Patent Application
20100022818
This invention concerns radiation therapy, especially electronic x-ray brachytherapy, for treating tissues which may have diffuse proliferative disease.
In brachytherapy, a radiation source (or a plurality of sources) is generally placed within a surgically created or naturally occurring cavity in the body. Such therapy often follows surgical treatment of cancer. The recently developed and now preferred radiation source for use with this invention is a miniature electronic x-ray source. Alternatively, selected low-energy radio-isotopes may be used with commensurate safety measures. One advantage of these sources, particularly the x-ray sources, is that their absorbed dose intensity can be conveniently manipulated with regard to output, on/off or modulated, and they can easily be attenuated locally.
Before the availability of miniature x-ray tubes, most brachytherapy was performed using high-energy isotopes like Iridium 192. Such high-energy isotopes are not capable in general of local attenuation because within the distances of interest for brachytherapy, their intensity will be basically unaffected by all practical attenuators.
With conventional intracavitary brachytherapy, a therapist prescribes a therapeutic dose of radiation to be administered to a volume of tissue (the target tissue) lying just outside the treatment cavity into which the radiation source or sources will be placed.
Generally the prescribed dose will specify a uniform minimum dose to be delivered at a preferred depth outside the treatment cavity (the prescription depth). Because by the laws of physics, radiation intensity falls off more or less exponentially with increasing distance from the radiation source, the prescription will include, either explicitly or implicitly, a maximum dose above which normal tissue may be subject to substantial necrosis. It is therefore generally desirable to create and maintain a predetermined space between the source of radiation and the first tissue surface to be treated (generally the cavity surface) in order to moderate the surface dose while still delivering the prescribed dose at the prescription depth.
This is usually accomplished by placing an applicator in the cavity which both fills and shapes the cavity into, most often, a solid figure of revolution (e.g., generally a sphere, cylinder or ellipsoid), and positions the radiation source along a central axis of the cavity so formed and along which the source may be traversed. It is common for the prescription to be delivered from several positions or dwell points along the axis of the applicator, often starting at the distal-most position, and sequentially withdrawn through the more proximal positions.
Some applicators are solid and of a polymer such as Noryl (GE Plastics), Radel (Solvay Advanced Polymers), or Ultem (GE Plastics), or comprising a composite structure of polymers, and of a shape to establish the necessary distances to effect proper delivery of the prescription (as in application Ser. No. 12/075,120, filed Mar. 5, 2008, which is incorporated herein by reference) while others have an inflatable element like a balloon (see, e.g., application Ser. No. 10/683,885, filed Oct. 10, 2003, incorporated herein by reference), which may be inflated to shape the cavity. It is preferable that the material of the solid applicator is substantially matched to attenuation of soft tissue. This simplifies treatment planning. If the applicator has a thin balloon such that attenuation due to the balloon may be substantially ignored, the balloon is preferably inflated using a fluid medium which has radiation attenuation properties similar to those of tissue. Water is such a medium, and again, this choice of medium simplifies treatment planning.
Traditionally, the radiation source is introduced into the applicator through a lumen positioned at a predetermined location in relation to the outer surface of the applicator (the inflated outer surface if of a balloon type). This location is usually along a central axis of the applicator, for example a lumen in the case of a solid type, or within a tubular source guide if a balloon type. These lumina are generally open at their proximal end to facilitate source insertion, but closed at their distal end.
As the source is withdrawn, an air gap is formed distally of the source within the lumen being traversed. The radiation attenuation of air is substantially less than that of the applicator material (or inflation medium if of a balloon type) so as to be near zero attenuation. Therefore, since the radiation distally down the applicator is only minimally attenuated, there is a radiation contribution from every sequential dwell point during the treatment, and a “hot spot” may be generated distal of the applicator lumen as the source is traversed away from its distal-most position.
It is an objective of this invention to overcome such hot spots in brachytherapy applicators. Other objectives will become apparent from the following discussion and figures.
We have discovered that an effective method to eliminate central hot spots distal of conventional brachytherapy applicators during radiotherapy is to incorporate a disc of attenuating material covering, and preferably somewhat overlapping the distal end of the central applicator lumen. A preferred disc is of Noryl (a blend of polyphenylene oxide and polystyrene) doped with Barium Sulfate (BaSO4). The overlap is preferable since the radiation “jets” from the more proximal dwell points tends to spread with increasing distance.
