PHSOR08 Presentation Time: 1:05 PM: Retrospective Comparison of TG43 vs. AcurosBV Model-Based Dose Calculation Algorithm (MBDCA) in Cervical Cancer Patients Treated with HDR Brachytherapy Boost

Abstract

<h3>Purpose</h3> To evaluate dosimetric impact of heterogeneity from applicators and patient tissue between plans treated with tandem and ovoid applicators with interstitial needles calculated with the TG43 formalism and Acuros®BV MBDCA (Varian Medical Systems (VMS), Inc). <h3>Materials Methods</h3> For this retrospective analysis, HDR plans prescribing 5.5 Gy to high-risk clinical target volume (CTV) D_90% for locally advanced cervical cancer patients were selected. The implants consisted of tandem and ovoid (T&O) with or without interstitial needles, treated using a GammaMedPlus™ iX afterloader with a ¹⁹²Ir source. Implants were stabilized by Alatus vaginal balloons (Angiodynamics) filled with a 5% IsoVue to 95% saline ratio. Imaging for applicator digitization and planning was done using either Acuity CBCT (VMS, Inc), or AIRO CT (BrainLab AG). For standard TG43 clinical dose calculation, all applicators were manually digitized on the CT or CBCT images. Solid renderings of Fletcher-Suit Delclos applicators included in BrachyVision (VMS, Inc) were matched to those used clinically. The composition of the applicators was assigned from the applicator library. Titanium needles were left unchanged, as there is no solid applicator model available. The dose was recalculated using Varian's Acuros®BV 13.5 and dose-to-medium-in-medium (D_m,m) was reported. Equivalent dose in 2 Gy per fraction (EQD2) values for targets and organs at risk (OARs) were compared between TG43 formalism and the Acuros®BV plans. D_90% and D_98% values were reported for the high and intermediate-risk CTVs, and D_2cm3 values were reported for OARs including bladder, rectum, sigmoid, bowel, and vagina. In a separate analysis, we compared dosimetric endpoint differences between TG43 and Acuros®BV plans imaged with CT versus CBCT, and also between plans with T&O applicators with and without needles. Linear regression and Bland Altman Analysis (BAA) were performed between the two plans to show the correlation and dosimetric differences between both methods. <h3>Results</h3> Thirty-two patients with a total of 156 fractions were included in this study, 70 planned on CBCT, and 86 planned on AIRO CT. Consistently, Acuros®BV plans showed lower dosimetric values for all plans compared to TG43. The average ± standard deviation of dosimetric reduction in D_90% 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 . Fig.1 shows an example BAA plot for differences between TG43 and Acuros®BV D_90% for the targets. The reduction to OARs D_2cm3 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. There was no statistically significant difference between TG43 and Acuros®BV plans calculated on CBCT versus CT images (p value = 0.24). A set of 5 patients with a total of 24 fractions planed with T&O applicators without needles was used for comparison with plans with interstitial needles, and no statistically significant difference between TG43 and Acuros®BV plans was observed (p value = 0.35). <h3>Conclusion</h3> This large patient cohort study confirms that plans calculated using Acuros®BV MBDCA show a reduction in dose to all structures when compared to the standard TG43 formalism. This is expected mainly due to more photon attenuation by high-density material of the applicators, as well as tissue heterogeneities. Clinical implementation of Acuros®BV can be challenging due to increased calculation time, alteration of the clinical flow, and increased planning complexity. These factors must be evaluated, and further investigation should be conducted in order to understand the effects of its utilization as a primary dose calculation method.