Therapeutic radiation dose delivered to the low axilla during whole breast radiation therapy in the prone position: Implications for targeting the undissected axilla

Purpose: One interpretation of the American College of Surgeons Oncology Group Z0011 trial is that whole breast radiation therapy, known to treat a portion of the low axilla when delivered in the supine position, can treat residual microscopic disease in patients with involved axillary nodes that were not removed by axillary dissection. The purpose of this study was to quantify radiation dose delivered to the axilla for patients treated in the prone position. Methods and Materials: We analyzed treatment plans from 40 consecutive patients who received radiation targeting the intact breast with tangent fields in the prone position. Axillary levels were contoured using Radiation Therapy Oncology Group (RTOG) definitions and radiation dose- volume calculations were made for axillary levels, heart, and lungs. We generated revised plans for 10 patients by modifying the tangent beams to increase axillary dose and compared original with modified plans. Results: The median proportion of the axilla covered by 90% of the prescription dose was 13% of level I (range, 0%-61%), 0% of level II (range, 0%-6%), and 0% of level III (range, 0%-0%). More of the level I axilla was covered in obese compared with nonobese patients (P = .013). Level I coverage did not differ significantly by laterality (P = .740) or tumor location (P = .527). Modification of the treatment plans significantly increased level I coverage (P = .005) with all modified plans delivering 90% of the prescription dose to at least 96% of the level I axilla. The modified plans had increased lung (P = .005) and heart (P = .028) dose, which were within acceptable RTOG normal tissue constraints. Conclusions: Most patients treated with standard whole breast tangential radiation in the prone position receive subtherapeutic dose to the level I and II axilla. Patients treated in the prone position who require therapeutic radiation dose to the low axilla need treatment field modification; this is feasible for many patients using tangent fields.