A Monte Carlo based dose verification tool for an HDR Ir-192 brachytherapy source and its evaluation on extremity malignancies.

BACKGROUND: TG-43 formalism dose calculation for brachytherapy with the homogeneous water-based assumption is widely used, which can lead to dose inaccuracy in treatment planning. PURPOSE: Extremity malignancies are also treated with high-dose-rate (HDR) 192Ir interstitial brachytherapy, a treatment site that has not been specifically investigated in prior comparative dosimetric studies, and the proximity of source dwell positions to bone and air introduces significant heterogeneity. BrachyPlanCheck, an in-house brachytherapy second dose verification program for Monte Carlo (MC) simulations, incorporates these heterogeneities and is used for a comparative dosimetric study on extremity malignancies. METHODS: BrachyPlanCheck was commissioned and validated in both a homogeneous water phantom and a heterogeneous breast phantom. The clinical evaluation included 8 patient-specific lower limb brachytherapy plans, comparing TG-43 and MC calculation using BrachyPlanCheck. Dosimetric differences were evaluated using dose comparisons, gamma index analysis, and dose-volume histogram (DVH) metrics for clinical target volume (CTV) and organ-at-risk (OAR), including bone and skin. RESULTS: In commissioning Level 1, MC simulations using BrachyPlanCheck demonstrated point dose agreement within ± 0.9%. Gamma passing rates exceeded 99.9% with criteria 1 mm/1% in commissioning Level 2. In clinical evaluation, TG-43 calculation overestimated CTV V150 by up to 10.50% and CTV D90 by up to 7.25%. OAR doses were generally higher with TG-43, with mean differences of 1.09% for bone D1cc (p = 0.109), 1.38% for bone D0.1cc (p = 0.078), 7.30% for skin D2cc (p = 0.008), and 5.68% for skin D0.1cc (p = 0.008). CONCLUSION: Validated by phantom and patient study, BrachyPlanCheck demonstrated robust performance for independent secondary dose verification. TG-43 calculation overestimated dose to CTV and OAR in extremity malignancy cases compared with BrachyPlanCheck.