Abstract
In recent years the Henschke applicator has been widely used for gynecologic patients treated by brachytherapy in Taiwan. However, the commercial brachytherapy planning system did not properly evaluate the dose perturbation caused by the Henschke applicator. Dose perturbation caused by the Henschke applicator is a major concern for the brachytherapy planning system (BPS). We use Monte Carlo N-Particle Transport Code eXtended ( MCNPX ) to evaluate the dose distribution of patient computed tomography (CT) images employing the Henschke applicator for intracavitary brachytherapy. This study included three scenarios. First, this Monte Carlo simulation has been validated by single source dose results comparing the dose parameters to AAPM TG-43 report. Second, to investigate dose impact owing to neglect of the metal shielding effect, Monte Carlo simulation, Brachytherapy Planning System (BPS) calculations, and film measurements have been performed for dose assessment in a water phantom. Third, the CT images are applied to MCNPX for dose calculation. In addition to this phantom study, the patient CT images are also applied to MCNPX for inhomogeneous assessments. The air kerma strength, radial dose function and anisotropic function were shown good agreement with TG43 dose parameters. The dose distribution of multiple sources without Henschke applicator was performed by isodose curves. The comparison among simulation, BPS results, and film measurements were also in good agreement. But, isodose distributions with Henschke applicator by the MC simulation showed significant deviation 80% from those by the BPS. Furthermore, the comparison of dose distribution between the patient CT and phantom CT, the discrepancy of dose was 16.7-26%. This discrepancy included that the applicator placement was not exactly the same with simulation condition. But the effect of tissue inhomogeneous is still smaller than the shielding effect of Henschke applicator. In the condition of homogeneous water phantom without applying Henschke applicator, a good agreement was found among MC, BPS, and film measurement. Comparisons of MC and BPS with applying Henschke applicator show significant differences, as reported previously in the literature. The maximum overestimation is 80% in treatment area. This results prove that BPS can not calculate the metal attenuation correctly. Further, the dose discrepancy derived from the tissue heterogeneities was less than 1%. The MC results demonstrate the inability of BPS to account for heterogeneities. Thus, dose distribution of applicator must be evaluated before using new applicator.