Dosimetric impact of tissue heterogeneity in brachytherapy: a comparative analysis of low and high dose rate sources

Abstract

The aim of this study is to determine the effect of tissue heterogeneity on the dose distribution of different brachytherapy seeds. For this purpose, the 131Cs, 125I, 60Co, and 192Ir brachytherapy sources were stimulated with Monte Carlo (MC) code, and their dose rate constant and radial dose function were compared with the published data. Then a cubic prostatic calcified tissue (20% 100%) as heterogeneity was simulated at spherical phantom of normal prostate tissue and the absorbed dose at various distances from the source center were calculated. In addition, the impact of the thickness of heterogeneous tissue on absorbed dose and photon flux was investigated. The result showed that the calcifications caused significant dose distortions for LDR sources of 125I and 131Cs, reducing the absorbed dose by up to 40% within and beyond calcified regions due to photoelectric interactions. Dose discrepancies are intensified with higher calcium content (>50%) and larger heterogeneity sizes. In contrast, HDR sources exhibited minimal sensitivity to calcifications (<5% dose variation), attributed to their higher photon energies and Compton-dominated interactions. For LDR, calcifications acted as shielding material, particularly when tumors lay beyond calcified zones, with dose fall-off worsening for thicker heterogeneities. So, the ignorance of tissue calcification by treatment planning systems incorporates a significant perturbation in delivered dose to the patient in brachytherapy with photon sources. This perturbation depends on the calcification percentages, brachytherapy source, as well as the distance from the source. These findings emphasize the importance of considering tissue heterogeneity in brachytherapy treatment planning for accurate dose calculations