Graft‐Versus‐Tumour Effect Following Allogeneic Haematopoietic Stem Cell Transplantation in a Paediatric Patient With Mogamulizumab‐Treated Primary Cutaneous Peripheral T‐Cell Lymphoma

ABSTRACT Introduction Primary cutaneous peripheral T‐cell lymphoma, not otherwise specified (pcPTCL‐NOS), is exceptionally rare in children, and evidence guiding diagnosis and treatment is limited. Case Presentation A 10‐year‐old boy with pcPTCL‐NOS refractory to chemotherapy received mogamulizumab prior to allogeneic haematopoietic stem cell transplantation (allo‐HSCT). Despite a 60‐day washout period, he developed prolonged steroid‐refractory acute graft‐versus‐host disease requiring ruxolitinib and extracorporeal photopheresis. Concurrently, transient skin lesions developed at previous lymphoma sites and regressed spontaneously, with biopsy supporting the interpretation of visible graft‐versus‐lymphoma. Conclusion This case underscores the need to optimise allo‐HSCT timing after mogamulizumab and preserve graft‐versus‐lymphoma effect while limiting graft‐versus‐host disease. Trial Registration : The authors have confirmed clinical trial registration is not needed for this submission.