Penile neurilemmoma: Utility of fine-needle aspiration cytology in diagnosis of a rare entity.

Subcutaneous lesions in the penis are of rare occurrence and encompass benign as well as malignant tumors. These include lipomas, leiomyomas, neurilemmomas and their malignant counterparts. A surgical excision at this site carries the risk of postoperative penile curvature and erectile dysfunction. We report a rare case of penile neurilemmoma which presented as a subcutaneous nodule on the dorsal surface of the penis. A fine-needle aspiration was performed which aided in the preoperative diagnosis and guided the extent of excision. We report this case to highlight the importance of needle aspiration as a simple outdoor procedure for penile lesions which can aid surgical approach and postoperative outcome.

Introduction

Neurilemmomas synonymously called schwannomas are benign, encapsulated neurogenic tumors that originate from the Schwann cells of the peripheral nerve sheath.[1] These lesions are eccentrically attached to the nerves and are commonly encountered in the head, neck, extremities, mediastinum and the retroperitonium. Despite the extensive innervations of the external genitalia, neurilemmomas are extremely rare in the penis. To our knowledge less than 30 cases of penile neurilemmomas have been reported and published in English literature.[12] We report a rare case of penile neurilemmoma diagnosed on fine-needle aspiration cytology (FNAC) and confirmed on excision biopsy.

Case Report

A 40-year-old male presented with a firm, small, nontender swelling on the dorsal surface of the penile shaft of 6 months duration. The swelling was insidious in onset with no prior history of trauma or sexually transmitted diseases. On examination, the swelling was 1.5 cm × 1 cm in size and present in the subcutaneous plane. The patient was referred to the laboratory for FNAC. The aspirate smears revealed a moderately cellular tumor composed of cohesive fragments of spindle-shaped cells with a moderate amount of eosinophilic cytoplasm. Focal palisading of the nuclei was noted. The nuclei were plump to spindle in shape with pointed ends and fine chromatin. Wavy eel-shaped nuclei were also seen [Figure 1]. The background was fibrillary. A cytodiagnosis of benign nerve sheath tumor was rendered.

Figure 1

Cellular aspirate smears from the lesion show spindled nuclei with focal palisading (arrows) (Pap, ×200). Inset shows wavy nuclei (arrowheads) (Pap, ×400)

An excision biopsy was performed. Macroscopically the specimen measured 1.5 cm × 1.5 cm, and the histomorphology revealed a benign neural tumor with a biphasic pattern. The lesion revealed nodules and cords of cellular tissue separated by poorly cellular zones (Antoni A and Antoni B areas). The cellular areas revealed palisading rows of plump fusiform nuclei in an eosinophilic background. Occasional Verocay bodies were also identified. There was no evidence of necrosis, increased mitotic activity or nuclear atypia. On immunohistochemical analysis, the tumor cells were strongly positive for S-100 [Figure 2] and negative for smooth muscle actin. The morphology and immunohistochemical findings were consistent with benign neurilemmoma. No clinical features or family history suggestive of neurofibromatosis could be elicited in the patient. He had an uneventful recovery postsurgery.

Figure 2

Tissue section showing nuclear palisades and Verocay bodies (H and E, ×100). Inset shows immunohistochemistry for S-100 protein showing diff use nuclear positivity (DAB, ×400)

Discussion

Dehner and Smith reported the first three cases of penile neurilemmomas in 1970.[23] Penile neurilemmomas are generally single and sporadic. Multifocality is commonly encountered in association with neurofibromatosis type 2 and schwannomatosis, with both having their basis in mutations in NF-2 gene (on chromosome 22q12). Malignant transformation is extremely rare and is seen in association with von Recklinghausen disease.[123]

Penile neurilemmomas are generally asymptomatic. Secondary symptoms like pain, dysuria and erectile dysfunction are encountered in large sized lesions with compression of adjacent structures.

The clinical and radiographic findings of male genital neurilemmomas are nonspecific, and the lesion is often misdiagnosed. The important differential diagnoses of penile neurilemmoma are Peyronie's disease, fibrosis associated with injections, benign soft tissue lesions like the lipoma, leiomyoma, fibroma and rarely malignant sarcomas. Radiological imaging techniques are useful in locating the plane of the lesion and in delineating the mass. Peyronie's disease and fibrosis due to injections is never encountered at the dorsum of the penis in the Buck's fascia which is the common location of neurilemmomas. Peyronie's disease is generally associated with erectile dysfunction and pain which is extremely rare in cases of penile neurilemmomas. Lipoma has a much softer consistency than neurilemmomas, however; histopathology is the gold standard to distinguish neurilemmomas from other benign soft tissue lesions like leiomyoma and fibroma.

Aspirate smears help in defining the presence of cellular tumor with benign morphology, and careful assessment of the presence of nuclear palisading and wavy nuclei can help differentiate from leiomyomas which show cigar shaped nuclei and do not show palisading. Penile leiomyoma is rare and uncommon. Most penile leiomyomas are benign, and surgery is implicated only when there is a rapid increase in size or presence of secondary symptoms due to compression of adjacent structures. Malignant sarcomas are characterized by rapid growth and symptoms associated with infiltration of adjacent surrounding structures.[2345] Rare malignant peripheral nerve sheath tumors have been reported in the penis and are known to recur and behave aggressively.[6] These require wide excisions unlike benign neurilemmomas occurring at the same site.

Complete surgical excision is the recommended modality of treatment. Partial penectomy is rarely required and performed in lesions with secondary symptoms. A preoperative cytology report of benign nerve sheath tumor may help in surgical decision making regarding extent of excision since preservation of tunica albuginea prevents penile curvature and erectile dysfunction. Due to the low rate of recurrence a regular postoperative follow-up is recommended in penile neurilemmomas.[45] The case is presented for its rarity and in emphasizing the utility of cytologic diagnosis in such lesions.