A 53‐Year‐Old Woman with a Subfascial Mass of the Back that Lasted for Years

Clinical History

A 53‐year‐old woman presented with a growing mass in the left lumbar region. The patient had become casually aware of it 10 years ago. Since then she performed radiological and clinical follow‐up; she refused surgical resection. In 2009, a cytological examination had displayed mesenchymal cells without nuclear atypia. In the last year, the tumor had been growing and had become symptomatic with pain during physical activity. Clinical examination revealed an unmovable swelling on the left lumbar region with intact skin. MRI showed a 9 × 4×5 cm T1 hypointense and T2 hyperintense mass, with a strong and heterogeneous enhancement after contrast infusion and a hypointense central core. It extended from the body of the second to the fifth lumbar vertebra. CT scan of the chest also revealed four pulmonary nodules enhancing with contrast, two of which were hypermetabolic during the whole‐body PET‐CT scan (max SUV 2.7 and 2.4). The patient underwent surgical resection of the lumbar lesion which was easily removed with sharp dissection from the surrounding muscle fibers.

Pathological Findings

At gross examination, the mass was encapsulated and its cut section was multinodular, glistening, mainly white and firm, with central areas of bone consistency (Figure 1A). Histopathological examination of hematoxylin and eosin stain showed a partially circumscribed tumor, with biphasic appearance. The main aspect was a proliferation of atypical spindle cells arranged in fascicles, mitotically active (7 mitoses/10 HPF) and focally embedded in a myxoid matrix (Figure 1B,C); several foci of necrosis were detected. The second aspect was more differentiated and made of elongated bundles, interweaving fascicles and loose whorls of cells with wavy nuclei, without pleomorphism and in absence of mitoses (Figure 1D,E). Bone metaplasia was present between these two areas. Immunohistochemical staining revealed diffuse reactivity to GLUT1 (Figure 1F), CD10 (Figure 1G), EMA (Figure 1H) and Bcl2 in the most differentiated area but absence of reactivity in the most atypical areas. Both of them were focally reactive to CD34. Negative were S100 and CD99.

Figure 1.

 

Ultrastructural examination revealed the presence of spindle cells containing prominent pinocytic vesicles and surrounded by a focally discontinuous external lamina (Figure 1I). What is your diagnosis?

Diagnosis

Malignant perineurioma [Malignant peripheral nerve sheath tumor (MPNST) with perineural differentiation]

Discussion

The term MPNST is inclusive of any sarcoma arising from the peripheral nerve, in most cases from the Schwann cells but not only from them. It is a rare form of sarcoma (3%–10%) 2 and those of perineurial origin are exceedingly uncommon 3, 4. Histopathological diagnosis of MPNST may be difficult because of the extreme morphological variability of this tumor. Conventional MPNST usually focally expresses S100 protein. In the present case, this was completely absent in both the sarcoma and the most differentiated areas. Although considered by some authors to be highly specific of perineurial cells, GLUT1 and EMA may be co‐expressed also in other tumors 1. Therefore, ultrastructural examination was necessary to show highly diagnostic features supporting the perineurial cell nature of the lesion.

50%–60% of MPNST cases are observed in patients with neurofibromatosis 1 and the risk of malignant transformation is higher in patients with deep neurofibromas. Another risk factor is represented by previous (4–40 years before) radiation exposure. In the present case, the patient did not show any of the two risk factors but the malignant transformation of a benign perineurial tumor that had lasted about 10 years.

Perineuriomas are benign peripheral nerve sheath tumors composed exclusively of perineurial cells, which surround individual nerve fascicles. They may be sclerosing (composed of cords of small epithelioid to spindle cells arranged in a dense collagenous stroma) or reticular, with anastomosing cords of elongated spindle cells, with lacy or reticular architecture. The malignant form has the same cytoarchitectural features but also hypercellularity, nuclear atypia, hyperchromasia and high mitotic rate.

Benign perineuriomas rarely recur. The malignant form may metastasize but its behavior is less aggressive than conventional malignant peripheral nerve sheath tumor 3.