ImportancePatients undergoing radiation therapy (RT) for head and neck squamous cell carcinoma (HNSCC) experience a range of debilitating adverse effects (AEs). Patient-reported outcome (PRO) measures to quantify these AEs are a necessary and important component of health care; however, currently available PRO options often measure only disease-related symptoms or AEs of non-RT treatments.ObjectiveTo develop a brief PRO measure of the most common AEs associated with RT for HNSCC.Design, Setting, and ParticipantsThis was a qualitative study that followed the US Food and Drug Administration guidelines to develop a brief measure of patient-reported RT-related AEs (the Functional Assessment of Cancer Therapy–Head and Neck Radiotherapy [FACT-HN-RAD] measure). The study included (1) a literature review of clinical trials; (2) secondary analysis of retrospective concept elicitation interviews (CEIs); (3) electronic surveys of practicing radiation oncologists; (4) mapping of existing items to inform the development of the draft version of the measure; and (5) validation of content and face validity via patient cognitive interviews. Analysis was performed of CEI data and interviews with practicing radiation oncologists. Data analysis was conducted from July 1, 2022, to April 21, 2023.ExposuresSurveys and qualitative interviews.Main Outcomes and MeasuresThe most common patient-reported RT-related AEs among patients with HNSCC.ResultsOf 19 CEI participants, 14 (mean [range] age, 67 [49-86] years; 12 [86%] men and 2 [14%] women) described RT-related AEs and were included in the secondary analysis. Eleven (79%) patients reported difficulty swallowing; 8 (57%), oral pain; 7 (50%), dry mouth; 7 (50%), weight loss; 6 (43%), skin burning; 5 (36%), loss of taste; 5 (36%), voice changes (36%); and 5 (36%), fatigue. Nine radiation oncologists (mean [range] time in practice, 8 [1-42] years; 5 [56%] men and 4 [44%] women) reported the most common AEs: 9 (100%) reported dysgeusia; 7 (78%), xerostomia; 7 (78%), mucositis or oral pain; 8 (89%), dysphagia or odynophagia; 6 (67%), dermatitis; and 3 (33%), fatigue. Together these data informed the development of an 8-item AE-focused measure of pain, dysphagia, xerostomia, dysgeusia, voice changes, dermatitis, fatigue, and weight loss. Cognitive interviews with 10 patients (mean [range] age, 61 [29-84] years; 8 [80%] men and 2 [20%] women) demonstrated strong face validity; all (100%) reported that the measure reflected their experience with RT and stated that the length of the questionnaire was “just right.”Conclusions and RelevanceThe 8-item FACT-HN-RAD measure captures the most common patient- and physician-reported AEs related to RT for HNSCC. This measure offers a means to serially monitor patient-reported treatment-related AEs and recovery over time in both clinical and research settings. Future work will evaluate the psychometric validity of the measure.