Solitary sacral plasmocytoma

The patient presented with a five-year history of low back pain which had become very severe and constant over the past ten days. The pain was mechanical in nature and he was unable to sleep due to the pain. There was associated radicular pain in the distribution of the first left sacral nerve root, but no symptoms relating to the bowel or bladder. Physical examination revealed mild tenderness in the upper aspect of the sacrum. Neurological and digital rectal examinations were normal.

The patient presented with a five-year history of low back pain which had become very severe and constant over the past ten days. The pain was mechanical in nature and he was unable to sleep due to the pain. There was associated radicular pain in the distribution of the first left sacral nerve root, but no symptoms relating to the bowel or bladder. Physical examination revealed mild tenderness in the upper aspect of the sacrum. Neurological examination was normal and so was digital rectal examination. His haematological and biochemical blood parameters and urinalysis were within normal limits. The patient was admitted for pain relief and further investigations.

Plain radiographs of the lumbo-sacral spine (Fig. 1) did not reveal any significant abnormality. A CT scan revealed a lesion in the body of the first sacral vertebra extending into the left lateral mass with encroachment of the spinal canal and compression of the cauda equina and bilateral first sacral nerve roots (Fig. 2). A CT-guided biopsy of the lesion was performed (Fig. 3). The patient's serum electrophoresis was normal and his urine was negative for Bence Jones proteins. There were no hot spots on his bone scan (Fig. 4), but the lower sacrum was not visible as he failed to empty the bladder. Five days later the histopathology of the lesion came back as plasma cell neoplasia. A skeletal survey did not reveal any abnormality other than degenerative changes in the lumbar spine. Bone marrow aspiration cytology from the iliac crest was also within normal limits. This being a solitary plasmacytoma in the sacrum with no bone marrow involvement, he was treated with radiotherapy to the lesion.

Sacral tumours often present as low back pain with no characteristic pattern or time course. These tumours can be large at presentation as a large volume of tumour can be accommodated within the pelvis (1). Pre-sacral tumours can sometimes be palpated on rectal examination. Encroachment into the spinal canal or the neural foramina can cause cauda equina compression. This is manifested as varying degrees of neurological deficit in the lower limbs with bowel or bladder dysfunction.

Solitary bone plasmacytomas are rare plasma cell proliferative disorders. Most often they involve the bones of the axial skeleton. In the spine, the vertebral body or the posterior elements can be involved.

Plain radiographs of sacral lesions may look normal in spite of extensive involvement of the bone. CT images demonstrate myelomatous mass with the same radiodensity as the adjacent musculature.

The diagnosis is based on histological confirmation of monoclonal plasma cell infiltration of a single disease site and on the exclusion of systemic myeloma by aspiration of marrow from an uninvolved bone (usually the iliac crest). Normal marrow contains about 3% plasma cells. If the aspirate contains a significantly greater proportion of undifferentiated or differentiated plasma cells, the disease is considered disseminated. But there exists a tendency in not accepting the 'solitary bone plasmocytoma' as a true isolated tumor mass. Although no or only very few tumor cells (smoldering myeloma) are visible in the bone marrow aspirate, all these 'solitary' tumors will generalize with time.

For solitary spinal lesions, the treatment of choice is localised radiotherapy (2). Chordoma is the most common primary bone tumour of the sacrum. Metastatic disease, giant cell tumours, lymphomas and myelomas are the other frequent neoplasms of the sacrum (3). Enostosis, osteoid osteoma, osteoblastoma, aneurysmal bone cyst, osteochondroma, chondrosarcoma, Ewing sarcoma, primitive neuroectodermal tumour, and osteosarcoma should also be considered in the differential diagnosis (3).