Radiation therapy can cause long-term dysphagia that seriously affects quality of life for survivors of head and neck cancer. 1-3 Numerous studies have linked pharyngeal constrictor dose to long-term dysphagia, but conclusions about the dose distribution that can be safely tolerated have been inconsistent. For example, a group from the Netherlands found that the mean dose to the superior pharyngeal constrictor muscle and the supraglottic larynx were each predictive of dysphagia. 4 A subsequent Vanderbilt study refuted these findings, reporting that these structures were not predictive but that dose to the inferior pharyngeal constrictor muscle was. 5 Other studies have connected late dysphagia with dose to the middle pharyngeal constrictor muscle, total larynx, oral cavity, contralateral submandibular gland, contralateral parotid gland, or a combination of these structures. 6-14 NRG Oncology trials commonly evaluate dose to the “uninvolved pharynx,” which is the total pharyngeal constrictor muscle volume minus the planning target volume (PTV) for the lowest dose target volume. NRG head and neck trials 3, 4, 5, 6, 8, and 9 all use uninvolved pharynx mean dose ≤ 45 Gy as a constraint to judge radiation plan quality.
Differences in methodology or patient population may explain the inconsistency of prior studies on dosimetric predictors of dysphagia, but it is possible that these studies did not evaluate the optimal metric for dysphagia. This study evaluates a novel organ at risk, the contralateral pharyngeal constrictor muscle, to determine whether dose to this structure is predictive of late swallowing function. The study also compares a constraint based on this structure to the NRG uninvolved pharynx constraint mentioned earlier.
Methods
This study is a retrospective review of patients treated at the Richard L. Roudebush Veterans Affairs (VA) Medical Center in Indianapolis, Indiana. Patients were identified by searching the VA Cancer Registry for patients treated for head and neck squamous cell carcinoma between September 1, 2016, and August 30, 2019. Eligible sites included cancers of the nasopharynx, oropharynx, hypopharynx, larynx and oral cavity, as well as head and neck cancer of an unknown primary site. Only patients treated with primary radiation with concurrent systemic therapy were included. Patients were excluded if they had prior surgery or radiation to the head and neck.
The pharyngeal constrictor muscles were contoured per the techniques described by Bhide and colleagues.11 The contralateral constrictor was defined as the half of the constrictor volume contralateral to the primary site. For midline tumors, the side of the neck with a lower volume of lymph node metastases was judged to be the contralateral side.
One-year dysphagia was defined as having a gastronomy tube (G-tube) in place or an abnormal modified barium swallow (MBS) ≥ 12 months after the completion of radiation. At the study institution, MBS is not routinely done after therapy but is ordered if a patient or clinician has concerns about swallowing function. MBS was considered abnormal if there was laryngeal penetration that reached the level of the glottis or was not ejected from the larynx.