Purpose: The purpose of this study was to develop a scale that can measure the concerns of cancer patients undergoing outpatient radiation therapy. Method: First, items were created based on a review of the literature, and a draft scale was created by examining content validity using the Item-level Content Validity Index (I-CVI). In the main study, researcher conducted an anonymous, self-administered questionnaire survey on 400 cancer patients undergoing outpatient radiation therapy at five institutions, and examined the reliability and validity of the scale. Result: Exploratory factor analysis identified nine items with two factors (concern about living with cancer and concern about living with irradiation). The Cronbach's alpha coefficient for the entire scale was 0.848. The goodness of fit in confirmatory factor analysis showed GFI=0.930, AGFI=0.879, CFI=0.926 and SRMR=0.058. No correlation was observed with STAI Y-1, and a certain level of discriminant validity was confirmed. Conclusion: We generally confirmed the reliability and validity results of a concern scale for cancer patients receiving outpatient radiotherapy.