Diagrammatic Approach to Delineate Phrenic Nerve in Central Lung Tumors

Incremental use of high-dose radiation therapy (RT) with SABR in thoracic tumors has led to identification of many unusual toxicities (chest wall pain, rib fractures, vascular perforation, brachial plexopathy) and consequently additional organs at risk (OARs; chest wall, ribs, bronchial wall, carotid artery, brachial plexus). Phrenic nerve is another structure that may be affected at any point during its long course from origin until end, although symptomatic toxicities have been reported rarely in the setting of reirradiation, large-volume irradiation such as mantle field, or SABR. We undertook a prospective study to describe the delineation of phrenic nerve course on RT planning computed tomography scan as an OAR. An anonymized RT planning computed tomography scan of neck and thorax (1.5-mm slice thickness, intravenous contrast) was used for the study. Radiology textbooks and publications were reviewed, and the course was delineated with the help of 2 radiologists and 6 radiation oncologists well versed with thoracic radiologic anatomy. A step-by-step description in the form of a pictorial essay is given. The adjacent structures including cervical vertebrae, cervical and mediastinal vessels, lungs, heart, and so on were identified, and the course of phrenic nerve on either side is described in relation to these structures. Delineation of the phrenic nerve as an OAR is challenging but feasible. We recommend routine delineation of the phrenic nerve as an OAR during thoracic RT. Although specific dose constraints are not known yet, unnecessary dose to the same during RT planning should be minimized.