PA26 Folliculotropic mycosis fungoides with large cell transformation in an adolescent patient

Abstract A 19-year-old man was referred to the cutaneous lymphoma clinic with a 3-year history of indurated plaques with follicular accentuation affecting the head, neck, upper and lower limbs, previously diagnosed as eczema. There was no palpable lymphadenopathy or B symptoms. Skin biopsies were diagnostic of folliculotropic mycosis fungoides (FMF), with histology showing a heavy periadnexal dermal lymphoid infiltrate with focal prominent folliculotropism. Lymphoid cells showed mild-to-moderate cytological atypia. Immunohistochemistry showed loss of CD7 in the T lymphoid cells. CD4 : CD8 ratio was > 20 : 1. Computed tomography (CT) staging showed bilateral axillary and right inguinal enlarged lymphadenopathy, but right axillary lymph node biopsy confirmed benign dermatopathic lymphadenopathy. Initial staging was IIa (T2bN1aM0B0a). He was given narrowband ultraviolet B, with excellent response, but relapsed after 2 months. Repeat CT scan showed marginal increases in the axillary and right inguinal lymph nodes. Repeat right axillary lymph node biopsy showed focal infiltration by a CD30+ T-cell lymphoproliferative disorder. Repeat skin biopsies showed FMF with large cell transformation including CD30 expression in 60% of cells with a high proliferation index. This confirmed FMF with large cell transformation and nodal involvement, stage IVA2 disease. He commenced first-line systemic therapy with methotrexate, but had no response. He was switched to brentuximab vedotin and has received eight cycles with partial response. Providing a near complete response can be achieved, he will receive an allogeneic haematopoietic stem cell transplant, with conditioning using total-skin electron beam, total lymphoid irradiation and antithymocyte globulin. Paediatric mycosis fungoides (MF) is rare, accounting for 0.5–7% of cases of MF. Diagnostic delay commonly exceeds 3 years. Hypopigmented MF is the most common form presenting in children. FMF is an aggressive variant, accounting for 3–36% of cases of paediatric MF. In a systematic review of 571 patients aged < 20 years with MF, 6.4% of patients had FMF. Only 2.5% of all patients developed large cell transformation and 4.7% progressed beyond stage IIA. FMF was not a risk factor for MF progression (Jung JM, Lim DJ, Won CH et al. Mycosis fungoides in children and adolescents: a systematic review. JAMA Dermatol 2021; 157: 431–8). Large cell transformation follows a more aggressive clinical course, requiring more intensive management. Advanced-stage MF in children and adolescents is extremely rare and carries a similarly poor prognosis to adult-onset disease. Management is challenging due to lack of evidence or structured protocols, meaning centres treat patients according to experience and drug availability. Recent international consensus-based recommendations advised an international registry for advanced-stage paediatric disease (Zvulunov A, Neale H, Stern J et al. Approach to mycosis fungoides in children: consensus-based recommendations. J Am Acad Dermatol 2024; 91: 1078–85).