gms | German Medical Science

Background: Most patients with plasma cell neoplasia show generalized disease at the time of diagnosis (multiple myeloma). However, about 5% of patients present with either a single bone lesion or, even less commonly, with an extramedullary plasmocytoma (SEP), both without signs of systemic spread. The SEP occurs more commonly in the upper respiratory tract (e.g., in the nasopharyngeal cavity in over 80% of cases) and is even more unusual in other parts of the body. 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: We report the case of a 63 years old patient who initially presented with an isolated tumour of the left knee. The patient had been treated for arthrosis of the knees for two years before a lump on his left knee was detected and further diagnosed via x-ray, CT-scan and fine needle aspiration. The anaplastic tumour was initially diagnosed as rhabdomyosarcoma (RMS) localized at the knee. An interdisciplinary approach with neoadjuvant radiotherapy (RT) was initiated, but shortly before commencing the radiation the patient suffered a stroke with hemiplegia. CT and MRI of the head revealed a single cerebral lesion on the left side. Assuming a solitary brain metastasis of the previously diagnosed RMS a surgical resection of this cerebral lesion was performed. After a comprehensive review of this specimen and the previous biopsy including clonality analysis the diagnosis was changed to anaplastic extramedullary plasmocytoma. A generalized disease was excluded by imaging, laboratory tests, and bone marrow aspiration. The patient was treated with definitive RT of the left knee with a total dose of 50 Gy and with additional postoperative RT of the cerebral metastasis with a total dose of 35 Gy. Surgery of the knee tumour was not performed.

Conclusion: Correct diagnosis of extramedullary plasmocytomas may be difficult, particularly as this disease is rare and can present with an atypical clinical picture and an atypical immunophenotype. Review by a histopathologist with a special interest in either bone tumours or lymphoproliferative disorders is strongly recommended. In this case two separate extramedullary lesions occurred both in uncommon sites, which posed additional difficulties.