The importance of assessing blood tumour burden in cutaneous T‐cell lymphoma

Mycosis fungoides (MF) and Sézary syndrome (SS) are the best‐studied subtypes of cutaneous T‐cell lymphoma. The level of blood tumour burden in patients is important for diagnosis, disease staging, prognosis and management, as well as assessing treatment response. Until recently, the assessment of blood involvement was made using manual counts of morphologically atypical T cells (Sézary cells), but this approach may be subjective, and is affected by interobserver variability. Objective and consistent approaches to accurately quantifying blood involvement are required to ensure appropriate stage‐related management of patients and to improve our understanding of the prognostic implications of blood tumour burden in these diseases. While assessment of blood involvement is common in SS and advanced‐stage MF, an improved understanding of the implications of blood involvement at early disease stages could help identify patients more likely to progress to late‐stage disease, and hence guide treatment decisions and frequency of follow‐up assessment, ultimately improving patient outcomes. This concise review discusses the development of flow cytometry‐based classifications for assessing blood involvement in MF and SS, and summarizes current recommendations for blood classification and assessment of blood response to treatment.