings, support 22 optionally has a diameter of less than 0.7 mm, less than 0.5 mm, less than 0.4 mm or even not more than 0.3 mm.

An amount of radiation supplied by radiotherapy device 21 to surrounding tissue depends on various parameters of the radiotherapy device. These include:

1) an amount of radionuclide atoms 26

2) a desorption probability of radionuclide atoms 26 upon decay, and

3) a rate of release of radionuclide atoms 26 by diffusion

The amount of radionuclide atoms 26 in radiotherapy device 21 is generally given in terms of activity per centimeter length of support 22. Typical values of activity are in the range of 0.1-20 micro-curie (μCi) per centimeter length.

The desorption probability depends on the strength of the bond of radionuclide atoms 26 to support 22 and/or to the type and thickness of coating 33. The bond of the radionuclide atoms 24 to support 22 is generally achieved by heat treatment of the radiotherapy device 21, and the strength of the bond is controllable by adjusting the temperature and/or duration of the heat treatment. In some embodiments, the desorption probability is between about 38-45%. Alternatively, higher desorption probabilities are achieved, for example using any of the methods described in PCT publication WO 2018/207105, titled: “Polymer Coatings for Brachytherapy Devices”, the disclosure of which is incorporated herein by reference. In other embodiments, lower desorption probabilities are used, such as described in U.S. provisional patent application 63/126,070, titled: “Diffusing Alpha-emitters Radiation Therapy with Enhanced Beta Treatment”, the disclosure of which is incorporated herein by reference.

The rate of release of radionuclide atoms 26, e.g., by diffusion, is in some embodiments very low and even negligible. In other embodiments, a substantial rate of diffusion of radionuclide atoms 26 is used, for example using any of the methods described in PCT publication WO 2019/193464, titled: “Controlled Release of Radionuclides”, which is incorporated herein by reference. The diffusion is optionally achieved by using a bio-absorbable coating which initially prevents premature escape of radionuclide atoms 26 but after implantation in a tumor disintegrates and allows the diffusion.

The required amount of activity on the sources in order to achieve tumor destruction varies dramatically for different types of tumors and source spacings. It is therefore important to identify for each type of tumor, the required activity for that specific tumor type. Methods for calculating the radiation reaching each point in a tumor, according to the activity of the implanted sources, are described in U.S. patent application Ser. No. 17/141,251, filed Jan. 5, 2021 and titled, “Treatment Planning for Alpha Particle Radiotherapy”, the disclosure of which is incorporated herein by reference.

Using those calculation methods, the required radon release rate can be calculated as a function of the diffusion length of lead-212 in the tumor, the diffusion length of radon-220 in the tumor, the spacing between the sources implanted in the tumor, the leakage probability of lead-212 from the tumor and the amount of radiation required to reach each location in the tumor.

FIGS. 6A-6D are graphs which illustrate the wide range of required radon release values for different values of the above parameters. FIG. 6A and FIG. 7B, which is an inset of FIG. 6A, show the required radon release rate as a function of the lead-212 diffusion length, for three different values of the radon-220 diffusion length, when the lead leakage probability is 80% and the spacing is 3.5 mm FIG. 6B is a similar graph, for a lead leakage probability of 40%. FIG. 6C shows the same graph for a spacing of 4 mm and lead leakage probability of 80%, while FIG. 6D shows the required radon release rate for 4 mm spacing and 40% lead leakage probability. The reader will appreciate that the range of possible radon release rate values is very large and the following discussion provides guidance as to narrow ranges to be used for specific tumor types.

In order to measure the diffusion length of lead-212 in different types of tumors applicant implanted sources inside tumors generated in mice and after a few days dissected the tumor and measured the actual activity that reached the various points in the tumor. These measurements are fit into the above equations and accordingly the diffusion length of lead-212 in the tumor is estimated.

The tumor was removed from the mouse and frozen so that the tumor can be sliced a short time after the removal of the tumor from the mouse. Thereafter, the tumor was sliced into slices of a thickness of about 10 microns. Fixation by formalin was done immediately after sectioning, and for a short duration (minutes), directly on the histological slices, placed on glass slides. After fixation, the slides were laid on a Fuji phosphor imaging plate in closed box for one hour. The slides were separated from the plate by a thin Mylar foil to avoid contaminating the plate by radioactivity. The plate was subsequently scanned by a phosphor imaging autoradiography system (Fuji FLA-9000) to record the spatial distribution of lead-212 inside the histological slices.

