A 63-year-old female with ulcerated scalp swelling (2.0 × 2.0 cm × 2.0 cm, not attached to the underlying bone, without regional lymphadenopathy) since 2 years with frequent bleeding on trivial trauma. Fine-needle aspirate showed features as shown in Figure 1.

Figure 1:: FNA aspirate showing squamous and basaloid cells (a, PAP ×100; b, MGG ×100) with blotchy keratinous material (c, MGG ×100). Squamous cells without atypia showed moderate cytoplasm. (d, MGG ×100).

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Q1: What is your interpretation?

Ulcerated pilar cyst

Pilomatrixoma

Trichoepithelioma

Proliferating trichilemmal tumor (PTT)

Squamous cell carcinoma d

d. Proliferating trichilemmal tumor.

Explanation: The fine-needle aspiration cytology (FNAC) smears were moderately cellular and showed anucleate and nucleated squamous cells, basaloid cells [Figure 1a], abrupt keratinization [Figure 1b], and blotchy keratinous material [Figure 1c]. Squamous cells did not show any significant cytological atypia [Figure 1d]. The basaloid cells and squamoid cells were almost equal in proportion. The basaloid cells did not show any peripheral palisading. Ghost cells, calcification, cholesterol clefts, and foreign body giant cells were absent.[1] A diagnosis of low-grade squamous neoplasm was rendered.

Wide local excision was advised for histopathological confirmation. On gross examination, the lesion was partly skin-covered; nodular, measuring 2.5 × 2.0 × 2.0 cm. The overlying skin was ulcerated. On the cut section, the tumor was solid and grayish-white. Microscopy revealed a lobulated intradermal mass of squamous epithelium [Figure 2]. Widespread trichilemmal keratinization was noted along with the absence of a granular cell layer [Figure 3]. The squamous cells did not show any atypia or mitotic figures. The diagnosis of a low-grade (benign) proliferating trichilemmal tumor was rendered. Post-surgery, 2 years follow-up did not show any recurrence.

Figure 2:: Lobules of squamoid cells with pushing borders and cystic spaces filled with keratinous material. No evidence of infiltration was seen (H&E 40×).

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Figure 3:: Trichilemmal type keratinization showing extensive glassy keratinous material without granular layer. Squamous cells do not show any atypia (H&E 100×).

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During fine-needle aspiration cytological evaluation of any skin nodule, the following differential diagnoses should be considered.

Pilar cyst: Pilar cyst is defined as a cyst containing keratin and its breakdown products. It arises preferentially in areas of high hair follicle concentrations; therefore, 90% of cases occur on the scalp. They are solitary in 30% of cases and multiple in 70% of cases. These are cystic nodular lesions with a smooth external surface. Young pilar cysts show abundant blotchy keratin with or without calcification and inflammation. Older cysts show necrotic debris with cholesterol crystals and inflammatory cells

Pilomatrixoma is a benign cutaneous adnexal tumor having differentiation toward the hair follicle matrix with a predilection for the head-and-neck region of children and young adults. However, a bimodal pattern with the first peak in the first decade and the second in the sixth decade of life, along with a female preponderance is observed.[2] Smears of pilomatrixomas are moderately cellular and comprised primarily of dense clusters of small- and medium-sized basaloid cells with overlapping nuclei, sheets of ghost cells, nucleate and anucleate squamous cells, and keratin fragments.[3] The background is dirty and rich in granular calcium and shows many multinucleated giant cells. PTT generally does not show many multinucleate giant cells. Ghost cells characteristic of pilomatrixoma is not seen in PTT

Conventional trichoepithelioma is usually seen in children and young adults as multiple, small, 2–4 mm papules. Giant solitary trichoepithelioma, however, occurs in elderly individuals and arises most commonly on the thigh and perianal region. On fine-needle aspiration cytology, it shows fronds of basaloid epithelial cells with abrupt keratinization, papillary mesenchymal body, and melanin pigmentation.[4] Although the present lesion also showed basaloid cells clusters with abrupt keratinization and melanin pigment, it did not show mesenchymal bodies

Proliferating trichilemmal tumor (PTT), also referred to as proliferating pilar tumor (PPT), is a tumor originating from the outer root sheath of a hair follicle. The histologic hallmark of PTT is the presence of trichilemmal keratinization.[5] It most commonly occurs on the scalp during the 4th–8th decades of life with a distinct predilection for women. Wilson-Jones first described PTT in 1966 as an entity that can clinically and histologically simulate squamous cell carcinoma (SCC).[6] A clinicopathological study of 76 cases divided PPT into three groups on the degree of stromal invasion and the level of cellular atypia.[7] Group 1 lesions (PPTs) behave benignly, Group 2 tumors (low-grade malignant PTT -LMPPTs) have a small risk of local recurrence, and Group 3 neoplasms (high-grade malignant PTT – HMPPTs) have the potential for regional recurrence and metastasis. Low- and high-grade malignant PTT shows additional marked nuclear anaplasia. The histological features are well recognized. However, characteristic fine-needle aspiration cytological findings of proliferating trichilemmal tumors are documented by a few authors only.[5,8] On cytology, diagnosis of PTT is sometimes difficult and misdiagnosed as adnexal neoplasm.[9]

The diagnosis of SCC was ruled out as squamous cells did not show any cellular atypia, pleomorphism, dyskeratotic cells, or tumor diathesis. Reactive epithelial atypia in inflamed pilar cysts may appear worrisome and raise the suspicion of SCC.

For cytological diagnosis of PTT, smears should show the presence of blotchy keratin, basaloid cells, squamous epithelial cells, and trichilemmal keratinization. However, these features may not be present in smears always and may lead to diagnostic dilemmas.

The presence of keratin material only may lead to misdiagnosis of epidermoid cysts or pilar cysts.

Reactive epithelial atypia in inflamed cysts can look worrisome. Smears with basaloid cells with abrupt keratinization may get erroneously diagnosed as trichoepithelioma or keratotic BCC.

It is not possible to differentiate on FNAC between benign and low-grade malignant PTT as smears in both have similar cytomorphological features. However, cytology of high-grade malignant PTT shows trichilemmal keratinization, blotchy keratin, basaloid cells, and atypical squamous epithelial cells.

The cytological diagnosis of PTT can be made in the presence of blotchy keratin, basaloid cells, squamous epithelial cells, and trichilemmal keratinization. However, these features may not be present in cytological smears and may lead to diagnostic dilemmas. Furthermore, the distinction between benign PPT (Group I) and low malignant PTT (Group II) is not possible on FNAC alone. FNA plays an important role to exclude SCC and high malignant PTT and to decide surgical management. PTT without atypia has a benign behavior, wide local excision of the lesion is recommended to prevent a recurrence.