This prospective study was designed to validate a questionnaire on patients' perception of care quality during respiratory-gated radiotherapy for breast or lung cancer. Psychometric tests were performed on selected patients. Confirmatory factorial analyses and capacity to discriminate the responses were achieved to validate the best model on 297 patients. Factorial analyses identified the following three scales: (a) perception of quality, (b) global satisfaction, and (c) physical or emotional experience. The scales were able to differentiate patients' responses according to radiotherapy modalities. The questionnaire presented adequate psychometric properties. This tool could be used for the assessment from the patient's point of view.