Comparison of Dose Distribution around Central Vaginal Applicator Brachytherapy Treatment Calculated with Water Based TG43 & Model Based TG186

Abstract

Aim: The focus of this research was to look at the dosimetric effect and evaluate the dose difference between TG-186 and TG-43 plans for cervical carcinoma using central vaginal applicators, in order to help clinicians to choose right algorithm. Introduction: TG-43 protocol is for calculation of dose-rate distributions around photon-emitting brachytherapy sources. The radiation is assumed to be carried out through an infinite homogeneous water phantom in the TG-43 dose calculation approach. Thus any heterogeneities within or outside the patient are not taken into account. As a result the accuracy of dose calculations in places near to air or bone may be impacted. The recently published AAPM Radiation Therapy Committee Task Group 186 (TG-186) report has provided new guidelines for dose calculation and reporting in brachytherapy to address the shortcomings of the current TG-43U1 dose calculation methodology. Heterogeneity correction algorithms have only recently been made available to the BT community in contrary to external beam radiotherapy. Materials and Methods: A retrospective study including 15 patients with cervical cancer who had undergone postoperative brachytherapy was performed. External Beam Radiotherapy (EBRT) was performed with a total dose of 50 Gy in 25 fractions as 2 Gy/per fraction using conventional fractionation schedule.Based on TG-43 and TG-186 algorithms plans were done for 15 cervical patients on the Nucletron- oncentra planning system and treated with Ir-192 brachytherapy source. The plans were not re optimized and therefore the dwell positions and dwell times were identical between the two plans. Retrospectively the plans were recalculated using the TG-186 algorithms. For each patient DVH is used to calculate dose in 0.1cc CGy, 1cc CGy, 2cc CGy and 5cc CGy of OAR doses as well as prescription point doses. Results: D w,m, found small changes in Prescription doses and in D0.1 cc, D1 cc, D2 cc, D5 cc for rectum and bladder with all dose parameters for individual patients differing from TG-43 values by < 1%. Conclusion: The dose difference between TG43 and TG186 algorithms is for most clinical cases not significant for target volumes and OARs. Cylinder applicators although TG43 algorithm overestimated the tissue dose the difference of dose distribution caused by the two algorithms was almost negligible because the difference of dose distribution was not much (less than 1%) and both were located around the applicator.