Cutaneous schwannoma masquerading as trichilemmal cyst over scalp in a young male.

Schwannomas are benign tumors derived from Schwann cells. They are comparatively rare on scalp and easily mistaken for tumors of hair. Trichilemmal cysts are common over scalp and present as dermal or subcutaneous growths over scalp. We present a solitary schwannoma of scalp clinically mimicking trichilemmal cyst. The tumor was surgically excised and the same was confirmed by histopathology.

What was known?

Cutaneous schwannomas are benign tumors of schwann cells, commonly seen along the major peripheral nerves.

Introduction

Schwannomas, also known as neurilemmomas and anaxonal, intraneural Schwann cell tumors are benign neoplasm of nerve sheath, originating from Schwann cells. The most common schwannoma is acoustic neuroma. Schwannomas arise from the nerve sheath of large peripheral or cranial nerves and occur at the level of subcutaneous tissue or deeper layers.[1] Most schwannomas present as intracranial, intraspinal, or deep tissue lesions.[2] Schwannoma enlarges slowly and follows a benign course, and malignant transformation is rare.[34] Cystic growths over scalp can be easily mistaken for common tumors like dermoid cyst and trichilemmal cyst. We present a case of asymptomatic solitary cystic growth over scalp and excision biopsy proved schwannoma.

Case Report

A 28-year-old male presented with asymptomatic cystic growth over vertex area of scalp of 6 years duration. The lesion started as small pea- sized cystic growth and gradually increased to the present size. There was no history of trauma, previous surgery, and similar growth over scalp. The growth was painless and there was no history of any surface changes over the cystic growth. Family history of similar complaints was negative. Examination revealed a 3 × 4 cm sized, mobile, soft to firm cystic growth over right vertex region with normal hairs over the surface [Figure 1]. Palpation revealed no tenderness and the cyst was not attached to the underlying fascia or bone. The growth was excised under local anesthesia. During excision, when skin was incised, a bilobed tumor with capsule was found and the tumor was sent for histopathology examination [Figure 2]. Histopathology showed, capsulated tumor with mixed Antoni A and Antoni B patterns in dermis. The Antoni A component is cellular, and Verocay bodies are a prominent feature. The loosely cellular, pale zones are Antoni B patterns [Figures 3 and 4].

Figure 1

Schwannoma: Soft to firm, round, mobile cystic growth seen over scalp

Figure 2

Schwannoma: A bilobed, capsulated tumor was seen during excision

Figure 3

(H and E, ×10) Mixed pattern was seen, Antoni A and Antoni B type patterns in dermis. The Antoni A component is cellular (black arrow), and Verocay bodies are a prominent feature. The loosely cellular, pale zones (red arrow) are Antoni B component

Figure 4

(H and E, ×40) Higher magnification showing Verocay bodies (green arrow)

Discussion

Schwannomas are benign, slowly growing, encapsulated tumors of Schwann cells. Schwannomas are not always anaxonal; there can be mixture of axonal and anaxonal elements.[5] They are common in flexor aspect of extremities and rarely seen over scalp.[1] Cutaneous schwannomas by nature are asymptomatic in presentation, rarely they may present with pain, tenderness and paresthesia localized to the tumor or radiating along nerve of origin. Males and females are equally affected and the peak incidence is 4th and 5th decades of life.[6] Clinically, schwannomas may present as solitary (most common), plexiform, cellular, cystic types[7] and ancient forms.

Histopathology of schwannoma is typical, encapsulated by perineurium, and characterized by two types of histological patterns: Antoni type A and Antoni type B.[8] Antoni type A is highly ordered cellular pattern in which spindle cells are arranged in compact fascicles and their nuclei are arranged in palisades. Verocay bodies are a common feature in Antoni type A pattern characterized by acellular processes that separate adjacent rows of palisaded nuclei. Antoni type B tissue is less cellular with pale zones of gelatinous matrix and admixed with cystic, edematous or myxoid degeneration. The mixed patterns can also be seen. Many rare forms of schwannomas are described including melanotic, Pacinian, neuroblastoma like, epithelioid and glandular types.

Immunohistochemistry of schwannoma reveals positive S100 and collagen type 4, and capsule is positive for epithelial membrane antigen.[910] Other immunohistochemical stains useful in the diagnosis of schwannomas are Leu-7; myelin basic protein; CD 34; CD 68; collagen types 1, 3, and 4; and vimentin.

Schwannoma over scalp is rare. When our patient presented to us with asymptomatic growth over s calp of 8 years duration, we had a differential diagnosis of trichilemmal cyst or dermoid cyst because of their common presentation over scalp. Trichilemmal cyst, also called as pilar cyst, presents as smooth, mobile, firm dermal nodule on the scalp, commonly seen in middle age. It is derived from the outer root sheath of the isthmus of the hair follicle. On histopathology, the cyst is lined by epidermal cells and contains homogenous keratinous material.[11] Dermoid cysts are ectodermal growths developing from epithelium, trapped along lines of embryonic fusion, commonly on scalp, forehead, or periorbital areas, characterized by asymptomatic, soft, or rubbery, round, subcutaneous cyst.[1213]

Among the other types of schwannomas, plexiform schwannomas constitute 5% of the presentation.[14] Plexiform schwannoma was first described by Masson, which he called “plexiform schwannogliosis”.[15] Few reports of malignant transformation in schwannomas have been reported in literature.[1617]

In conclusion, cutaneous schwannomas are rarely seen and should be considered in the differentials of asymptomatic, mobile, soft to firm, round cystic swelling over scalp.

What is new?

In a case of subcutaneous swelling over scalp, cutaneous schwannomas should be considered as one of the differentials.