Abstract P3-19-17: Radiation of the low axilla in the prone position

Abstract

Abstract Background: Whole breast irradiation (WBI) after a positive sentinel lymph node biopsy (SNB) is recommended to be treated in the supine position to facilitate inclusion of the low axilla with “high tangents” when regional nodal irradiation is not planned. Treatment in the prone position has several advantages over supine positioning including minimizing heart and lung doses for many and decreased skin toxicity for larger breasted women. We hypothesized that, using three dimensional conformal radiation therapy (3DCRT), the low axilla can be safely and adequately treated in the prone position with minimal toxicity and good outcomes. Methods: We identified patients who underwent post lumpectomy whole breast and low axilla irradiation in the prone position using 3DCRT from 2014 to 2020. Standard 3DCRT treatment planning included delineation of surgical cavity, breast and low axillary clinical target volumes (CTV) with 5 mm expansion to planning target volumes (PTV). The “low axilla” CTV was generally defined as the level I axilla according to the RTOG Breast Cancer Atlas. Dosimetric data for both targets and organs at risk (OARs) was extracted from approved treatment plans’ dose-volume histograms (DVHs). Toxicity and cancer outcomes were collected from the electronic medical records. Descriptive statistical analysis was performed. Results: Seventy patients were identified. Median age was 61 years (range 34-87), median body mass index (BMI) was 30.4 kg/m2 (range 22.1-49.1), and 88.6% (N=62) had hormone sensitive, HER2 negative breast cancer. The median tumor size was 1.35 cm (range 0.07-4.5cm). For 56 patients (80.0%), a SNB was done with median of 2 (range 1-7) sentinel nodes removed - 19 (34%) with macro-metastasis (median size 4 mm, range 2.2-13mm), 21 (37.5%) with micrometastasis, and 16 (28.6%) with isolated tumor cells. Three patients had an additional node with isolated tumor cells. Thirteen (18.6%) were Nx (no nodal evaluation) and 1 had an unsuccessful SLNB with no lymph nodes obtained. Hypofractionation was used in 97.1% (N=68): 4256 cGy in 16 fractions (N=44, 62.8%) or 4000 cGy in 15 fractions (N=24, 34.3%). All targets were covered adequately. The median V95/V90 of the PTVbreast_eval, PTVlump_eval, and PTVAx were 96%/98.3% (range 76.2/91.9% - 99.6/101.4%), 100.1%/101.2% (range 87.6/94.9%-102.8/103.3%), and 95.3%/97.5% (range 82.4/91.6%-100.4/101.7%) respectively. The mean heart dose for all patients was 83.5 cGy; 82.7 cGy for right-sided tumors and 83.8 cGy for left-sided tumors. The median V16 of the ipsilateral lung was 4.25% (range 0.2 - 11.3%). Overall, toxicity was low with no grade 3 or higher events. For acute toxicity, most patients (N=54, 77.1%) reported grade 1 fatigue and had either grade 1 (N=52, 74.2%) or grade 2 (N=15, 21.4%) dermatitis. For late toxicity, 14 patients (20%) were referred to physical therapy after radiation, 7 (10%) for range of motion, 5 (7%) for arm lymphedema evaluation and 4 (6%) for other reasons. With a median follow-up of 18.5 months (range 0-63 months), 1 patient recurred both locally and regionally (1.4%) and one other patient recurred distantly. Conclusions: Patients with a positive SNB or are Nx who are recommended to have post-lumpectomy whole breast and low axilla irradiation can be safely and adequately treated in the prone position using 3DCRT with minimal toxicity and good outcomes. Citation Format: Victoria Doss, Erin Healy, Sasha Beyer, Sachin R. Jhawar, Jose G. Bazan, Julia White. Radiation of the low axilla in the prone position [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 P3-19-17.