P047 A rare case of diffuse large B-cell lymphoma, arising atop a congenital melanocytic naevus

Abstract

Diffuse large B-cell lymphoma (DLBCL) is an aggressive subset of non-Hodgkin lymphoma. To date, cutaneous DLBCL has seldom been described in the medical literature arising on top of a giant congenital melanocytic naevus (CMN). A 53-year-old man who was fit and well presented to dermatology in 2024 with tender, fluctuant swelling and ulceration over the right parietal scalp, at the site of his giant CMN. He had previously reported cystic lumps up to 10 years ago underneath his CMN, but these had resolved prior to biopsy. Initial histology in 2024 demonstrated a congenital naevus, with occasional marked lymphocytic infiltrate.

A second incisional biopsy was taken 6 months later, after re-presentation with progressive symptoms over the CMN. Histology revealed sheets of pleomorphism in the dermis and frequent mitotic figures. Diffuse strong CD45, CD20 and CD79a infiltrate was expressed in the neoplastic infiltrate on immunohistochemistry, with the majority of neoplastic cells showing expression of BCL6 and BCL2. This was consistent with a diagnosis of a high-grade DLBCL arising atop the CMN. The patient reported no B symptoms and had no palpable lymphadenopathy. Positron emission tomography–computed tomography (PET-CT) revealed an avid subgaleal mass at the right temple, adenopathy around the diaphragm and a lytic lesion at the left fifth rib. Baseline bloodwork was normal. He was promptly initiated on rituximab, polatuzumab vedotin, doxorubicin, cyclophosphamide and prednisolone (R-Pola-CHP) every 3 weeks over a 4-month period, for stage IV DLBCL. Despite adverse effects of fatigue, peripherally inserted central catheter-associated deep-vein thrombosis and thrombophlebitis, the patient had a striking response, with evidence of remission of the lymphoma and adenopathy within 4 months, evident clinically and on PET-CT scans. This case highlights a rare presentation of DLBCL, arising on a background of CMN, which demonstrated an excellent and rapid response to R-Pola-CHP chemotherapy.