The patient is a 72-year-old man with refractory diffuse large B-cell lymphoma (DLBCL) originally diagnosed more than 10 years ago and treated with fludarabine-based chemoimmunotherapy. He had multiple relapses and achieved a complete response to pembrolizumab plus vorinostat but ultimately relapsed. He was then given CD19-targeted chimeric antigen receptor (CAR) T cells and, within 1 week, he was noted to have an erythematous rash and growing mass on his shin at the site of his known lymphoma, which was suggestive of progressive disease and was biopsied.

The biopsy showed a diffuse, sheet-like infiltrate composed of medium-to-large atypical cells (panel A; hematoxylin and eosin stain, original magnification ×40) suggestive of persistent DLBCL. Stains showed that the infiltrate was negative for B-cell markers, including PAX5 (panel B, original magnification ×40) and positive for CD3 (panel C, original magnification ×40) with predominately CD4 (panel D, original magnification ×40) compared with CD8 (panel E, original magnification ×40). The infiltrate lacked BCL2 (not shown), which is suggestive of T-cell lymphoma. However, 15 days after the biopsy, this lesion completely resolved without intervention, and the positron emission tomography scan confirmed complete remission. This likely represented the CAR T cells that were homing to this particular site but eventually dissipated. These can mimic persistent disease or even a T-cell lymphoma and should be evaluated with caution.