Milea Timbergen

259 specific HRQoL-tool in future studies. The response rate was lower than we had hoped for, but similar response rates have been published in studies describing more common diseases such as cancer 23 . In the current study the relatively low response rate may have been due to the length of the questionnaire, the single centre setup (one centre in each country), the timing of sending out the questionnaire (mid-summer) and/or the overall reluctance to participate in a survey study. Furthermore, many patients also need to complete questionnaires as part of their regular health care; therefor patients might be less willing to complete questionnaires for research purposes. Sending out a reminder to patients would have been a valid option to increase the response rate. Selection bias may have led to an overestimation of HRQoL- problems in our cohort. As the primary aim was to identify the most relevant issues in this patient group, the effect of this overestimation is less relevant. A population-based cohort is required to determine true prevalence of issues and perhaps a more representative result. Lastly, interpretation of the questions is influenced by the current health situation of each patient. We tried to eliminate such influencing factors by excluding patients with a diagnosis of cancer and FAP-associated DTF. However, patients HRQoL might also be influenced by disease stage, tumour location and treatments, and by other comorbidities and personal circumstances. This impact on HRQoL-issues could be evaluated in a future population-based cohort study and stresses the need for validation of our findings in a large, international DTF cohort to evaluate the prevalence of HRQoL-issues. Today, solely one DTF-specific questionnaire, the Gounder/DTRF Desmoid Symptom / Impact Scale, is available, and currently mainly used in the setting of clinical trials 24-26 . The findings of our study will be used for the development of a DTF-specific tool, according to the EORTC guidelines, which can be used accompanied by the EORTC QLQ-C30 HRQoL-instrument and will be useful for observational studies, clinical trials and clinical care. Implementation of this tool and action on abnormal findings, concerns or poor experiences of patients might improve satisfaction with health care, symptom management and HRQoL 27 . Health care providers may benefit from being able to anticipate and identify problems earlier, thereby improving work efficiency and promoting patient-centred care through shared decision-making 28-30 . In order for a tailored HRQoL-tool to work in clinical practice, this tool should add value to the clinical workflow without disrupting it 31 . Our results will be used in the development of an international, multicentre, population-based study in line with the EORTC guidelines for developing a questionnaire 14 . This study includes pre-testing and content validation of a DTF- specific questionnaire. This questionnaire will assess the prevalence of HRQoL-issues and will identify risk factors for the development of HRQoL-issues patients experience. Patients will receive an invitation to participate in an online survey and one reminder for completing 9