Generalised eruptive histiocytosis mimicking lymphoma relapse in a patient with previous diffuse large B-cell lymphoma

A 92-year-old man with a history of diffuse large B-cell lymphoma treated with chemoradiotherapy, and later with radiotherapy for an isolated cutaneous relapse with complete response, presented with a 1-month history of multiple asymptomatic erythematous-to-violaceous papules and nodules on the face, trunk, and upper limbs. Given his oncological history and previous skin involvement, lymphoma relapse was the leading diagnostic concern, with cutaneous metastases and sarcoidosis also considered. However, skin biopsies showed a non-Langerhans cell histiocytic infiltrate expressing CD45, CD68, and CD163, without CD1a, CD20, PAX5, S100, or Langerin expression. Skin and blood flow cytometry identified no abnormal cell populations, and a PET-CT scan showed no abnormal metabolic activity. Clinicopathological correlation narrowed the differential to group C (cutaneous) histiocytosis, and subsequent spontaneous regression of the lesions over 3 months without treatment supported the diagnosis of generalised eruptive histiocytosis. At 15 months of follow-up, there was no recurrence or evidence of active disease. This case illustrates that cutaneous histiocytoses can mimic several inflammatory and neoplastic conditions, including lymphoma relapse, reinforcing the need to actively resist cognitive bias in people who have had cancer through reflective reassessment, broad differential diagnosis, and biopsy confirmation.