Epidemiology of cutaneous lymphoma
Authors: M. Vermeer
Journal: British Journal of Dermatology (2021)

Linked Article: Dobos et al. Br J Dermatol 2021; 184:1059–1067. Cutaneous lymphomas (CLs) represent a rare and heterogeneous group of lymphomas that present in the skin without extracutaneous manifestations at the time of diagnosis. As the diagnosis and classification of CLs requires integration of clinical, histological, immunophenotypical and molecular characteristics, and is complicated by the rareness of these diseases, clinical care of patients with a CL is often concentrated in national expert centres. Data on the epidemiology of CLs are primarily based on patient cohorts seen in these expert centres and in national pathology registries. However, patient cohorts from expert centres can be influenced by referral bias, while pathology registries are prone to misclassification of patients as central review with the integration of clinical and pathological features is not routinely performed. However, an accurate understanding of the incidence and the prevalence of CLs is important for clinicians, patients, pharma and regulatory policymakers. In this issue of the BJD, Dobos et al. present a retrospective analysis of epidemiological data from the French Cutaneous Lymphoma Registry (FCLR).1 Based on 779 patients included in 2018 by the FCLR, the authors report an incidence of CL of 0·96 per 100 000 population. By comparing patients included in the FCLR database from 2005 to 2009 with patients included from 2010 to 2014, and with patients included from 2014 to 2019, several changes in the prevalence of CL subtypes were found. Previous epidemiological studies on cutaneous T-cell lymphoma (CTCL) performed in Wales and Norway, based on 120 and 337 patients, respectively, reported a much lower prevalence for CTCL of 0·29–0·39/100 000 population.2, 3 In contrast, studies from the USA based on much larger groups of patients (range 4310–6230) reported a prevalence of 0·64–0·87/100 000 population.4, 5 Therefore, based on the results of these latter studies and the current paper by Dobos et al.,1 a similar prevalence of CL of around 0·90/100 000 population seems most likely in both Europe and the USA. In line with previous studies, CTCL represented a much larger proportion (75%) of the included patients than cutaneous B-cell lymphoma (CBCL; 25%).6 Also, the proportional distribution of CL over different diagnostic entities and the demographic characteristics observed in this study, including age at diagnosis, a female-to-male ratio of 0·6 for mycosis fungoides (MF), diffuse large B-cell lymphoma (leg type) being more frequent in females and the observation that the large majority of patients with MF presented with early-stage disease, are in line with previous studies.6, 7 The strong points of the paper by Dobos et al. include the large number of collaborating centres, the large number of included patients, the long duration of data collection and the consistent use of the World Health Organization–European Organisation for Research and Treatment of Cancer’s classification system, allowing for the observation of changes in the epidemiology of CL over time.1 Whereas for some well-described CL subtypes, such as classical MF and lymphomatoid papulosis, the number of cases remained relatively stable over time, a large (sometimes spectacular) increase was observed in other CL subtypes. For example, for CTCL, an increased number of included patients was seen for those with folliculotropic MF (10 in 2005–09 and 325 in 2015–19), primary cutaneous CD4+ small/medium T-cell lymphoproliferative disorder patients (35 in 2005–09 and 244 in 2015–19) and primary cutaneous anaplastic large-cell lymphoma (one in 2005–09 and 94 in 2015–19). For CBCLs an increase was seen in primary cutaneous marginal zone B-cell lymphoma (113 in 2005–09 and 446 in 2015–19) and primary cutaneous large B-cell lymphoma (leg-type; two in 2005–09 and 111 in 2015–19). Whether the increase in patients diagnosed with these relatively new and rarer diagnostic entities relates to more accurate diagnosis combined with greater awareness of the FCLR and greater coverage, leading to more referrals, or a truly increased incidence, remains to be seen. Having built the infrastructure, the FCLR is in a perfect position to observe how these epidemiological developments will further evolve in the future and answer these questions. In addition, description of treatment results and survival in this well-described cohort of patients will also be of great interest.