Towards Dosimetric Improvement in HDR Brachytherapy, an Investigation of a Model Based Dose Calculation Algorithm - Acuros

Purpose: To evaluate dosimetric impact of heterogeneity from applicators and patient tissue between plans treated with tandem and ovoid applicators and interstitial needles calculated with the traditional Task Group 43 formulism and a model-based dose calculation algorithm (MBDCA) approach - Acuros®BV (Varian Medical Systems (VMS), Inc). Methods: For this analysis, patients with locally advanced cervical cancer treated with high dose rate (HDR) brachytherapy with a prescription of 5.5 Gy to D90% high-risk clinical target volume (CTV-HR) were selected. Fletcher-Suit Delclos style tandem and ovoid (T&O) applicators and interstitial needles were used and stabilized by vaginal balloons filled with a 95% saline and 5% IsoVue solution. Imaging techniques for applicator delineation and planning was done using Acuity CBCT or AIRO CT, depending on the modality used at the time of treatment. Standard planning for TG43 based dose calculation consisted of manually digitizing applicators on the CT or CBCT images. For plans calculated with Acuros®BV, solid applicator renderings were used, and the interstitial needles were left unchanged due to no solid model being available. Dose to medium in medium Dm,m was reported for Acuros plans and compared to dose to water in water D w,w from TG43 plans. Equivalent dose in 2Gy per fraction (EQD2) values were reported for target structures and organs at risk (OAR) including bladder, sigmoid, rectum, bowel, and vagina. As an additional comparison we investigated dosimetric changes based on the imaging modality used (CBCT vs. CT) and the presence of interstitial needles versus plans with T&O applicators alone. To investigate correlation and regression between DCAs both linear regression and Bland Altman Analysis (BAA) was performed. Conclusions: Based on data presented in this retrospective study, the average ± standard deviation of dosimetric reduction in D90% was 4.33 ± 0.09% (0.41 ± 0.13 Gy) for CTV-HR and 4.12 ± 0.09% (0.21 ± 0.07Gy) for CTV-IR . The reduction to OARs D2cm3 was: 4.99 ± 0.15% (0.34 ± 0.17 Gy) for bladder, 7.87 ± 0.16% (0.20 ± 0.09 Gy) for rectum, 5.79 ± 0.17% (0.21 ± 0.09 Gy) for sigmoid, 6.91 ± 0.14% (0.25 ± 0.13 Gy) for bowel, and 4.55 ± 0.14% (0.46 ± 0.27 Gy) for vagina. This indicates that plans calculated using Acuros®BV showed a dosimetric reduction to all structures when compared to TG43. This trend is attributed to heterogeneities within the treatment volume including applicators, tissue, and presence of air.