55‐YEAR‐OLD MAN WITH A PARASPINAL MASS

CASE

A 55‐year‐old man presented to an outside institution for a routine preoperative chest X‐ray in preparation for a knee replacement. The chest X‐ray demonstrated a mass in the right lower lobe. Additional imaging with computed tomography (CT) and a positron emission tomography scan demonstrated a 1.5 cm extrapleural paraspinal mass in the region of T8, which also had mild increase in glucose uptake. Transthoracic fine needle aspirate was performed.

The patient reported no history of melanoma and no change in any moles or other pigmented lesions on his skin. He was asymptomatic, with no reports of shortness of breath, chest pain, neurologic symptoms, night sweats, weight loss, nausea/vomiting, bone pain or weakness. Medical history was significant for hepatitis C and atrial fibrillation. Both parents had histories of colonic carcinoma. There was no other history of cancer in the family.

Physical examination was unremarkable with no point tenderness over the thoracic spine and no sensory deficits. Neurologic examination was unremarkable. Total body skin examination failed to reveal any suspicious pigmented lesions.

A magnetic resonance imaging of the thoracic spine with and without contrast demonstrated a well‐defined mass in the right paraspinal soft tissue at T8 level (Figure 1). The mass was 2.2 × 2.0 cm and demonstrated fairly homogenous contrast enhancement with extension into the adjacent right neural foramen, suggesting tumoral extension into this region. No cord compression was noted.

Figure 1.

The patient elected to have the lesion surgically removed. The surgeon noted the mass was in close connection with the exiting T8 nerve root and somewhat adherent to the sympathetic chain. The mass was separated from the pleura and other surrounding structures and removed within an intact capsule.

PATHOLOGY

Fine needle aspirate

Papanicolaou‐stained smears and cellblock from the CT‐guided biopsy contained fragments of cohesive epithelioid tumor cells with mild to moderate nuclear atypia and prominent cherry red nucleoli (Figure 2). Brown pigment seen in both the tumor cells and the macrophages was confirmed to be melanin by a positive Fontana–Masson stain and a negative Perl's stain for hemosiderin. The tumor cells showed strong positivity for S100 and HMB‐45. These cells were negative for immunohistochemical markers Mart‐1, desmin, glial fibrillary acidic protein (GFAP) and pan‐keratin.

Figure 2.

Resection Specimen

The specimen was an encapsulated, ovoid, tan‐white to brown mass, 2.5 × 1.7 × 1.0 cm. Cut sections revealed a dark‐brown nodule, 1.8 × 1.5 × 0.9 cm. A representative section submitted for frozen section analysis revealed benign appearing peripheral nerve sheath components consistent with melanotic schwannoma.

Permanent sections displayed multiple growth patterns (3, 4). The central portion of the nodule was large epithelioid cells, prominent red nucleoli, intranuclear inclusions and intracellular brown pigment. The lesion had a high mitotic rate (4/10 HPF). The peripheral portion of the lesion was predominantly spindle cells with features of benign peripheral nerve sheath tumor and peripheral nerve. Within the benign‐appearing portion of the lesion were multiple scattered pigmented cells and clusters of bland epithelioid cells. The atypical tumor cells were strongly immunoreactive for S100 and pan‐melanoma marker (5, 6) and focally reactive for Melan A and HMB45. The adjacent large and small peripheral nerve segments were also immunoreactive for S100. The lesional cells and the adjacent nerve and nerve sheath segments were also immunoreactive for CD57 (Leu 7) and collagen type IV ( 7, 8). Electron microscopy demonstrated melanosomes in various stages of maturation, intercellular junctions and a well‐defined basal lamina (9, 10).

Figure 3.

Figure 4.

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Figure 10.

DIAGNOSIS

Malignant melanotic schwannoma.

DISCUSSION

Melanotic schwannomas (MS) were first described in 1932 by Millar(8). These tumors are rare, with approximately 80 cases reported in the literature to date (14). They are of neural crest origin and are believed to arise from a common precursor for Schwann cells and melanocytes 12, 13. MS occurs about a decade earlier than other types of schwannomas with a peak in the fourth decade (range 10–92 years) with no sex predominance. Most patients present with symptoms of nerve compression 10, 12, 14. The cranial and spinal nerve roots, along with the parasympathetic chain, are common locations for MS, with cases also reported in the respiratory and alimentary tracts among other sites 1, 11. Approximately 20% of patients have multiple MS and 10% of cases undergo malignant transformation (9).

