Characterization of gypsum attenuators for radiotherapy dose modification.

Gypsum has significant value as a compensator material for use with high energy x rays in radiation therapy. Compensator thickness may be calculated by using an effective attenuation coefficient (mu eff). Detailed measurements using narrow and broad beam geometry were collected to determine this effective attenuation coefficient as a function of energy (4-15 MV), field size, depth in tissue, and thickness of the compensator. An effective attenuation coefficient relation was defined using a least-square method. It was then determined that extrapolating the broad beam data to a 0 x 0 cm2 field size resulted in a good approximation to the measured narrow beam attenuation coefficient. The variations in surface dose produced by gypsum attenuators were compared to open beam results. For the energies studied, it was determined that the increase in surface dose was acceptable for clinical application.