BACKGROUND: Mycosis fungoides (MF) is a rare malignancy that is characterized by the presence of circulating tumour cells (CTCs) in a subgroup of patients. Reliably distinguishing MF from inflammatory skin conditions is challenging. OBJECTIVES: To evaluate the potential benefits of next-generation sequencing (NGS)-based T-cell receptor rearrangement repertoire analysis in detecting clonal rearrangements in MF and inflammatory skin conditions. METHODS: Skin biopsies and blood samples from 33 patients with MF and 10 patients with inflammatory skin conditions were analysed using TRB and TRG NGS. Twenty-seven patients had early-stage IA (n = 19) and IB (n = 8) MF, and six had advanced-stage disease (IIB, n = 5
IIIA, n = 1). RESULTS: Analysis applying standard abundance thresholds identified at least one clonal rearrangement in the skin DNA of 97% (n = 32/33) of patients with MF and in 90% (n = 9/10) of those with inflammatory skin conditions. To enhance specificity, an abundance and distribution-based approach was applied, which considered only rearrangements that significantly stood out from the physiological background as clonal (MF, n = 29/33
inflammatory skin conditions, n = 1/10), allowing for highly sensitive (88%) and specific (90%) discrimination between MF and other inflammatory skin conditions. CTCs were detected in 46% (n = 11/24) of patients with early-stage MF and in 60% (n = 3/5) of those with late-stage MF. CONCLUSIONS: NGS-based T-cell receptor repertoire analysis is a highly sensitive and specific method for the differential diagnosis of early-stage MF vs. inflammatory skin conditions, and for the sensitive molecular detection of CTCs.