Further details of the measurement of the diffusion length of lead-212 are discussed hereinbelow in appendix A.

The diffusion length of radon-220 in different types of tumors was determined in a similar manner, but rather than waiting several days, the tumor was removed about half an hour after source insertion, as the spatial distribution of radon-220 stabilizes very fast, and the measurement requires that the contribution arising from lead-212 buildup inside the tumor would be minimal (lead-212 buildup increases from zero to a maximal value about 1.5-2 days after source insertion, and is sufficiently low 30 minutes after source insertion). Details of the measurement of the diffusion length of radon-220 are discussed hereinbelow in appendix B.

The results of the above described measurements of the diffusion length of lead-212 are summarized in the following table 1, in which for each cancer type and average lead-212 tumor length is provided along with an extent of a standard deviation from the average.

TABLE 1

212Pb diffusion length

Tumor type
in millimeters

Squamous cell carcinoma
0.50 ± 0.12

Colon
0.45 ± 0.06

Glioblastoma (GBM)
0.40 ± 0.09

Melanoma
0.38 ± 0.05

Prostate
0.35 ± 0.05

Breast (triple negative)
0.35 ± 0.05

Pancreatic cancer
0.30 ± 0.08

Measurements of the diffusion length of radon-220 found values of between 0.23 and 0.31 mm. The number of measurements, however, is relatively small and therefore in the following discussion, room is left for the possibility that the value of the diffusion length of radon-220 is as low as 0.20 mm.

Based on clinical results, the inventors believe that a radiation level of 10 Gray throughout the tumor is sufficient.

The lead-212 leakage probability is relatively low in the center of the tumor, but reaches about 80% on the periphery of the tumor. In order to ensure tumor destruction throughout the tumor, applicant has used the 80% leakage probability value in selecting the radon release rate of the sources.

Another factor which may be used in determining the radon release rate of the sources is the spacing between the sources. If the sources are to be placed with a relatively high spacing between them, such as 4.5 mm or 5 mm, the sources should have a high radon release rate, such as above 1.5 microcurie per centimeter length. In contrast, when the spacing between the sources is below 4 mm, the sources may be assigned a relatively low radon release rate.

FIG. 7A is a graph showing the value of the required radon release rate for a 4 mm spacing, 80% lead-212 leakage, and a radiation dose of 10 Gray, over a limited range of interest, in accordance with embodiments of the invention. FIG. 7A is an inset of FIG. 6C, which will be referred to when values go beyond the boundaries of FIG. 7A. As can be seen, the required radon release rate varies substantially for different tumor types, which have different diffusion lengths. Therefore, in order to properly select the radon release rate of the sources, it is crucial to have a good estimate of the radon-220 and lead-212 diffusion lengths in the tumor type. In selecting the radon release rate, applicant has considered that inaccuracies may occur in the placement of the sources, such that some sources may be separated by an extent larger than the prescribed spacing. In addition, the tumor may be non-homogenous such that some areas of the tumor may require more radiation than others.

As stated in Table 1, the lead-212 diffusion length for colon cancer is estimated to be about 0.45 mm with a standard deviation of 0.06. The radon-220 diffusion length is between 0.2-0.3 mm. As can be seen in FIG. 7A, using a radon release rate of between 0.7 and 2.2 microcurie per centimeter length would achieve sufficient destruction of the tumor, regardless of the exact values of the radon-220 diffusion length. Further taking into account that it is unlikely that both the radon-220 diffusion length and the lead-212 diffusion length will have values at the ends of their expected ranges, a radon release rate of between 0.75 and 1.8 microcurie per centimeter length could be considered sufficient. For a long term treatment, this corresponds to a cumulated activity of released radon of between about 3.5 MBq hour per centimeter and 8.4 MBq hour per centimeter.

