Depth-related differences in bone marrow cell inactivation with 60Co γ rays

Cell survival was compared to physical radiation doses at different depths in a tissue equivalent phantom. Mouse hematopoietic stem cells irradiated at 3-mm depth increments were assayed by spleen colony formation. Maximum energy deposition for 60Co γ rays occurred at 0.5 cm below the surface followed by exponential attenuation with depth. In contrast, the maximum stem cell inactivation was approximately 2 cm below the surface. This phenomenon is independent of source distance and field size and suggests that changes in beam composition may be reflected in differential survival over the proximal few centimeters of depth