Impact of tissues heterogeneities on monitor units calculation and ICRU dose point: analysis of 30 cases of prostate cancer treated with 18-MV photons after three-dimensional planning.

Abstract

Analysis of dose specification of tissue heterogeneities. Three-dimensional dose distribution analyses of 30 patients with localized prostate cancer were reviewed with and without tissue heterogeneity correction. The number of monitor units for each portal entrance (more than 300 different fields) was calculated and the impact of targeting and number of portal entrances was also integrated. The presence of gas in the rectum induces an overdosage of 0.6%, pubic bone induces an underdosage of -1.5%, and femoral heads are responsible for 6% underdosage. For the treatment as a whole, the underdosage is correlated with targeting techniques and weighting of each portal entrance (range, -0.5% to -3.2%). Dose calculation must take into account tissue heterogeneities and more precise guidelines for dose prescription are mandatory for further intercomparison.