SPB is a rare hematologic malignancy, which is defined by the presence of a single osteolytic lesion due to monoclonal plasma cell infiltration, with or without soft-tissue extension [2]. The median age at the diagnosis is 55 years and males are more frequently affected than females. The lesion is often present for many years as a single lesion; however, it can eventually progress to multiple plasmacytomas within 3 years [5]. The most common site of SBP involving is the vertebrae, especially thoracic vertebrae, and the back pain is a common clinical feature due to lesion compression [6, 7]. Diagnostic criteria of a SPB include pathological proven solitary lesion, normal bone marrow with no evidence of clonal plasma cells, normal skeletal survey and MRI ( or CT ) of spine and pelvis ( except for the primary solitary lesion ), absence of end-organ damage [1]. Our case meets all these criteria for the diagnosis of SPB. Nonetheless, clinical aspects overlap, thus making directed diagnostic and therapeutic strategies difficult. Besides the above criteria of diagnosis, imaging studies play an important role in the diagnostic, treatment considerations and prognostic aspects [1]. However, imaging studies may not always provide a conclusive diagnosis. SBP has an osteolytic appearance on plain radiographs, and an expansile osteolytic lesion with marked enhancement on CT or MRI [2]. In the published finding, SBP of spine could show curved coarse trabeculae with hypertrophic sclerosis, forming“mini brain sign”which had certain characteristics. This appearance may be correlated with the less aggressive nature of SBP comparing with other malignant tumors that aggressively destroy the bone with no radiological evidence of bone repair features such as sclerosis and thickening [8, 9]. In our case, we could not observe this similar signs. To the best of our knowledge, most studies did not report the imaging presentation of SBP involving adjacent disc space, there was only one reported case of SBP with involving adjacent disc space by direct extension [4]. Afonso and colleagues reported a 41-year-old woman with SBP, her spine CT revealed areas from T12 to L2 with vertebrae and disc space destruction. The case report mainly focused on lesion’s CT and MRI findings, without describing differential diagnosis in diseases with involving adjacent disc space, such as spinal neoplasms and infections.
Usually spinal lesions associated with a poorly defined vertebral body endplate, involvement of intervertebral disc space, presence of paravertebral abscesses and involvement of two contiguous vertebral bodies are suggestive of spinal infection, whereas spinal lesions associated,well-preserved disc space, destroyed vertebral bodies with solid extraosseous soft tissue, or non consecutive multifocal involvement of spine are suggestive of spinal neoplasm [10, 11]. However, one study found 2% malignant spinal lesions with disc involvement [12], SBP with adjacent disc space destruction may be due to aggressive traits of primary plasmacytoma presenting infiltration and destruction of adjacent bone, muscle, fat and vascular encasement. [13, 14]. In an approach to such patients with involving adjacent disc space, differential diagnosis should include neoplasm and bacterial infection. In some conditions of spinal infections, patients’ symptoms with no fever can be very similar to those of other spinal lesions, and imaging findings can be non-specific or atypical, laboratory examinations including white blood cell count (WBC), erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are inconclusive, thus differential diagnosis is warranted [15, 16]. In these cases, biopsy is paramount to confirm diagnosis. However, a study reported the presumed aetiology in 18% of the cases was not confirmed on pathological examination [17]. A recent metanalysis reported the sensitivity of CT-guided percutaneous needle aspiration biopsy was 52.2% (95% CI, 45.8–58.5) for the diagnosis of spinal infections [18]. Additionally, the presentation of disc involvement is nonspecific feature of spinal infection, but is compatible with solid neoplasms, as shown the case in the present report. Hence, differential diagnosis between spinal neoplasms and infections is more difficult and complicated. The diagnostic approach for the patients with spinal lesions should include a complete medical history, physical examination, necessary laboratory examinations and imaging evaluations during which possible risk factors for infection and neoplasm must be investigated and identified. Spine imaging evaluation should be essential to careful observation at or around the each portion of vertebral bodies, such as facet joints, epidural space, and muscular structures of spine. In some cases, repeat or open biopsy may be required as it is the only reliable method to distinguish neoplasms and infections.