APBI Protocol for Early Breast Cancer

29 patients (30 plans due to one patient with bilateral disease) previously receiving supine APBI treatment for early stage breast cancer (stages 0-2) at our institution between 2019 and 2022 were utilized in this Institutional Review Board (IRB-1207033005) approved study. Nine patients (10 plans) were used to iteratively tune an optimization template and an independent 20 patient cohort was utilized to validate the template via automatic replanning without intervention or reoptimization. Seven patients met RAD 1802 inclusion criteria and were simulated and contoured according to trial protocol. Inclusion criteria consisted of age ≥ 50, estrogen receptor (ER) positive, and negative margins of at least 2mm for invasive histology or 3mm for ductal carcinoma in situ, carcinoma in situ, or T1 disease. Patients receiving neoadjuvant chemotherapy or having multifocal cancer, pure invasive lobular histology, surgical margins< 2mm, a lumpectomy cavity within 5mm of the body contour, or unclear cavity delineation on the planning scan were excluded. Additionally, patients with evaluation PTV (PTV_Eval) volumes exceeding 124cc were excluded based on fat necrosis observed by Timmerman et al. above this threshold (8 (link)). 23 patients were not included in the RAD 1802 study, but were simulated, contoured, and planned with the same methods and intent. For all patients, an isotropic 1cm gross tumor volume (GTV) expansion was utilized for clinical target volume (CTV) generation and an isotropic 3mm CTV expansion was utilized for PTV generation. PTV_Eval volumes were created by carving out the PTV at anatomical boundaries (i.e., lung, rib, chest wall, and 5mm from the skin). PTV_Eval volume ranged from 28.6cc to 217.9cc, with an average of 85.2cc. Patients were prescribed 30Gy in five fractions, with an average 98.3% of the PTV_Eval receiving 30Gy in the original clinical plans. Patient characteristics are summarized in Table 1.