Delineating brachial plexus, cochlea, pharyngeal constrictor muscles and optic chiasm in head and neck radiotherapy: a CT-based model atlas

Background and purpose: Sparing of the organs at risk is one of the primary end-points of radiotherapy. The effects of organ-at-risk delineation on the dosimetric parameters can be critical and can influence treatment planning and outcomes. The aim of our study was to provide anatomical boundaries for the identification and delineation of the following critical organs at risk in the head and neck district: brachial plexus, cochlea, pharyngeal constrictor muscles and optic chiasm. Patients and methods: One patient was initially selected to elaborate our atlas. This patient was subjected to a planning computed tomography of the brain and head and neck district; axial images of 3-mm thickness at 3-mm intervals were obtained. In the same set-up a magnetic resonance imaging study was also performed. The obtained images were fused based on anatomical landmarks and used by a radiation oncologist, supported by a neuroradiologist, to provide anatomo-radiological limits for the identification of the brachial plexus, cochlea, pharyngeal constrictor muscles and optic chiasm. These limits were further verified on three consecutive patients. Results: A computed tomography-based atlas was developed with definition of cranial, caudal, medial, lateral, anterior and posterior limits for each organ considered. Conclusions: This study allows improvement of definitions of anatomic boundaries for the brachial plexus, cochlea, pharyngeal constrictor muscles and optic chiasm. Our multidisciplinary experience led to the production of an institutional reference tool that could represent a useful aid for radiation oncologists in clinical practice.