Abstract P4-12-17: Initial clinical experience and acute toxicity report of patients treated with accelerated partial breast irradiation (APBI) using the GammaPodTM
Authors: Nichols Elizabeth M , McAvoy Sarah , Becker Stewart 
Published: March 2020

Abstract
Introduction/Background: Recent reports of APBI show low rates of local recurrence when used for appropriately selected patients as defined by the ASTRO consensus guideline. Multiple techniques of APBI exist which vary in invasiveness and side effect profiles. The GammaPod was FDA cleared for use in breast cancer in 2017. The device includes a novel breast cup system which immobilizes and localizes the breast using a vacuum and stereotactic coordinate reference frame. It also includes a unique table top for CT simulation, planning system and algorithm and the treatment unit itself which comprises 25 rotating Co-60 sources. Here we report the initial clinical experience and toxicity report of patients treated with the GammaPod using APBI. Materials and Methods: Fourteen consecutive patients treated from November 2018 - March 2019 were retrospectively reviewed under institutional IRB approval. All patients underwent lumpectomy followed by adjuvant radiation therapy using a dose of 30 Gy delivered in 5, 6 Gy fractions prescribed to the planning target volume (PTV). The PTV constituted the lumpectomy cavity as defined by clips and post-operative changes plus a 1 cm expansion for microscopic disease (clinical target volume) and 0.3 cm expansion for set up uncertainty not to include the first 0.5 cm of skin or the chest wall (PTV). Patients were evaluated for side effects before each fraction and 1 month following completion of therapy. Toxicities were evaluated using CTCAE v 4.03 criteria. Descriptive statistics were used to evaluate patient toxicity outcomes. Results: Thirteen of fourteen patients completed treatment. One patient discontinued therapy due to spiritual beliefs. A total of 66 treatments were delivered. Median patient age was 67 (range: 40-76). Three patients had stage 0 disease while the remainder had stage I disease. All had estrogen receptor positive disease. Radiation therapy started a median of 52 days (range: 34-130) following surgery and was completed in a median of 10 elapsed days (range: 8-15). During treatment, one patient developed petechiae, one patient mild discomfort and three patients small subcentimeter superficial blisters related to the vacuum suction applied from the immobilization device. These were all self-limited and resolved without medical intervention. Five patients each experienced one grade 1 toxicity: two patients reported grade 1 breast pain, two patients grade 1 erythema and one patient grade 1 hyperpigmentation. No patient reported fatigue during their treatment. There were no grade 2 or higher toxicities reported during or within the first 30 days after completion of treatment. Conclusions: APBI using the GammaPod was well tolerated with a low acute toxicity profile compared to published rates using whole breast irradiation or APBI using three-dimensional conformal RT. While these results are encouraging, increased patient numbers and longer follow up are needed to more accurately compare this novel APBI modality with other APBI techniques.
Citation Format: Elizabeth M Nichols, Sarah McAvoy, Stewart Becker. Initial clinical experience and acute toxicity report of patients treated with accelerated partial breast irradiation (APBI) using the GammaPodTM [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P4-12-17.