Rare aggressive presentation of primary cutaneous diffuse large B-Cell lymphoma, leg type, in the upper arm with concurrent non-small cell lung cancer.

A female patient in her early 90s presented with a rapidly growing erythematous plaque on the right upper arm. Histopathological analysis revealed dense dermal lymphoid infiltrate with a non-germinal centre phenotype (CD20+, MUM1+, BCL2+, BCL6+, CD10-). Histology confirmed the diagnosis to be primary cutaneous diffuse large B-cell lymphoma, leg type (PCDLBCL-LT). Elevated Ki-67 (90%) indicated aggressive disease. Subsequent Positron Emission Tomography (PET)-CT staging revealed right axillary lymphadenopathy and coincidentally diagnosed recurrent non-small cell lung cancer (NSCLC). The PCDLBCL-LT was treated with localised radiotherapy after the patient declined chemotherapy. This case report highlights an unusual presentation of PCDLBCL-LT on the upper limb. It further emphasises the challenges posed in diagnosing and treating solitary skin lesions among older patients with a prior oncological history while highlighting the importance of an individualised multidisciplinary approach for safe and efficacious treatment in the context of advanced age and multiple comorbidities.