In some embodiments, in order to increase the probability of success of the treatment, a radon release rate of at least 0.9 microcurie per centimeter length, at least 1.05 microcurie per centimeter length, at least 1.2 microcurie per centimeter length or even at least 1.3 microcurie per centimeter length is used for colon cancer. On the other hand, in some embodiments, in order to reduce the amount of radiation to which the patient is exposed, the radon release rate is not greater than 2.0, not greater than 1.8, not greater than 1.6 or even not greater than 1.4 microcurie per centimeter length.

Alternatively or additionally, the sources optionally include at least 3.9 MBq hour per centimeter, at least 4.4 MN hour per centimeter, at least 4.9 MBq hour per centimeter or even at least 5.3 MN hour per centimeter. On the other hand, the sources optionally include less than 8 MBq hour per centimeter or even less than 7.5 MBq hour per centimeter.

In some embodiments, the exact radon release rate within the range is selected responsive to the duration of the treatment. If a long duration of at least about 10 days or even at least 14 days is used, a lower radon release rate is used. In contrast for short treatment durations, such as below 100 hours or even below 50 hours, the sources are assigned a higher radon release rate.

CONCLUSION

It will be appreciated that the above described methods and apparatus are to be interpreted as including apparatus for carrying out the methods and methods of using the apparatus. It should be understood that features and/or steps described with respect to one embodiment may sometimes be used with other embodiments and that not all embodiments of the invention have all of the features and/or steps shown in a particular figure or described with respect to one of the specific embodiments. Tasks are not necessarily performed in the exact order described.

It is noted that some of the above described embodiments may include structure, acts or details of structures and acts that may not be essential to the invention and which are described as examples. Structure and acts described herein are replaceable by equivalents which perform the same function, even if the structure or acts are different, as known in the art. The embodiments described above are cited by way of example, and the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Therefore, the scope of the invention is limited only by the elements and limitations as used in the claims, wherein the terms “comprise,” “include,” “have” and their conjugates, shall mean, when used in the claims, “including but not necessarily limited to.”

APPENDIX A

²¹²Pb Diffusion Length Measurement

A single DaRT seed (6.5 mm length, 0.7 mm outer diameter), carrying 2-3 uCi ²²⁴Ra, is inserted to the center of a mice-borne tumor 7-20 days after tumor inoculation, when the tumor transverse diameter is ˜6-15 mm Four to five days later, the tumor is excised (as a whole) and cut in two halves, at the estimated location of the seed center, perpendicular to the seed axis. The seed is then pulled out using surgical tweezers and placed in a water-filled tube for subsequent measurement by a gamma counter. The tumor is kept for one hour at −80° C. It is then taken, in dry ice, for measurement in the same gamma counter to determine the ²¹²Pb activity it contains. The measurements of the seed and tumor activity are used to determine the ²¹²Pb leakage probability from the tumor.

Immediately after the gamma measurement, both halves of the tumor undergo histological sectioning by a cryostat microtome. Sections are cut at 250-300 μm intervals with a thickness of 10 μm, laid on positively charged glass slides and fixed with 4% paraformaldehyde. Typically, there are 5-15 sections per tumor, spanning a length of 1.5-5 mm Shortly after their preparation, the glass slides are placed, faced down, for a duration of one hour, on a phosphor imaging plate (Fujifilm TR2040S) protected by a 12 μm Mylar foil and enclosed in a light-tight casing. Alpha particles emitted from the sections in the decays of ²¹²Pb progeny atoms, ²¹²Bi and ²¹²Po, penetrate through the foil and deposit energy in the active layer of the phosphor imaging plate. The plate is then read out by a phosphor-imaging scanner (Fujifilm FLA-9000).

