Diffuse large B-cell lymphoma mimicking metastatic melanoma: the importance of biopsies in the era of immune checkpoint inhibitors.

Positron emission tomography (PET)/computed tomography (CT) imaging is an established tool in diagnosing and staging for various malignancies, however, during immune checkpoint inhibitor (ICI) therapy not only inflammatory changes may mimic disease progression, but also secondary malignancies should be considered in the setting of unusual clinical and radiographic findings. Here, we present the case of a 64-year-old man with a lymphogenic metastatic malignant melanoma treated with ipilimumab/nivolumab, in whom PET/CT indicated tumor progression of an intra-abdominal mass. Biopsy revealed an unusual reactive T-cell expansion without clonal expansion, pathologically consistent with ICI-induced immune response. As the patient's general condition worsened, we switched to targeted therapy, which had to be discontinued due to increasing fatigue. Follow-up PET/CT at 6 months showed further intra-abdominal progression. Subsequent histopathology of the extirpated mesenteric lymph node conglomerate now revealed diffuse large B-cell lymphoma. Our case highlights the importance of repeated histologic examinations of radiologic pathologies to distinguish secondary malignancies from ICI-induced inflammatory reactions or progressive disease.