Blastic plasmacytoid dendritic cell neoplasm: a rare external ear lesion presenting with leukaemia.

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare and aggressive haematological malignancy, typically characterised by cutaneous lesions and bone marrow involvement. We present a unique case of a woman in her 70s, initially seen for a spontaneous swelling on her left external ear resembling a haematoma, which recurred after initial treatment, triggering further evaluation.Diagnostic challenges arose as the patient displayed positive markers for Myeloperoxidase (MPO) (p-ANCA), suggesting vasculitis. Dermatology considered various differential diagnoses, but imaging and tests ruled out significant pathology. Steroid treatment led to improvement, but coincided with a surge in white cell count (WCC), prompting an urgent haematological review.Subsequent investigations, including a punch biopsy of the external ear and a bone marrow biopsy revealed BPDCN concurrent with chronic myelomonocytic leukaemia. This case highlights the challenging diagnostic journey, emphasising the need for multidisciplinary collaboration and the potential for unique BPDCN presentations, expanding our understanding of this malignancy.