PurposeTo create a Dutch translated short version of the Breast Cancer Treatment Outcome Scale (BCTOS) and validate it in patients who have completed both breast conserving surgery and adjuvant radiotherapy.MethodsThe BCTOS consists of items comparing the treated with the untreated breast. After forward and backward translation, we tested the BCTOS-12 plus 5 additional items. Two-hundred breast cancer patients treated with breast conserving therapy (BCT) between January 2016 and December 2017, were asked to complete the BCTOS items twice with a 2 week interval. The EORTC QLQ-BR23 breast and arm symptoms subscales were completed once in parallel. Feasibility was assessed by missing or non-unique answer rates and content validity with floor and ceiling effect analysis. Construct validity was evaluated with 1) principal component analysis (PCA) 2) convergent validity and 3) known groups comparison (clinical validity differentiating between patients with and without locoregional side effects). From all potential items with good feasibility, content and construct validity, items were selected for the Dutch BCTOS based on clinical validity. The relation to the EORTC QLQ-BR23 subscales and reliability was tested for the new Dutch BCTOS.ResultsHundred and one of 200 (50.5%) approached patients participated in this study, with follow-up after surgery ranging from 5 to 29 months. Feasibility was high (1.5% missing answers). Content validity testing showed a floor effect > 20% in all 17 items. PCA showed that all items loaded well (> 0.4) into the assigned subscale and revealed two distinct subscales: cosmesis and function. Based on clinical validity, item “breast shape” was replaced by “breast elevation/position” and “overall skin appearance”. Very good clinical validity (Cohen’s d = 1.38) was found for the new Dutch BCTOS-13. Correlation to the EORTC QLQ-BR23 subscales was high (ICC = 0.65–0.85) for both subscales. Test-retest reliability (Cohen’s d = 0.105) and internal consistency (Cronbach’s α =0.90) were excellent.ConclusionsPsychometric evaluation of a newly developed Dutch BCTOS-13 questionnaire in BCT patients showed excellent results, that were slightly better than the original BCTOS-22 and the shortened BCTOS-12. The good clinical validity makes the BCTOS-13 a useful tool to identify patients with unfavourable cosmetic and functional outcomes, requiring specific attention.Electronic supplementary materialThe online version of this article (10.1186/s41687-018-0085-y) contains supplementary material, which is available to authorized users.

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