3043 - iADAPT Boost: Dosimetric Benefits of a Hybrid CT-Guided Adaptive Radiotherapy Regimen for Pelvic Malignancies

12:30pm - 01:45pm ET

Poster Hall - Exhibit Hall A

Screen: 2

POSTER

Presenter(s)

Antonia Kubiatowicz, PhD - Mass General Brigham Cancer Institute, Harvard Medical School, Boston, MA

A. Kubiatowicz^1,2, S. Quirk³, C. Hancox³, D. M. McClatchy III⁴, A. Elmotayam⁵, J. E. Leeman⁶, and H. Elhalawani⁶; ¹Mass General Brigham, Boston, MA, ²Harvard Medical School, Boston, MA, ³Department of Radiation Oncology, Brigham and Women’s Hospital/Dana-Farber Cancer Institute, Boston, MA, ⁴Department of Radiation Oncology, Massachusetts General Hospital/Mass General Brigham and Harvard Medical School, Boston, MA, ⁵Brigham and Women's Hospital/Dana Farber Cancer Institute, Boston, MA, United States, ⁶Department of Radiation Oncology, Mass General Brigham, Harvard Medical School, Boston, MA

Purpose/Objective(s): To quantify organ-at-risk (OAR) sparing achievable using a hybrid approach integrating conventional image-guided radiotherapy (IGRT) with a CT-guided online adaptive radiotherapy (CT-ART) cone-down boost for pelvic malignancies. We hypothesized that a hybrid approach would provide meaningful OAR sparing while allowing a resource-efficient strategy to extend adaptive benefits beyond ultra-hypofractionated regimens thus minimizing clinical resource burden, and supporting broader clinical implementation.

Materials/Methods: Four patients with pelvic malignancies were evaluated. Three cases with CBCT imaging were retrospectively simulated using an emulator, and one patient was treated prospectively on our clinical system using the hybrid workflow. Treatment consisted of conventional IGRT for initial fractions, followed by daily adapted boost fractions. For each adaptive fraction, daily anatomy was recontoured on CBCT, with AI-generated auto-segmented contours reviewed and edited by the attending MD. Two plans were generated per fraction: a scheduled plan (original plan recalculated on daily anatomy) and an adapted plan reoptimized on daily anatomy. Standardized planning directive templates ensured consistent target objectives and OAR prioritization. Dose accumulation and dosimetric comparisons were performed in commercial software. Endpoints included clinically relevant dose-volume metrics for bowel, bladder, rectum, testes, femoral heads, and spinal structures.

Results: Across 19 evaluable OAR metrics, 73% showed dose reduction with adaptive boost delivery. The largest improvements were seen in large bowel V54Gy (mean 29% reduction, up to 55%), small bowel V45Gy (19.4% reduction), and bladder V54Gy (11.8% reduction). Testes maximum dose decreased by an average of 4.9% (up to 13%). Additional reductions were observed in small bowel D0.1cc (2.2%), rectum V45Gy (1.8%), and femoral head doses. Target coverage was maintained in all adapted plans while improving OAR sparing relative to established dose-toxicity thresholds.

Conclusion: This hybrid approach resulted in meaningful OAR sparing while preserving target coverage. By concentrating adaptation during the cone-down phase—when target volumes are smaller and anatomy more variable—this approach integrates conventional IGRT with selectively deployed online adaptive boost, leveraging AI-assisted contouring with physician oversight and standardized directives for a reproducible, scalable workflow. Clinically, it achieves consistent reductions in bowel, bladder, and gonadal dose; operationally, it lowers barriers to adaptive radiotherapy adoption, potentially improving patient quality of life without disproportionate resource utilization and expanding access in routine pelvic cancer care.