Abstract P3-19-18: Feasibility and short term toxicity of a consecutively delivered 5-fraction stereotactic body radiotherapy in early stage breast cancer patients receiving accelerated partial breast irradiation

Abstract Background: For early stage breast cancer, breast conservation is an appealing option for many women. Interest in reducing the duration and volume of RT has continued in the form of accelerated partial breast irradiation (APBI). Recent work demonstrates that for appropriately selected patients, APBI yields equivalent rates of ipsilateral breast tumor recurrence (IBTR) with improvements in patient rated cosmesis compared to whole breast radiotherapy. When using external beam radiotherapy for APBI, appropriate techniques to reduce setup uncertainty and overall treatment volume are critical to reduce long term adverse cosmesis. Stereotactic body radiotherapy (SBRT) is an attractive technique to deliver APBI because of its excellent accuracy and potentially superior sparing of uninvolved breast tissue. We report here the initial results of our IRB-approved prospective clinical trial investigating feasibility, safety, and cosmetic outcomes of daily, 5-fraction SBRT for APBI in women meeting ASTRO consensus suitability criteria for partial breast irradiation. Methods: We enrolled twenty patients with early stage breast cancer after lumpectomy who met APBI suitability. During lumpectomy, a breast surgeon placed a bioabsorbable 3D fixed array tissue marker (BioZorb™, Hologic, Marlborough, MS) for enhanced visualization of the cavity boundaries. We defined the clinical target volume (CTV) as the delineable cavity plus a 1 cm isotropic expansion. We placed a 3mm isotropic planning target volume (PTV) expansion around the CTV. We treated each woman with 3000cGy delivered in 5 consecutive, daily, free-breathed fractions in either prone or supine positioning depending on individual anatomy. Patient treatment position was free-breathing in the prone position for 17 patients and supine in 3 patients using triggered imaging and fiducial tracking. A maximum PTV of 124cc was allowed to minimize incidence of fat necrosis. Plans used 10MV flattening filter free (FFF) beams delivered on a Varian Edge linear accelerator. We documented the following: adverse events (CTCAE 4.0), causality of the event to SBRT, and nurse- and patient- scored cosmesis (i.e., 1=excellent, 2=good, 3=fair, 4=poor) at pre-treatment baseline, 1 month post-treatment, and at each proceeding 6 month interval. We also clinically surveilled for disease recurrence. Results: We completed accrual and treatment of twenty patients. At time of submission, fourteen patients had completed 6-month follow-up, and eight patients had completed 12-month follow-up. No patients experienced grade ≥3 toxicity. Adverse events reported were mostly grade 1. Eighty-five percent of patient-scored cosmesis were excellent or good at baseline, with no evidence of deterioration at 1-month post-treatment (Baseline = 1.9, 1 month = 1.9; p = 0.914). Eighty-five percent of nurse-scored cosmesis reports were excellent or good at baseline, with no evidence of deterioration between baseline and 1-month post-treatment (Baseline = 1.95, 1 month = 1.90; p = 0.564). Eighty-six percent of patients had excellent or good patient- and nurse-scored cosmesis at 6 months, with either mean score of 1.71. Eighty-eight percent of patients had excellent or good patient- and nurse-scored cosmesis at 12 months, with mean cosmesis score of 1.50 and 1.63, respectively. Local control follow-up is immature, but no patients developed IBTR. Conclusions: Our study demonstrated that consecutive daily 5-fraction breast SBRT for APBI is a safe, efficient, well-tolerated, and cosmetically favorable means of delivering partial breast irradiation in suitable women. As follow-up duration matures, we will report long term cosmetic outcome and recurrence. Citation Format: Yilan Liu, Christopher Veale, Diana Hablitz, Helen Krontiras, Michael Christian Dobelbower, Rachael Lancaster, Markus Bredel, Catherine Parker, Kimberly Keene, Evan Thomas, Drexell Hunter Boggs. Feasibility and short term toxicity of a consecutively delivered 5-fraction stereotactic body radiotherapy in early stage breast cancer patients receiving accelerated partial breast irradiation [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P3-19-18.