Cutaneous T-cell lymphoma (CTCL) is a malignancy of skin-homing T cells associated with increased risk of second malignancies. CAR-T lymphoma is a mature T-cell malignancy arising in cancer patients treated with chimeric antigen receptor T-cell (CAR-T) therapy. Because of its frequent cutaneous involvement, CAR-T lymphoma can mimic CTCL, which in turn can resemble cutaneous sequelae of oncologic therapies. Careful evaluation of these entities in cancer patients is therefore essential, yet little guidance exists regarding optimal methods of screening, diagnosis, and treatment. Clinicians managing CTCL in cancer patients should consider how oncologic treatment toxicities may overlap with CTCL treatment toxicities or affect patients’ risk for other skin cancers. Where possible, immune-boosting or at least immune-preserving treatments are preferred, with immune checkpoint inhibition emerging as an important option in selected patients. Patients with a history of CAR-T therapy and new, persistent skin findings should undergo careful work-up with molecular screening for lymphoproliferative processes. Presence of a CAR in lesional tissue can be suggestive but is not diagnostic of a CAR-T-associated malignancy. Treatment for any patient with a cutaneous T-cell or CAR-T lymphoma and a second cancer should be an interdisciplinary effort tailored to the patient’s specific medical history and molecular profile. Further research is needed to establish better evidence-based guidelines for treatment of these patient populations.

Patients most often present with hypopigmented patches, though folliculotropic disease is also common. Prognosis is usually excellent. Most patients are diagnosed with early-stage disease, and less than 5% of cases progress to late-stage disease. Pediatric MF can be difficult to recognize. It often mimics more common benign inflammatory and pigmentary dermatoses. Furthermore, clinicians may be hesitant to perform skin biopsies in younger patients, contributing to delays in diagnosis. Evaluation and treatment guidelines largely follow those of adult MF, with special considerations regarding age and expected lifetime exposure to therapy. Adaptations include limiting unnecessary ionizing radiation, relying more heavily on ultrasound as an imaging modality, and balancing disease control with the long-term safety of treatments, including topical corticosteroids and mutagenic therapies such as PUVA, mechlorethamine gel, and radiation. Despite favorable outcomes, the chronic nature of MF can lead to substantial psychosocial burden on both the patients and their families. Caregivers must navigate frequent visits, uncertainty about recurrence, and concerns about long-term adverse effects. Management should therefore incorporate developmentally appropriate communication, psychosocial support, and long-term follow-up. This chapter reviews the epidemiology, clinical and pathologic features, diagnostic approach, staging, and treatment of pediatric MF. It highlights the distinctions between pediatric and adult disease, recently developed recommendations, and the importance of individualized, family-centered care. Because the clinician evaluating a child with an atypical or persistent eruption must distinguish pediatric MF from other rare cutaneous lymphoproliferative disorders, this review also discusses hydroa vacciniforme lymphoproliferative disorder (HV-LPD). Although HV-LPD is a distinct Epstein–Barr virus-–associated entity rather than a variant of MF, it is included here because it is one of the few primary cutaneous lymphoproliferative disorders of childhood. It therefore shares the diagnostic challenges of mimicking benign dermatoses and requiring careful clinicopathologic and immunophenotypic correlation, and represents an important consideration in the pediatric differential diagnosis.