Uncovering the Armpit of Axillary SBRT

Abstract

The growing use of stereotactic body radiotherapy (SBRT) in metastatic cancer has led to applications in new and unique anatomic locations, highlighting the importance of effective, safe, reproducible treatment delivery. The objective of this study was to review our institutional SBRT experience for axillary metastases (AM), focusing on outcomes, safety and process. In this ethics approved single-institution retrospective review, patients treated with SBRT to AM from 2014-2022 had tumor, treatment planning, and dosimetric variables abstracted. Toxicity was assessed per Common Terminology for Adverse Events V5.0. Cumulative incidence functions were used to estimate the incidence of local failure (LF), with death as competing risk. Kaplan-Meier method was used to estimate progression-free (PFS) and overall survival (OS). We analyzed 37 patients with 39 AM who received SBRT. Patients were predominantly female (60%), Eastern Cooperative Oncology Group performance status 0-1 (62%), and elderly (median age: 72), with a median follow-up of 14.6 months. Common primary sites included breast (n = 16, 43%), skin (n = 7, 19%), and lung (n = 5, 14%). Treatment indication included oligoprogression (n = 18, 46%), oligometastases (n = 14, 36%) and symptomatic progression (n = 7, 18%). A minority had prior overlapping radiation (n = 7, 18%) or regional surgery (n = 4, 11%), while most had prior systemic therapy (n = 26, 70%). Significant heterogeneity in simulation, planning and treatment was identified. Immobilization included 5-point thermoplastic mask (n = 12, 32%), Vacloc (n = 12, 32%) arms-up thorax bag (n = 11, 30%). 4-D CT scans were obtained in 46%, MR simulation in 21%, and intravenous contrast in 10%. Median dose was 40 Gy (interquartile range (IQR): 35-40) in 5 fractions, (BED10 = 72 Gy), over a median of 12 days (IQR: 9-14). Seventeen cases (44%) utilized a low-dose elective volume to cover remaining axilla; 14% used a high dose clinical target volume. Median planning target volume margin was 5mm (range: 3-10mm), and plans were generated with 5 different dose constraint protocols. At first radiographic assessment, 87% had partial or complete response, with a single progression. Of symptomatic patients (n = 14), 57% had complete symptom resolution and 21% had improvement. One and 2-year LF rate were 19% and 31%, respectively. Median OS was 21.0 months (95% [Confidence Interval (CI)] 17.3-not reached) and median PFS was 7.0 months (95% [CI] 4.3-11.3). Acute and late toxicities were uncommon, with two grade 3 events (1 plexopathy in a case with tumor involving brachial plexus, 1 skin ulceration) identified, and no grade 4/5. In this series of AM SBRT, low rates of toxicity, and good rates of LF and symptom improvement were observed. As treatment was delivered with a variety of individual treatment differences, an institutional protocol is under development to standardize technique, optimize efficiency and improve evaluability.