Overlapping is not required to achieve substantial results using the concept of the invention, however. With electronic radiation sources, the lumen might be on the order of 6 mm in diameter. A preferred disc would then be on the order of 7 mm diameter. The disc thickness would be dependent on loading and the resulting attenuation, but would be sized to produce the prescribed distal dose cumulatively considering the contribution from all dwell points. An example might be a Noryl disc of 2 mm thickness with a 6% preferred loading of BaSo4by weight.
In another embodiment water is used as a radiation attenuator in the distal axial direction. The water can be pure or a solution of selected compounds, such as one or more salts.
Other disc materials and loadings will occur to those of skill in the art and are to be considered within the scope of the invention. Present applicators have been largely developed for use with high-energy isotope sources and do not include axial attenuators.
The disc 14 is aligned with the lumen 16 and with axially emitted radiation emanating from the lumen. The disc thickness and BaSO4 filling level are adjusted such that the cumulative dose delivered distally from each dwell point is commensurate with the prescription dose. The disc diameter is preferably, though not necessarily, from 10% to 20% greater than the lumen diameter, making necessary the size of the counterbore. A radiation source 24 on the end of a source catheter 26 is shown within the lumen 16.
The preferred solid applicator embodiment is as described above, but other variations or designs of solid applicators can be improved by applying similar methods to those described above. For example, multi-lumen solid applicators can be fitted with separate discs beyond the end of and in line with each lumen, or collectively fitted with a larger, single attenuating disc overlapping all lumina. Also, the attenuating disc can be of different sizes, even of diameter equal to the lumen, if desired, although slightly larger is preferred for reasons explained above. Further, the attenuating disc may comprise simply a plug such as the plug 20 bonded into a bore 18 in the end of the solid applicator, and being of greater attenuating property than the solid applicator, in lieu of the two-piece arrangement 14, 20 shown in
The incorporation of an attenuating disc at the end of the lumen 36 in a balloon applicator can be accomplished in several ways. The balloon itself can be doped in a preferably circular area at a spot surrounding the distal end of the lumen, i.e. the end of the source guide shaft or body 32 which forms the lumen. See copending applications Ser. Nos. 10/683,885, 10/962,247 and 11/471,013), in which selected areas of balloons are doped with attenuating materials in balloon applicators. The balloon thickness can be a limiting factor as to whether sufficient attenuating material can be incorporated in the balloon, but this will reduce the “hot spot” effect. The shaft 32 itself could be fitted with a cap over its end, the cap having attenuating material, or, in cases in which the diameter of the attenuating element is deemed sufficient at the same diameter as the lumen 36, a plug having attenuating material can be fitted into the lumen at the end of the shaft 32. Other methods and apparatus are also possible.
Other balloon applicator designs may be substituted for that shown, including incorporation of other functionality, for example drainage, with appropriate conventional details of construction as would be apparent to those of skill in the art.
With either embodiment, the distal axial dose can be engineered to deliver the prescribed dose throughout the target tissue without a hot spot discontinuity centrally at the distal tip of the applicator. For example, we have found that using solid applicators with outer diameters according to the table below, a concentric inner lumen of 5.6 mm and disc outer diameters of 6.6 mm, a conformal dose of radiation can be generated from those disc thicknesses corresponding in the table.
A conformal dose of radiation has isodose surface shapes that mimic the outer surface of the applicator. Isodose surfaces are imaginary surfaces where the dose intensity is uniform over the entire surface. It is a simple matter to determine disc dimensions for balloon conformal doses once the balloon shapes are selected.
Although the description above assumes an electronic radiation source such as the preferred miniature x-ray tube of, for example, U.S. Pat. Nos. 6,319,188 and 7,158,612, incorporated entirely herein by reference, the radiation source (or sources) could equally comprise low-energy isotopic seeds of Iodine 125 or Palladium 103 mounted on a wire or wires (not shown).
From the description of preferred embodiments above, other embodiments will be apparent to those of skill in the art and are to be considered as within the scope of this invention as defined in the following claims.
