High Tangents in the Prone Position: A Pilot Report on Its Feasibility

Abstract

Prone breast irradiation is a widely used technique to limit the dose to the heart and lungs. Yet, prone irradiation generally precludes inclusion of the axillary lymph nodes (LN). In this study we examine the feasibility of delivering high tangents with the patient in the prone position. Patients treated in a single institution from 2012 to 2016 for whom the radiation oncologist prescribed high tangents and treated the patient in the prone position were identified. Any patient treated with a third field was excluded. The levels I-III axillary LN were re-contoured using the RTOG breast contouring atlas. The dose delivered to the LN, breast, and organs at risk was calculated using the original treatment fields. For the LN, coverage was defined as >90% of the prescribed dose. Fifteen patients treated with high tangents using two tangentially opposed beams in the prone position were identified. The median patient age was 62; 53% of patients had left-sided breast cancer. Most patients had invasive ductal carcinoma (66%), T1 tumors (73%), intermediate grade (73%) and estrogen receptor (ER) positive (93%), HER2 negative disease (86%). Of the 15 patients, 11 underwent sentinel lymph node biopsy alone (with a median of 3 LN removed), 64% had macroscopic disease, 27% had microscopic disease. No patients had extranodal extension. Four patients had no axillary staging. Six patients (40%) received chemotherapy prior to radiation and 64% of patients with ER positive disease received endocrine therapy. Nine patients were treated with conventional fractionation and 6 with a hypofractionated regimen. The median distance from the superior border of the tangent field to the humeral head was 0.9cm (range: 0-1.5cm). The mean coverage of the level I and level II axilla was 80.6% and 31.4% respectively. Axillary level I coverage for distance from the humeral head <1cm was 94.7%, whereas distance > 1cm was 64.5% (p=0.006). Axillary level II coverage for distance from the humeral head <1cm was 42.1%, whereas distance > 1cm was 19.2% (p=0.11). Only 3 patients had any coverage of level III (mean 1.8%). The average mean cardiac dose was 92cGy (128.0cGy for the left breast and 50.8cGy for right breast). The median ipsilateral lung V20 was 2.9%. The median maximum point dose was 111.7%. This series represents the first report of delivering two tangential beams in the prone position intending to cover the breast and lower axillary nodes in the treatment field. This series demonstrates that this technique offers good coverage of level I LN, particularly with <1cm distance from the superior border of the tangents to the humeral head, and partial coverage of level II LN. The mean cardiac dose is low for all patients.