2999 - Implant-Sparing VMAT for Hypofractionated Postmastectomy Radiotherapy Using ESTRO-ACROP Target Delineation: A Paired Comparative Dosimetric Study
Presenter(s)
S. Fathy1, T. Nageeti2, D. Alhawi3, M. A. G. Al-Gaoud4, N. Shorbagi5, O. Kalantan3, R. Zatar3, and E. Rashaidi1; 1King Abdullah Medical City, Oncology Center, Jeddah, Saudi Arabia, 2King Abdulla Medical City- Holy Capital, Makkah, Saudi Arabia, 3King Abdullah Medical City, Jeddah, Saudi Arabia, 4King Abdullah Medical City Radiation Oncology Department, Makkah, Saudi Arabia, 5King Abdullah Medical City-Medical Physics Department, Makkah, Saudi Arabia
Purpose/Objective(s): Immediate implant-based reconstruction is increasingly performed in patients undergoing postmastectomy radiotherapy (PMRT). However, conventional VMAT often exposes implants to near-prescription doses, which may increase the risk of reconstruction-related complications. We hypothesized that implant-directed inverse optimization during volumetric modulated arc therapy (VMAT) could reduce implant dose without compromising target coverage or organ-at-risk (OAR) constraints. Such dose reduction may be clinically relevant given prior associations between implant dose and capsular contracture risk. A within-patient paired dosimetric comparison of implant-sparing and standard VMAT was therefore conducted for hypofractionated PMRT using ESTRO-ACROP target delineation.
Materials/Methods: Fifteen eligible patients with immediate implant-based reconstruction were retrospectively replanned on identical CT datasets. Chest wall and regional nodal targets were contoured per ESTRO-ACROP guidelines. Left-sided cases were planned using deep inspiration breath-hold; right-sided cases under free breathing. For each patient, two single-isocenter VMAT plans were generated; (1) standard VMAT without implant optimization and (2) implant-sparing VMAT, in which the implant was contoured as a dedicated OAR and assigned high optimization priority. Across all 30 plans, a prescription dose of 40 Gy in 15 fractions was delivered to the planning target volume (PTV), with normalization to ensure =95% PTV coverage. Dosimetric endpoints included PTV V95%, PTV D2%, and OAR metrics for heart, lungs, contralateral breast, thyroid, spinal cord, and implant. Paired t-test was used for statistical comparisons.
Results: Both techniques achieved comparable target coverage; PTV V95% (95.68 ± 0.70 vs 96.18 ± 1.04%, p = 0.074). A small but statistically significant difference was observed in hot spot dose (PTV D2%: 104.79 ± 1.27% vs 104.39 ± 1.25%, p = 0.014) in implant-sparing and standard VMAT, respectively. Implant-sparing optimization resulted in substantial reductions in implant dose. Mean implant dose decreased from 40.31 ± 0.42 Gy to 22.90 ± 2.81 Gy (p < 0.001), representing an absolute reduction of 17.4 Gy and 43.2% relative reduction that was consistent across laterality. Ninety-three percent of implant-sparing plans achieved a mean implant dose =25 Gy. High-dose exposure was also significantly reduced, including maximum dose (42.45 ± 0.91 Gy vs 43.02 ± 0.71 Gy, p = 0.018), V20 (57.99 ± 13.59% vs 100%, p < 0.001), and V30 (28.89 ± 11.45% vs 100%, p < 0.001). Heart and lung doses, as well as other OAR constraints, were consistently met without significant differences between approaches.
Conclusion: Implant-directed VMAT optimization significantly reduces implant dose without compromising target coverage or OAR constraints. This technique is immediately implementable within standard planning workflows and provides a strong rationale for prospective clinical evaluation.