Patent Application
20070075277
This invention concerns the absorption of radiation, such as x-ray radiation, using a flexible shield. Particularly, the invention is concerned with a lightweight, very thin and flexible non-lead radiation shield, worn against a patient while radiation therapy is administered internally to the patient, and with protection against the effects of backscatter radiation on the patient.
Shields for protection of patients and medical workers against excessive doses of radiation, particularly in dentists' offices and other x-ray imaging or therapy situations, are well known. Heavy and relatively stiff lead shields have been typical for this purpose.
Shields of lighter weight and greater flexibility have also been used. U.S. Pat. Nos. 4,938,233, 6,048,379 and 6,674,087 disclose various radiation shields, some of which employ tungsten or other heavy metal particles suspended in a polymeric flexible medium, such as silicone.
Experimental results have indicated that radiation at, for example, 50 kVp, absorbed in a shield formed of such heavy metal particles, generates an undesirable backscattered radiation dose. For the situation where radiation is administered from a source within the patient, the backscattered radiation dose is absorbed in adjacent tissue, particularly the patient's skin adjacent to the shield. The current disclosure includes improvements to the flexible absorber design to minimize this undesirable and potentially damaging effect.
The invention now described encompasses a lightweight, very thin and flexible radiation shield which includes, in flexible media, a layer including high atomic number particles and a layer including mid atomic number particles.
Measurements indicate that backscattered radiation is largely limited to low-energy photons. The invention includes the incorporation of a thin layer or layers of solid mid atomic number absorber particles carried in a polymer incorporated into the patient side of the absorber panel. In use, impinging high energy x-ray photons pass into the absorber through the thin layer of mid atomic number particles. Backscattered radiation from this thin layer is minimal. As x-rays pass into the heavy atomic number absorber, they are absorbed, and any backward-emitted low energy backscatter radiation is in turn largely absorbed by the mid atomic number layer or layers of the invention.
A preferred embodiment of the invention involves the use of a first, patient-adjacent layer with a thin silicone polymer carrier that is loaded with fine metal particles. Ideally these metal particles have significant content of the mid atomic number elements Fe, Co, or Ni due to their inherent radiation absorption edges. As the layer should also remain non-toxic, food grade Fe, Fe oxides, and/or stainless steel powders are ideal. The powders are mixed with liquid silicone rubber, and applied to the absorber device in a thin film.
A second layer more remote from contact with the patient includes high atomic number particles, such as tungsten, again in a flexible medium such as silicone. The entire composite of multiple layers, in a preferred embodiment, is not greater than about 2 mm in thickness.
In one specific embodiment of the invention the flexible shield is used in conjunction with one or more dosimeters, placed adjacent to the patient's skin. The dosimeters can be incorporated into the shield, at or very close to the patient side of the shield. These dosimeters can provide feedback for verification of dose at the skin, and for control of the dose.
It is thus among the objects of this invention to improve in the convenience of use and in the performance and effectiveness of non-lead flexible radiation shields, particularly for the case where radiation is administered inside the patient and backscatter is an important concern. These and other objects, advantages and features of the invention will be apparent from the following description of preferred embodiments, considered along with the drawings.
In the drawings,
In one preferred implementation the overall thickness t of the flexible radiation shield 10 is no more than about 2 mm, and can be even less.
Of the two layers 14 and 16, these in one preferred embodiment are both soft silicone, such as very soft Shore A5 medical grade silicone. In one preferred embodiment the layer 14, more remote from the patient, is filled with ninety percent by weight tungsten powder, carried in the silicone host. The tungsten powder in one embodiment is minus 100 mesh sintered tungsten metal, mixed with the liquid silicone and molded into sheets or shapes suitable for the absorber application. Breast shapes, i.e. cup shapes, have also been produced of this material. Such a layer alone, only about 1 millimeter in thickness, has been shown to attenuate x-rays of 45 kvp by a factor of greater than ten thousand.
