Tissue compensators made of solid water or lead for megavoltage X-ray radiotherapy

Curvature of the body within the treatment field of a patient undergoing radiotherapy can result in a non-uniformity of the dose distribution within the tumor volume which, for a 20 cm X 20 cm field and 6 MV X-rays, could exceed 30% of the prescribed tumor dose. The use of bolus is usually not satisfactory as skin sparing is lost. The use of a compensator at the position of the block holder assembly attenuates the primary beam while not significantly effecting the scattered radiation in the patient's body. Consequently, the necessary compensator thickness depends on the type of material used, the field size, beam energy, and depth of the point of interest. Using a compensator with thickness equal to that of the missing tissue would result in significant underdosing of the tumor. In this paper, measurements of the necessary thickness of solid water and lead required to reduce the non-uniformity of dose on the central axis to within 3% of the prescribed dose are presented for 6 MV and 10 MV X-rays.