For each tumor section, the result is a two-dimensional intensity map, proportional to the local ²¹²Pb activity. The intensity (in units of photo-stimulated luminescence) is converted to ²¹²Pb activity using suitable calibration samples measured concurrently with the slides. The point where the seed crosses the section is identified either by the appearance of a “hole” in the activity map, or by taking the center-of-gravity of the activity distribution. Examples are shown in FIGS. 8A-8D. We define a region of interest (ROI) centered at the estimated seed location, and divide it into 0.1 mm-wide concentric rings, with radii in the range 0.5-3 mm. For each ring, we calculate the mean value of the activity. If the ROI extends beyond the area of the tumor section, or includes a region with degraded tissue or image quality, the ring average is taken over a limited azimuthal sector. The resulting curve of the activity as a function of the radial distance from the origin (estimated seed location), is then fitted numerically by a function describing the radial activity distribution from a seed, based on the diffusion-leakage model. The calculation describes the seed as a line source perpendicular to the image. The source is divided into a large number of point-like segments, each contributing, to a given pixel in the plane of the image, an activity A

$\frac{\exp (- r / L_{Pb})}{r} .$

In this expression r is the distance between the source segment and the pixel under consideration, and A and L_Pbare free parameters, whose values are adjusted to optimize the fit to the entire curve (FIGS. 8A-8D). The value obtained for L_Pbis taken as an estimate for the ²¹²Pb diffusion length of the section. The average value of L_Pbover all sections is taken to represent the ²¹²Pb diffusion length of the tumor, with an uncertainty equal to the standard deviation of the values obtained in all sections.

FIG. 8B is a graph of the radial activity distribution for the sampled data of FIG. 8A, fitted by the diffusion-leakage model.

FIG. 8C shows a PSL spatial distribution in another histological section from the same tumor where the seed position is determined automatically by calculating the intensity center-of-gravity.

FIG. 8D is a graph of the radial activity distribution for the sampled data of FIG. 8C, fitted by the diffusion-leakage model.

FIG. 9 shows the measured values of ²¹²Pb diffusion length as a function of tumor mass for pancreatic tumors.

FIG. 10 shows the measured values of ²¹²Pb diffusion length as a function of tumor mass for prostate tumors.

FIG. 11 shows the measured values of ²¹²Pb diffusion length as a function of tumor mass for melanoma tumors.

FIG. 12 shows the measured values of ²¹²Pb diffusion length as a function of tumor mass for squamous cell carcinoma tumors.

FIG. 13 shows the measured values of ²¹²Pb diffusion length as a function of tumor mass for triple negative breast tumors.

FIG. 14 shows the measured values of ²¹²Pb diffusion length as a function of tumor mass for GBM tumors.

APPENDIX B
Rn Measurement Methodology

A DaRT seed in inserted to a tumor for a relatively short time—30 minutes, after which the seed is removed (in order to prevent the Pb buildup inside the tumor). The tumor is then set to freeze and cut into 10 μm-thick sections perpendicular to the seed axis. These are placed on glass slides and are fixed using Formaldehyde. The tumor sections are than taken to a digital autoradiography system (iQID alpha camera, by QScint Imaging Solutions, LLC), which records alpha particle hits one-by-one, providing their xy coordinates (with an accuracy of ˜20 μm), a timestamp and a signal proportional to the deposited energy.

An example for an image, consisting of four histological sections of a DaRT treated tumor and acquired using the iQID system is shown in FIG. 15, which shows four histological sections of a DaRT treated tumor, acquired using the iQID autoradiography system. For the analysis, the image is cropped so that each section is analyzed independently, as can be seen in FIG. 16A, which shows a single histological section used for the analysis. For each section, the center is chosen (either by a center-of-gravity method, or by identifying a “hole” in the activity map), and the average number of alpha particle counts is calculated at increased radial distances from the center. The resulting plot is then fitted numerically, by assuming that the recorded activity map is a superposition of infinitesimal segments along the DaRT seed, where each segment is calculated using eq. 1:

$\begin{matrix} Γ (r) = \frac{A}{r} \cdot e^{(- \frac{L_{Rn}}{r})} & (1) \end{matrix}$

In this expression, r is the radial distance between the seed segment and the point-of-interest on the image, L_Rnis the radon diffusion length and A is a free parameter. These two parameters (L_Rn, A) are found by a least-squares-fit approach.

The fit is performed over a limited region of the activity distribution, to avoid the artificial “hole” in the center (where the DaRT seed was) and the far end of the distribution, where the statistical variation are too large. An example of a fitted curve is shown in FIG. 16B, which shows the calculated average counts as a function of distance from the seed location, including the fitted function.

Diffusing alpha-emitter radiation therapy for colon cancer

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