Normal tissue anatomy for oropharyngeal cancer: contouring variability and its impact on optimization.

PURPOSE To evaluate the variability of organ at risk (OAR) delineation and the resulting impact on intensity modulated radiation therapy (IMRT) treatment plan optimization in head-and-neck cancer. METHODS AND MATERIALS An expert panel of 3 radiation oncologists jointly delineated OARs, including the parotid and submandibular glands (SM), pharyngeal constrictors (PC), larynx, and glottis (GL), in 10 patients with advanced oropharynx cancer in 3 contouring sessions, spaced at least 1 week apart. Contour variability and uncertainty, as well as their dosimetric impact on IMRT planning for each case, were assessed. RESULTS The mean difference in total volume for each OAR was 1 cm(3) (σ 0.5 cm(3)). Mean fractional overlap was 0.7 (σ 0.1) and was highest (0.8) for the larynx and bilateral SMs and parotids and lowest (0.5) for PC. There were considerable spatial differences in contours, with the ipsilateral parotid and PC displaying the most variability (0.9 cm), which was most prominent in cases in which tumors obliterated fat planes. Both SMs and GL had the smallest differences (0.5 cm). The mean difference in OAR dose was 0.9 Gy (range 0.6-1.1 Gy, σ 0.1 Gy), with the smallest difference for GL and largest for both SMs and the larynx. CONCLUSIONS Despite substantial difference in OAR contours, optimization was barely affected, with a 0.9-Gy mean difference between optimizations, suggesting relative insensitivity of dose distributions for IMRT of oropharynx cancer to the extent of OARs.