Feasibility of penumbra compensating filters for conformal prostate radiotherapy

Radiation therapy beams demonstrate a gradual dose fall off at the field edges, due to factors affecting the physical penumbra and transport of radiation. Adequate target coverage requires an increase in field size larger than the target volume itself for a uniform dose to be delivered to that target volume. A method is presented for the design and fabrication of penumbra compensating filters (PCFs) which essentially sharpen the penumbra on a field-by-field basis and are used in conjunction with custom shielding blocks. We have explored the feasibility of using PCFs to reduce the field margins required for our four-field conformal prostate treatments. The penumbra compensation is designed based on a profile measured along the direction perpendicular to the blocked field edge that shows the greatest 50% to 95% isodose distance for a typical conformal prostate patient. Rigid foam material is milled and filled with a low melting point alloy material to create a filter which provides dose compensation in the field periphery of the custom shielding block. The accuracy of our methodology has been established using film dosimetry. By employing PCFs, the reduction in the rectal margin ranges from approximately 4 mm in the posterior region to 13 mm in the superior-posterior region, as compared with the shielding blocks alone. The reduction in bladder margin ranges from approximately 4 mm in the superior-anterior region to 10 mm in the superior region. Dose-volume histograms for an idealized cylindrical rectum indicate a substantial reduction in the volume treated to high doses. The calculated normal tissue complication probability values were 8.7% and 10.5% with and without PCFs included in the blocked fields respectively. The advantages of using PCFs, compared with multileaf collimator based techniques, are discussed.