Two-dimensional film dosimetry application in heterogeneous materials exposed to megavoltage photon beams

Measurement of dose within tissues and tissue interfaces having sharp density discontinuities and heterogeneities (such as in the lung, esophagus, and rectum) is essential for treatment plan verification and accurate prediction of the prescribed dose. This study examines the feasibility and utility of simplifying standard film dosimetry to measure dose distributions deposited by megavoltage beams in tissue substitutes (such as cork for lung) and anthropomorphic phantoms which closely resemble human tissues having large density heterogeneities and having sharp tissue interfaces. In addition, film dosimetry determined the dose distribution involving superposition of multiple radiation fields and helped evaluate the accuracy of a commercial treatment planning program which incorporates tissue heterogeneity effects through the "effective path length" algorithm. This study shows that these treatment planning programs and simple calculations overestimate the dose delivered within the lower density material in heterogeneous regions.