2988 - CBCT-Guided Online Adaptive Radiation Therapy for Accelerated Partial Breast Irradiation: A Single-Institution Experience
Presenter(s)
C. G. Reid1, W. S. Hotsinpiller1, J. A. Pogue2, J. Harms2, M. Soike2, K. S. Keene1, M. C. Dobelbower3, M. Bredel4, D. N. Stanley2, and H. Boggs2; 1University of Alabama at Birmingham, Birmingham, AL, 2Department of Radiation Oncology, University of Alabama at Birmingham, Birmingham, AL, 3University of Alabama at Birmingham Department of Radiation Oncology, Birmingham, AL, 4Department of Radiation Oncology, Sylvester Comprehensive Cancer Center, University of Miami, Miami, Florida, United States
Purpose/Objective(s): Accelerated partial breast irradiation (APBI) using stereotactic body radiation therapy (SBRT) is increasingly utilized for early-stage breast cancer (ESBC) due to its shorter treatment duration, comparable local control, and improved cosmesis. Cone beam computed tomography (CBCT)-guided online adaptive radiation therapy (OART) enhances target coverage and minimizes normal tissue exposure by accounting for lumpectomy cavity variations. This retrospective, single-institution study evaluates clinical characteristics and outcomes of ESBC patients treated with APBI using OART.
Materials/Methods: Forty patients (two with bilateral disease) with ESBC were treated with SBRT using OART from January 2022 to May 2024. For CT-simulation (1-mm slice thickness), patients were immobilized supine using standard breast simulation setup. The treatment planning volumes included the lumpectomy cavity (GTV), a 1 cm expansion of the GTV cropped 3 mm from the surface and the chest wall (CTV), and a 3 mm expansion of the CTV cropped 3 mm from the surface (PTV). 95% of the PTV received 30 Gy in 5 non-consecutive fractions. The OART delivery process, supervised by a physician and physicist, involved a deformable image registration of daily CBCT to the planning CT, aligning to the lumpectomy cavity. Target volumes and organs-at-risk were modified each session, and plans were re-optimized. Either the adaptive or scheduled plan was chosen for each fraction. Clinical characteristics and outcomes were assessed at one month and most recent follow-up using CTCAE criteria and physician documentation.
Results: Patients (ages 49-83) had primarily right-sided (59.5%) and upper outer quadrant tumors (61.9%), with invasive ductal carcinoma as the most common pathology. The median tumor size was 0.9 cm. The median time from lumpectomy to radiation initiation was 62 days, and the median treatment duration was 9 days. The median PTV-to-whole-breast ratio was 0.12 (maximum of 0.39). The average treatment time using OART was 32 min (median of 30 min), and the adapted plan was selected for treatment 90% of the time. Seven patients underwent reirradiation, three to the same breast. At one month, 47.6% had no acute toxicities, and all reported toxicities were CTCAE grade 1, including breast pain (9.5%), seroma (9.5%), hyperpigmentation (28.6%), lymphedema (2.4%), and fatigue (16.7%). At median follow-up of 14 months, most toxicities remained grade 1 (86.2%), with breast pain (23.8%), seroma (9.5%), hyperpigmentation (14.3%), atrophy (4.8%), and lymphedema (4.8%). Two patients who underwent reirradiation to the same breast developed grade 2 atrophy and breast pain. A third patient, with no prior treatment, experienced a grade 2 rib fracture. No local, regional, or distant recurrences were observed.
Conclusion: This study represents the largest cohort of ESBC patients treated with APBI using OART, demonstrating excellent cosmesis, minimal toxicity, and oncologic outcomes comparable to other techniques.