LAUSR

Background: The GammaPod is the first stereotactic body radiation therapy (SBRT) system optimized for treating breast cancer. It is composed of the treatment delivery system, the stereotactic localization and immobilization device, prone patient loading system and a treatment planning system, streamlining radiation therapy for both patient and provider. The GammaPod allows for higher doses in one or several large fractions which differentiates SBRT from conventional techniques. Patients benefit from shorter treatment timelines and more precise targeting. This project investigated the clinical benefit of the GammaPod compared to accelerated partial breast irradiation (APBI) with external beam radiation. Methods: Consecutive patients from 2018-2021 were retrospectively reviewed. All patients were treated with a lumpectomy +/- sentinel lymph node biopsy followed by adjuvant GammaPod stereotactic partial breast irradiation. All patients had at least 1 month of follow up from SBRT completion. The treatment protocol included 30Gy delivered over 5 fractions. Patient, tumor, and dosimetric characteristics were analyzed. Toxicities and cosmesis were recorded prospectively by treating physicians and reviewed retrospectively. Local recurrence was defined as in-breast, or skin/chest wall; regional recurrence was defined as a nodal failure. Results: Forty-nine patients were included. One hundred percent were female and 78% were White. The median age was 63 years, AJCC 8th edition anatomic staging was predominantly stage 0 (29%) and stage 1 (69%), and median tumor size was 8mm. Fifty-five percent of patients were treated for left-sided disease. Seventy-one percent of lesions were invasive carcinomas; 97% were ER+/Her2- and one patient was Her2+. Twenty-nine percent had ductal carcinoma in situ. The most common GammaPod cup sizes were L00 (24%), L01 (16%), M00 (14%), L04 (8%), and L06 (8%). All patients received surgery prior to radiation therapy and the median days elapsed from surgery to radiation therapy was 53 days (34-272). One patient underwent adjuvant chemotherapy prior to radiation therapy for her2+ disease. Median total RT dose was 30Gy (27-43). All patients were treated in 5 fractions with a median beam time of 14.57 minutes and a median variation of 2.56 minutes between fractions. The median days elapsed in treatment was 10 days (4-20). Five patients were undergoing reirradiation. The median follow up time was 8.25 months (.9-27mo) and one patient had regional reoccurrence. Of the 41 patients with recorded cosmesis results, 98% experienced positive (excellent or good) cosmesis outcomes. Eighty-eight percent experienced excellent cosmesis at follow up and no patients experienced poor cosmesis. All recorded toxicities were grade 1 or less. Thirty-three percent of patients experienced blisters, 27% experienced some degree of hyperpigmentation/hypopigmentation, and 14% experienced contact dermatitis from the immobilization device. Other toxicities ≤ 6.1% included erythema, pain, desquamation, cellulitis/mastitis, and petechiae. Conclusion: We present an initial analysis of the novel GammaPod for SBRT adjuvant radiation treatment for early stage breast cancer. The use of the GammaPod yielded positive cosmesis results and was well tolerated with a minimal side effect profile compared to external beam APBI. Therefore, the GammaPod offers a promising alternative to external beam ABPI in terms of post radiation cosmesis and patient satisfaction. Citation Format: Kayla Paulosky, Sarah Ruff, Stewart J Becker, Mariana Guerrero, Sarah McAvoy, Elizabeth M Nichols. Clinical and cosmetic outcomes of adjuvant stereotactic partial breast irradiation using the GammaPod [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 P3-19-22.