Mycosis Fungoides With Large Cell Lymphoma Transformation: A Report of a Rare Case.

BACKGROUND Mycosis fungoides, the most common cutaneous T-cell lymphoma, can transform into large cell lymphoma in some cases, leading to worse outcomes and reduced life expectancy. CASE REPORT We report the case of a 38-year-old Brazilian man with a history of multiple skin lesions refractory to medical treatment, requiring repeated hospitalizations, procedures, chemotherapy, phototherapy, and immunotherapy. The patient was followed by the same hospital's pathology and dermatology services for 17 years, from the diagnosis of mycosis fungoides to subsequent transformation to large cell lymphoma and eventual death. CONCLUSIONS We report a rare condition with a poor prognosis, highlighting the challenges associated with disease transformation, which markedly reduces patients' life expectancy and quality of life. The histological findings and association with the clinical manifestations were vital to the diagnosis and treatment of the disease; thus, early biopsy - even in young patients - is recommended, as it may reveal a new differential diagnosis and influence prognosis. Since the evolution of mycosis fungoides to large cell lymphoma after the transformation process is extremely aggressive, we emphatically recommend initial and follow-up biopsies even in younger patients without prior comorbidities, as in the present case. Increased awareness among clinicians regarding atypical presentations, particularly in populations not typically considered at risk, such as our patient, may also contribute significantly to timely intervention and more favorable outcomes.