Extramedullary Plasmocytoma: A Rare Case with Bifocal Manifestation at Uncommon Sites

Background: Primary extramedullary plasmocytomas (EMPs) are plasma cell tumours that arise outside the bone marrow. As these tumours are rare and can present without a typical clinical picture, correct diagnosis is difficult to confirm, particularly when the lesions occur at uncommon sites. Case Report: This is a case report of a 63-year-old patient who initially presented with an isolated lump on his left knee. Biopsy showed an anaplastic tumour which was first diagnosed as localised rhabdomyosarcoma; therefore, a combined approach with neoadjuvant radiotherapy followed by surgery was recommended. Shortly before radiotherapy started, the patient suffered a stroke with hemiplegia. CT and MRI of the head revealed a single brain lesion. A resection of this cerebral lesion was performed and surprisingly revealed a cerebral EMP. After comprehensive review of this specimen and the previous biopsy, the diagnosis was changed to anaplastic bifocal EMP. Generalised myeloma was excluded; therefore the patient was treated with definitive radiotherapy of the left knee region and postoperative partial brain irradiation. Unfortunately, the patient died several months later due to fulminant progressive disease at both treated sites despite rapidly initiated chemotherapy. Conclusion: Correct diagnosis of EMP may be difficult, particularly as this disease is rare and can present with atypical clinical picture and immunophenotype. Review of the specimen by a histopathologist with special interest in soft tissue tumours or lymphoproliferative disorders is strongly recommended.