Contouring of the swallowing apparatus
During prospective treatment planning, contouring of the swallowing muscles (as one structure) and the larynx was performed on 3‑mm axial planning CT slices (Somatom, Siemens Healthcare, Forchheim, Germany) using Eclipse (v15.1, Varian Medical Systems, Inc., Palo Alto, CA, USA). For a more precise distinction, the substructures of the swallowing musculature (scm, mcm, icm, cphm, and eim) were retrospectively contoured by the same physician. Anatomical organ delineation was performed in accordance with Levendag et al. (scm: mid C2 to upper C3; mcm: upper C3 to upper C4/caudal part of the corpus of the hyoid bone; icm: upper C4 to mid C6; cphm: mid C6 to esophageal junction) [Besides the gross target volume (GTV), the following dose characteristics were extracted for all substructures of the swallowing musculature and the larynx: Dmax (maximum dose), Dmean (in Gy), V40Gy (organ volume (%) that received ≥ x Gy), V50Gy, V60Gy, and V65Gy (in %).