BP06 Presentation Time: 4:45 PM: Dosimetric Evaluation of Accelerated Partial Breast Irradiation (APBI) with the TRIUMPH-T Trial Regimen Using SAVI Applicator—Single Institution Retrospective Analysis.

To present dosimetric outcomes of Strut Adjusted Volumetric Implant (SAVI) brachytherapy treatment plans delivered over 4 years using Tri-fraction Radiation Therapy Used to Minimize Patient Hospital Trips (TRIUMPH-T) Trial at our institution; and investigation into a plan-coverage-complexity indicator. Retrospective analysis on dosimetric parameters derived from clinically delivered treatment plans using different size SAVI (Cianna Medicals, Aliso Viejo CA) applicators during 2019 and 2023 were performed. Treatment plans were prepared using Ir-192 source modeled in BrachyVision (Varian Medical Systems, Inc.) and Oncentra (Elekta Brachy, Veenendaal, The Netherlands) treatment planning systems. Triumph-T trial dose regimen (22.5 Gy in 3 fractions, Bi-daily, 6 hours apart) and dosimetric criterion were employed for APBI delivery. Target [PTV_Eval] and organs at risk (OAR) [chestwall and ribs] were delineated in accordance with NSABP-B39/RTOG-0413 and Triumph-T trials. A 3 mm skin rind was contoured, as institutional preference, for skin-sparing. V 90% ≥ 90% was desired for PTV_Eval with V 150% < 40cc and V 200% < 17 cc for high dose metrics and D 0.03cc < 100% and D 0.03cc < 110% for skin and chestwall/ribs, respectively. A PTV-ratio was defined in this study as the ratio of PTV_Eval to PTV 1cm (volume encompassing 1 cm expansion around lumpectomy cavity) and was analyzed for linear correlation with the PTV_Eval coverage (V 90%) metric to determine feasibility of serving as a plan-coverage-complexity indicator. Pearson correlation coefficient (with a two-tailed t-test of significance; p<0.05) was used. Our center treated 147 patients with median PTV_Eval volume of 67.1 cc [35.9 cc-116.1 cc] using SAVI brachytherapy, following the Triumph-T trial planning technique. Amongst these, twenty (n=20) cavities were treated with 6-1-mini SAVI, fifty-two (n=52) cavities with 6-1 SAVI, fifty (n=50) with 8-1 SAVI, and twenty-five (n=25) with 10-1 SAVI, and these cavity groups had a median volume of 9.7 cc [7.7 cc - 15.4 cc], 18.1 cc [14.9 cc - 22.8 cc], 33.3 cc [15.3 cc - 41.3 cc] and 53.1cc [21.9 cc - 65.4 cc], respectively. Excellent coverage was achieved for PTV_Eval with mean V 90% at 97.0% ± 2.3%. Mean V 150% and V 200% were at 28.2cc ± 3.5cc and 12.7cc ± 2.4cc, respectively. From 6-1-mini to 10-1 SAVI, V 150% and V 200% values increase from 20.4cc to 35.7cc and 10.8cc to 14.1cc, respectively. For OARs, the mean D 0.03cc to skin, chestwall, and ribs were 86.9% ± 15.5%, 88.7% ± 30.6% and 59.5% ± 23.4%, respectively, and were within tolerance. Average PTV-ratio values were 0.92 ± 0.08 for all plans evaluated. Starting from 6-1mini to the 10-1 SAVI, the Pearson correlation coefficients between PTV-ratio and V 90% were 0.86, 0.90, 0.77, and 0.72, (p<0.05) indicating a high correlation. Retrospective analysis from 147 patients treated at our institution show that SAVI- brachytherapy using a 3-fraction Triumph-T trial regimen is an efficient alternative for APBI. Treatment plans delivered met dosimetric constraints outlined in the trial with excellent target coverage, minimal hot-spots, and OAR sparing. A novel metric, PTV-ratio, serving as a plan-coverage-complexity indicator showed a positive correlation to V 90% for all SAVI applicators. [ABSTRACT FROM AUTHOR]