Update on the psychometric properties and minimal important difference (MID) thresholds of the FACT-M questionnaire for use in treatment-naïve and previously treated patients with metastatic Merkel cell carcinoma

Objectives: For valid and reliable assessment of patients' Health-Related Quality of Life (HRQoL), it is crucial to use psychometrically robust instruments. In the context of rare diseases such as Merkel cell carcinoma (MCC), validated disease-specific instruments are often not available. The Functional Assessment of Cancer Therapy - Melanoma (FACT-M) was originally developed for use in melanoma. Its psychometric performance for use in MCC and minimal important difference (MID) thresholds have been previously reported based on a cohort of metastatic MCC patients who had disease progression following one or more prior line of chemotherapy (NCT02155647 Part A; n = 70). Since then, new data from the phase II JAVELIN Merkel 200 trial among treatment-naïve patients are available (NCT02155647 Part B; n = 102). This study aims to increase accuracy and precision of previously established psychometric properties and MID thresholds of FACT-M in metastatic MCC patients.

Methods: Published qualitative research suggests that patients with metastatic MCC had similar experiences and described similar concepts associated with their disease independent of whether they were treatment naïve or had prior treatment. Therefore, it was deemed appropriate to pool FACT-M data from Part A (previously treated) and Part B (treatment-naïve) cohorts for this study. Construct validity was assessed by evaluating item-factor correlations (convergent validity) and known-groups validity using ECOG performance status 0 versus 1. Concurrent validity was assessed using EQ-5D items. Internal consistency reliability was assessed using Cronbach's α. Anchor- and distribution-based approaches were used to derive MID thresholds.

Results: Overall, psychometric tests based on various validity (convergent, known-groups, concurrent) and reliability (Cronbach α) analyses confirmed previous findings in that FACT-M performs well in MCC patients. MID thresholds derived from this study are largely in line with previously established thresholds with some minor adjustments.

Conclusions: In the context of rare diseases, which often have limited data available for psychometric testing, a reasonably large MCC patient sample was available for this study, enhancing accuracy and precision of previously established FACT-M psychometric properties and MID thresholds with only small deviations for use in metastatic MCC patients. Results suggest that the FACT-M is suitable for Merkel cell carcinoma regardless of patients' treatment status.

Trial registration: This study is a pre-planned post-hoc analysis conducted on data collected in Part A and Part B of the JAVELIN Merkel 200 trial. This trial was registered on 2 June 2014 with ClinicalTrials.gov as NCT02155647.