Because a single layer such as the layer 14 described above tends to generate an undesirable backscatter radiation dose to adjacent tissue when x-rays at about 45 to 50 kVp are primarily being absorbed, the flexible radiation shield of the invention includes the layer 16, also preferably a layer with a soft silicone host. The layer 16 comprises at least one layer having solid mid-atomic number absorber particles, and this layer (or layers) 16 is placed against the patient. In one preferred embodiment the mid-atomic number particles comprise about fifty percent by weight of the entire layer, the balance being the same soft medical grade silicone described above relative to the layer 14. The mid-atomic number particles preferably are at least as small as minus 100 mesh (149 microns in diameter), and more preferably about 400 mesh (37 microns). A preferred size range is about 35 to about 150 microns. They may be, for example, any of the following metals alone or in mixtures, including compounds of any of the metals: iron, nickel and cobalt and other elements of similar atomic number. Iron, nickel and cobalt match have absorption that matches the absorption and re-emission of characteristic lines and radiation of tungsten. Since the layer should remain non-toxic, food grade iron oxides and/or stainless steel powders are advantageously used. These powders are mixed with liquid silicone rubber, and can be applied against the layer 14 in a thin film, essentially integrating the two silicone layers together. Alternatively, the layer 14 can be applied against a previously produced layer 16.
Tests of a composite flexible radiation absorber shield 10, produced in accordance with the example given above, revealed, at 50 kVp radiation, a significant reduction of backscatter. Most of the x-ray radiation at 50 kVp appears to pass through the patient-adjacent layer 16, and of the radiation which does, nearly all is absorbed in the layer 14 (with greater than 10,000 to 1 reduction based on radiation which is able to transmit through the entire shield 10). As noted above, a small percentage of the radiation striking the high molecular weight layer 14 is backscattered back toward the patient, and nearly all of this backscatter is absorbed as it travels back through the mid-molecular weight layer 16 adjacent to the patient. Backscattered radiation from the mid-molecular weight layer 16, from the initially impinging radiation, is minimal.
In other embodiments other polymers can be used as carriers or hosts for the layers of high molecular weight and mid-molecular weight absorber materials. Wax layers have been produced, for disposable use and preferably shaped to the patient's breasts or other organ or body feature where radiation is being internally administered. This type of shield is castable to the shape desired and produces a semi-hard absorber structure, of relatively low cost. Also, shields can be produced with much lower proportions of radiation attenuating metals, and these structures may be used in contrast enhancing, marker or filter applications.
The absorber 10 constructed as in
In another embodiment, the flexible radiation shield structure 10 shown in
A graded absorber shield structure may be produced for certain applications. In this form the shield is created with co-bonded regions that have tungsten filler adjacent to regions that have no filler. The result is an absorber with selective absorption which may be of value in certain radiation treatment applications.
Functionally composite structures including adhesives can form an integral part of the shield. For example, adhesive (covered by a releasable backing sheet) can be in selected areas of the skin side of the shield, where the surgeon is likely to cut the shield to make the patient incision. The adhesive helps permit closure of any gaps.
The flexible radiation shield for the breast application covers the breast and reduces the dose leaving the patient during the treatment. This shield will allow the doctor, attending staff and friends to be with the patient during treatment. The shield has features that reduce the secondary scattering dose at the interface between the high Z material absorber and the patient's skin. Placing a miniature dosimeter on the patient's skin over the applicator will allow a verification of the does delivered and especially the dose to the skin. Due to the backscatter dose that is developed because of the high Z shield, obtaining an accurate dose at the skin surface depends on how the x-rays interact with the dosimeter. Having optimized low and intermediate Z materials surrounding the detector is critical to achieving accurate dosimetry. The miniature dosimeter 22 or dosimeters, can be integrated into the flexible shield so that they are one component, as shown in
More than one detector can be installed in the shield, as indicated in
The dosimeters on the surface, between the skin and shield, can also be used for mapping and feedback control. In the mapping mode the x-ray source or sources can be run at their intended high voltage but at a reduced source current, to reduce the dose, but to indicate the dose that would be delivered at full source current. The sources would be run as indicated at all dwell positions and the total delivered dose would be recorded. This mode can accurately predict the total dose that will be delivered at the skin at selected locations when the source or sources are run at full power, time and dwell positions.
In the feedback control mode, the dosimeter readings can be used in real time to control the source's output to achieve a desired total dose. When the dose at a given dosimeter reaches the desired level, the source can be changed in current or position.
The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to these preferred embodiments will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims.
