Standardization of Head and Neck Contouring Using the Acanthiomeatal Line

Abstract 

The purpose of this study was to determine the perceived and actual chin position(s) used for radiotherapy of head-and-neck cancers in a variety of clinical settings. Dosimetrists were asked to describe the external landmarks used to set the chin position. The lateral treatment planning radiographic figures in Ang's textbook, Radiotherapy for Head and Neck Cancers: Indications and Techniques, were analyzed for chin position by drawing a horizontal line from the tip of the chin to the cervical spine. The physicians at 7 departments were asked to rate the chin positions used in their departments for head-and-neck simulations. Choices included: (1) mildly flexed, (2) neutral, (3) mildly extended, and (4) hyperextended. In addition, each center was asked to select 2 representative cases to show routine chin position. The dosimetrists fixed the chin in neutral position by placing a virtual plane defined by 3 points (the base of the nasal septum [acanthus] and the external auditory canals) perpendicular to the table top. The type of head holder was irrelevant. Eighty-two percent (31/38) of the figures in Ang's text showed positioning in the neutral position (tip of the chin intersected the cervical spine between C2-3/C3-4). Most (71.4%) of the radiotherapists thought their patients were treated in the hyperextended neck position but, in fact, 85.7% (12/14) of the simulations showed a neural neck position. Reproducible chin positioning can be obtained by using the acanthiomeatal line. Consistent use of this technique will create a uniformly positioned set of axial co-images that have consistent appearance of avoidance and lymphatic areas. This will simplify contouring on axial computed tomography (CT) images of the neck. Standardizing the chin position is an important step to developing a standardized atlas and developing an information tool for automated contouring.

Key Words: IMRT, Set-up, Standardized contouring, Automated contouring, Target delineation, Radiotherapy simulation