Clinical validation and applications for CT-based atlas for contouring the lower cranial nerves for head and neck cancer radiation therapy

Objectives Radiation induced cranial nerve palsy (RICNP) involving the lower cranial nerves (CNs) is a serious complication of head and neck radiotherapy (RT). Recommendations for delineating the lower CNs on RT planning studies do not exist. The aim of the current study is to develop a standardized methodology for contouring CNs IX-XII, which would help in establishing RT limiting doses for organs at risk (OAR). Methods Using anatomic texts, radiologic data, and guidance from experts in head and neck anatomy, we developed step-by-step instructions for delineating CNs IX-XII on computed tomography (CT) imaging. These structures were then contoured on five consecutive patients who underwent definitive RT for locally-advanced head and neck cancer (LAHNC). RT doses delivered to the lower CNs were calculated. Results We successfully developed a contouring atlas for CNs IX-XII. The median total dose to the planning target volume (PTV) was 70 Gy (range: 66-70 Gy). The median CN (IX-XI) and (XII) volumes were 10 c.c (range: 8-12 c.c) and 8 c.c (range: 7-10 c.c), respectively. The median V50, V60, V66, and V70 of the CN (IX-XI) and (XII) volumes were (85, 77, 71, 65) and (88, 80, 74, 64) respectively. The median maximal dose to the CN (IX-XI) and (XII) were 72 Gy (range: 66-77) and 71 Gy (range: 64-78), respectively. Conclusions We have generated simple instructions for delineating the lower CNs on RT planning imaging. Further analyses to explore the relationship between lower CN dosing and the risk of RICNP are recommended in order to establish limiting doses for these OARs.