Applying the reciprocal dose principle to heterogeneous phantoms: practical experience from Monte Carlo studies

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Abstract
The reciprocity theorem states that for any pair of regions in a uniform isotropic or uniform scatterless model, the specific absorbed fraction (.PHI.) is independent of which region is designated source and which is designated target. Although the conditions of these models are not met by phantoms heterogeneous in tissue composition and density, calculations with a Monte Carlo radiation transport code show that in many cases the reciprocal .PHI. from photon radiation are approximately equal. Apparently, the reciprocity principle is valid within a factor of 2 (Snyder) or within about 20% (Mayneord and Clarke), except when 1 of the organs in the pair considered is the skeleton, where the reciprocal .PHI. may differ by as much as a factor of 4. The principle evidently holds within at least 10% when both organs are of near-unit-density. When 1 of the organs is near the tissue-vacuum boundary (skin, breasts or testes), the reciprocal .PHI. may differ by as much as 10% or so at energies where scattering is marked. When 1 of the organs is the lungs or the whole body, a comparable error may occur at some energies. More extensive use of the reciprocity principle in internal dose computations [in humans] in heterogeneous phantom apparently is warranted.