A Radiation Therapy Contouring Atlas for Delineation of the Level I-II Axilla in the Prone Position

Abstract

When patients are treated prone for whole breast irradiation (WBI), the axilla typically receives less dose than when patients are treated with WBI in the supine position. There are situations, however, where the axilla is a target as in a situation where the physician intends to treat with "high tangents" to provide good coverage of both the breast and level I-II axilla. In these scenarios, ideal target delineation when the patient is in the prone position is not well-defined. While different consensus guidelines exist for delineation of the nodal areas in the supine position, to our knowledge there are no contouring guidelines for the regional nodes in the prone position based on bone, skin, vascular and muscle landmarks. Forty-three patients treated with high tangents in the prone position from 2012-2018 were identified as representative cases. The level I and II regional nodal contours from the Radiation Therapy Oncology Group (RTOG) breast cancer atlas were adapted for prone position by a radiation oncologist and a breast radiologist based on anatomic considerations and changes observed from supine to prone positioning on diagnostic imaging. The revised nodal contours were reviewed by an expanded expert multidisciplinary panel including additional breast radiation oncologists and surgical oncologists to delineate the level I and II axilla on noncontrast computed tomography (CT) scans. We adapted the RTOG breast cancer atlas, supported by detailed figures, in order to create a CT based atlas of the level I and II axillary lymph node stations in the prone position. For the level I axilla, the cranial and caudal anatomic boundaries remain unchanged. With transition to the prone position from supine, tenting of the pectoralis major occurs displacing the muscle from the chest wall and shifting the axillary space anteromedial to the lateral border of the pectoralis major. Therefore, the anterior boundary is now defined by the plane of the anterior extent of the pectoralis major to skin. The medial boundary is defined by the plane of the lateral border of the pectoralis major and pectoralis minor including to ribs and intercostal muscles. The lateral boundary is defined by the skin, cropped by 5mm. The posterior boundary is defined by the plane of the anterior surface of the latissimus dorsi and subscapularis muscle to skin. For the level II axilla, the cranial, posterior, medial and lateral boundaries remain unchanged. With transition from supine to prone, the axilla exhibits an anterolateral shift, now laterally abutting the lateral border of the pectoralis minor. The anterior boundary is now defined as the posterior aspect of the pectoralis major muscle. The caudal boundary is where the pectoralis minor inserts into ribs. The adaptations to the RTOG breast cancer atlas for prone positioning will enable radiation oncologists to more accurately target the level I and II axilla when treating patients prone in whom the axilla is a target in addition to the breast.