Background and purpose: The human body is composed of various inhomogenous tissues with a variety of physical and radiological properties. These inhomogeneities could change isodose distributions, increase the probability of geometric errors, and eventually lead to missing of the target irradiation or incorrect isodose coverage in conformal radiation therapy (CRT) due to the uncertainties resulted from the effect of such inhomogeneties on isodose curves. In this study, the effect of inhomogeneities on dosimetric accuracy of the CRT of prostate was investigated using Monte Carlo simulation method.

Materials and methods: An anthropomorphic pelvis phantom was used in two modes representing a homogeneous and heterogeneous phantom. Three dimensional (3D) CRT planning composed of five fields were performed on the CT images of the phantoms using the CorPLAN treatment planning system (TPS) of a 2100C/D Varian linac. Delivery of 3DCRT was also simulated by EGSnrc Monte Carlo code for the same conditions used in the CorPLAN TPS. The dose distributions resulted from the MC simulations were compared with those of the CorPLAN TPS for both of the phantoms based on the "dose difference (DD) percentages" and "distance to agreement (DTA)" parameters.

Results: Findings indicated that the heterogeneity leads to an overdose estimation at the target location up to 2.8% and 4.4% for the 6 and 18 MV energies, respectively by the CorPLAN TPS compared with the MC simulation data. The average DD and DTA for the homogeneous phantom were 2.7%-3.4mm and 1.6%-2.3 mm while for the inhomogeneous phantom they were 5.5%-3.7 mm and 6.0%-2.5 mm at the 6 and 18 MV energies, respectively.

Conclusion: Our results indicates that the accuracy of the dosimetry parameters estimated by the CorPLAN TPS at 18 MV is more than that of 6 MV energy. But, the heterogeneity deteriorates more the accuracy level of such dosimetry parameters estimated by this commercial TPS at 18 MV energy.