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

The effect of small ring-shaped cavities on the dose delivered by a cobalt-60 beam to a homogeneous medium was studied experimentally and theoretically. Changes as small as 1 x 10(-4) of the total dose were measured. Experimental results show that, 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 to our knowledge is not predicted by any of the presently available inhomogeneity dose correction algorithms used in treatment planning. Calculations were based on the perturbations of first and second scatter dose contributions and show that the presence of a cavity in the medium introduces three processes that decrease scatter dose and five 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. Results from this study demonstrate two constraints that must be satisfied by future dose calculation algorithms: (i) they must correctly determine dose in a homogeneous non-unit density material, and (ii) they must account for the change in dose due to small inhomogeneities in the medium.