TOTAL-BODY IRRADIATION WITH AN ARC AND A GRAVITY-ORIENTED COMPENSATOR

Purpose: To deliver uniform dose distributions for total-body irradiat ion (TBI) with an are field and a gravity-oriented compensator. This t echnique allows the patient to be treated lying on the floor in a smal l treatment room. Methods and Materials: Through the sweeping motion o f the gantry, a continuous are field can deliver a large field to a pa tient lying on the floor. The dose profile, however, would not be unif orm if no compensator were used, due to the effects of inverse square variation of beam intensity with distance as well as the slanted depth in patient. To solve this problem, a gravity-oriented compensator mad e of cerrobend alloy was designed. This compensator has a cross-sectio n of an inverted isosceles triangle, with the apex always pointing dow nward, due to gravity. By properly selecting the thickness of the comp ensator, the width of the base, and the distance between the pivots to the base, the difference in the path length through the compensator c an be made just right to compensate the effects of inverse-square and slanted depth, thus producing a uniform dose profile. Results: Arc fie lds with a gravity-oriented compensator were used for 6, 10, 15, and 1 8 MV photon beams. The arc field can cover a patient with a height up to 180 cm. The field width was chosen from 32 to 40 cm at the machine isocenter. The optimal thickness of the compensator was found to be 2. 5 cm, and its base was 25 cm wide. The distance from the pivot points to the hat surface of the compensator proximal to the beam ranges from 13 to 14 cm for different beam energies. The dose uniformity at a dep th of 10 cm is within +/-5% for all beam energies used in this study. Conclusions: Highly uniform dose profiles for TBI treatments can be de livered with an are and a gravity-oriented compensator. The proposed t echnique is simple and versatile. A single compensator can be used for all energies, because the amount of compensation can be adjusted by c hanging the distance to the pivot and/or the field size. (C) 1997 Else vier Science Inc.