Histologically, melanotic schwannomas are characterized by the typical spindle‐shaped neoplastic Schwann cells and melanosomes in various stages of maturation. These tumors are grossly pigmented and are immunophenotypically marked for both Schwann cell markers and markers of melanocytic differentiation (10). They contain melanosomes that are typically in stages II, III (premelanosomes) and IV (mature melanosomes). MS has both non‐psammomatous and psammomatous variants. The non‐psammomatous variants usually involve the spinal nerves and the paraspinal ganglia (10). The psammomatous variants tend to affect nerves of the intestinal tract, heart and cranial nerves (9).

Approximately 50% of the patients with the psammomatous variant have Carney's complex, and these cases generally occurs in younger patients 2, 3. Carney's complex is an autosomal dominant, multisystem disorder characterized by myxomas (primarily involving the heart, skin and breasts), pigmented skin lesions, endocrine abnormalities and melanotic schwannomas (3). Myxomas are the most serious findings. Endocrine abnormalities include nodular adrenal cortical hyperplasia and acromegaly caused by pituitary adenomas 3, 9.

Cytologic examination of MS reveals elongated cells with long branching cytoplasmic projections, abundant melanin pigment and fusiform or rounded nuclei (7). The tumor is reactive for melanoma markers including S100, HMB45 and Melan A (14). Immunohistochemical markers S100 and HMB45 are commonly positive in MS, and GFAP and neurofilament protein are sometimes positive. Lamin and collagen type IV are positive in MS and may be useful in distinguishing MS from malignant melanoma (MM)(14).

As previously mentioned, 10% of MS undergo malignant transformation. Useful criteria to diagnose malignancy in a MS include the presence of necrosis, an increased mitotic rate and nuclear atypia. However, one should be careful in evaluating nuclear atypia, as benign MS can also appear rather atypical.

Distinguishing malignant melanotic schwannoma (MMS) from melanoma can be challenging. MMS typically occurs in younger patients (mean age of 33) whereas melanoma is more common in the fifth decade (2). The presence of nuclear palisading, high cellularity, prominent spindle cells and lack of a cutaneous primary may be helpful in rendering a diagnosis of MMS. Electron microscopy can contribute significantly to the differentiation of MMS vs. melanoma. MMS cells demonstrate cell processes, have a prominent basal lamina, melanosomes and most importantly, intercellular junctions (11). Melanomas have cellular interdigitation without intercellular junctions, indented nuclei and prominent nucleoli (4). Our case demonstrated prominent intercellular junctions in addition to the external lamina, cell processes and melanosomes in all stages of maturation, supporting our diagnosis of MMS.

The importance of thorough sampling of tumors was illustrated in this case. The periphery of the lesion, where the frozen section was taken, contained only benign appearing nerve sheath components. This was confirmed by retrospective review of the frozen slides following examination of the permanent sections. With the surrounding benign peripheral nerve sheath tumor and the central malignant appearing zone, the differential diagnosis included melanotic schwannoma (benign vs. malignant) and melanoma (either a sequestered primary or metastatic). Supplemental studies with immunohistochemistry were not able to adequately distinguish between MMS and melanoma. Electron microscopy was useful in rendering the diagnosis of MMS.

The reported recurrence rate for MMS ranges from 10% to 24% 5, 6, 12. Most paraspinal lesions do not metastasize (reported rate of metastasis of 5%–26%) 5, 6, 12. Incomplete excision appears to have a negative impact on prognosis; however, radiotherapy has been shown to be beneficial even with incomplete excision. MMS involving the cranial nerves has a worse prognosis (5). Fifteen percent of patients with MMS died of their tumor (9).

Postoperatively, the patient healed well from the surgery and was neurologically intact. It was decided that close follow‐up would be the best course of action. The complete resection and the small size of the lesion also weighed against radiation therapy. Follow‐up imaging 3 months after surgery demonstrated soft tissue changes consistent with post‐surgery, but no evidence of recurrence of the paraspinal mass.

ABSTRACT

A 55‐year‐old man presented for a routine preoperative chest X‐ray in preparation for a knee replacement. Imaging revealed an extrapleural paraspinal mass in the region of T8. Fine needle aspiration of the mass was interpreted as melanoma. Neurologic examination was unremarkable. Total body skin examination failed to reveal any suspicious pigmented lesions. The mass was in close connection with the exiting T8 nerve root and somewhat adherent to the sympathetic chain. The mass was removed within an intact capsule. Immunohistochemical staining and electron microscopy of the resected specimen revealed a malignant melanotic schwannoma. The features of melanotic schwannoma, both benign and malignant variants, and their distinction from melanoma are discussed.