Effect of small inhomogeneities on dose in a cobalt‐60 beam

Abstract

The effect of small ring-shaped cavities on the dose delivered by 60Co beam to a homogeneous medium was studied experimentally and theoretically. Changes as small as 1 .times. 10-4 of the total dose were measured. Depending on the position of the cavity, replacing water with a small cavity can either increase or decrease scatter dose to a point in the medium. The increase in scatter dose was not anticipated and is apparently not predicted by any of the presently available inhomogeneity dose correction algorithms used in treatment planning. Calculations were based on the perturbations of 1st and 2nd scatter dose contributions and show that the presence of a cavity in the medium introduces 3 processes that decrease scatter dose and 5 that increase it. The calculated net effect is in good agreement with experiment. Additional calculations show that the effect of a single small inhomogeneity cannot be easily extrapolated to larger inhomogeneities and that multiple inhomogeneities do not act independently. Two constraints were demonstrated that must be satisfied by future dose calculation algorithms; they must correctly determine dose in a homogeneous nonunit density material, and they must account for the change in dose due to small inhomogeneities in the medium. [This research has applications for radiotherapy treatment planning and the calculation of dose distributions within a patient.].