Proliferating Pilar Tumor of the Cheek Misdiagnosed as Squamous Cell Carcinoma.

Proliferating pilar tumours, also known as trichilemmal tumours, are rare tumours that arise from the external root sheath of hair follicles. These lesions usually have a firm-to-soft texture and form small nodules, but may grow gradually, causing pressure ulceration or hyperkeratinisation. Because of this feature, care should be taken to differentiate proliferating pilar tumours from squamous cell carcinoma. An 89-year-old woman presented with a protruding horn-shaped mass on her left malar area, which was first misdiagnosed as squamous cell carcinoma and then revealed to be a low-grade malignant proliferating pilar tumour. We report this case due to its rarity and clinically atypical characteristics. Copyright:

Introduction

Proliferating pilar tumours are rare cutaneous neoplasms derived from the outer root sheath of the hair follicle. These tumours are usually observed as intradermal or subcutaneous nodules on the scalp.[12] Most proliferating pilar tumours arise from within a pre-existing trichilemmal or pilar cyst. These common intradermal cysts occur in 5%–10% of the population, and 2% of those cases develop into a proliferating pilar tumour. The most characteristic histological finding of proliferating pilar tumours is trichilemmal keratinisation, which refers to compact amorphous keratinisation of the squamous cells that cover the cyst wall without a granular layer.[34] Because of similarities in the location and appearance of these tumours and squamous cell carcinoma (SCC), the differential diagnosis must carefully distinguish between a proliferating pilar tumour and SCC.[3] Proliferating pilar tumours have been classified into three categories according to their biological behaviour as benign, low-grade malignancies and high-grade malignancies.[5]

We report a patient who was diagnosed with a low-grade malignant proliferating pilar tumour. This case is noteworthy because of its rarity and atypical features.

Case Report

An 89-year-old woman presented to our clinic with a protruding horn-shaped mass on her left cheek. She had been diagnosed with SCC through an incisional biopsy at another dermatology clinic. The lesion had been present for several years and was gradually increasing in size. There was a history of recurrent swelling over the past 6 months. The swelling was painless and non-tender. The patient did not give a history of previous trauma or infection. The lesion constituted a single, hard, erythematous, movable mass measuring 1 cm × 1.5 cm × 1.2 cm over the left malar area, with a cutaneous horn [Figure 1]. On examination, there was no neck node lymphadenopathy. The rest of the systemic examination was normal, except that the patient had Parkinson's disease. Wide excision was performed, and the margins were found to be tumour-free.

Figure 1

A protruding horn-shaped mass. (a) Frontal view. (b) Lateral view

A histopathological evaluation showed that the tumour was composed of lobulated nests of squamous cells, and the encapsulated tumour pushed into the adjacent stroma [Figure 2a]. The nests showed proliferating squamous epithelium with trichilemmal keratinisation [Figure 2b]. The squamous cells had mild atypia, as shown by an increased nucleus-to-cytoplasm ratio, nuclear polymorphism and the presence of few atypical mitoses [Figure 2c]. Based on the histopathological findings, the lesion was diagnosed as a low-grade malignant proliferating pilar tumour. The patient had no symptoms of recurrence or metastasis after 6 months of follow-up.

Figure 2

Photomicrographs showing (a) lobulated nests of squamous cells and an encapsulated solid and cystic mass with a border pushing into the adjacent stroma (H and E, ×10); (b) proliferating squamous epithelium with trichilemmal keratinization (H and E, ×40); (c) peripheral nuclear palisading with mild atypia and parakeratosis (Hand E, ×100)

The study was approved by the local ethics committee of Hanil general hospital (approval no.: HGH-2020-OTH-002). This study adhered to the ethical principles outlined in the Declaration of Helsinki, and all patients provided consent for the procedure.

Discussion

Proliferating pilar tumours most commonly occur in women aged 50–75 years, with a mean age of 64.1 years. These tumours present in the head and neck region in 80% of cases, and 90% of those cases appear on the scalp.[34] These lesions are known to be rare tumours of the scalp and are more commonly observed in areas with excess hair growth. These tumours develop over time from the foci of epithelial cells in trichilemmal cysts. Therefore, pilar tumours arise from within pre-existing trichilemmal cysts. Their growth may be provoked by an unknown trigger, such as trauma, irritation or inflammation.[3]

These tumours usually form a single, firm-to-soft nodule that can vary in size, but they sometimes show rapid and extensive growth, which may cause ulceration of the underlying tumour. The clinical differential diagnoses for proliferating pilar tumours include epidermoid cyst, keratoacanthoma, SCC, pilomatrixoma, sweat gland tumours, dermatofibrosarcoma protuberans, cylindroma, basal cell carcinoma and angiosarcoma.[5] Because of the morphological similarity of proliferating pilar tumours and SCC, the differential diagnosis should carefully distinguish between these two conditions. Trichilemmal keratinisation is an important differentiator of proliferating pilar tumours from SCC.[34]

Proliferating pilar tumours are classified as benign, low-grade malignant and high-grade malignant tumours based on their clinicopathological characteristics.[5] Benign lesions show no infiltration of the surrounding stroma and minimal nuclear atypia. Low-grade malignant lesions show multiple lobulated and expansive masses of squamous epithelium separated by loose stroma. Squamous cells show atypia such as nuclear enlargement, an irregular nuclear membrane and hyperchromasia. High-grade malignant lesions display nuclear pleomorphism, atypical mitosis and lymphovascular invasion. Immunohistochemical indicators, such as Ki67, p53 and CD34, have been considered as important factors in the diagnosis of these lesions.[678]

Benign lesions do not pose a risk for recurrence, whereas local recurrence may be observed in cases of low-grade malignant lesions. High-grade malignant lesions have a high rate of recurrence and have been found to show a tendency for metastasis. Immunostaining is important for distinguishing benign lesions from malignant lesions. The treatment of benign and low-grade tumours is wide excision with a tumour-free margin. For high-grade tumours, surgical excision should be combined with adjuvant radiotherapy to reduce the risk of recurrence or metastasis.[9]

The clinical presentation of a hyperkeratotic horn-shaped mass on the cheek in an elderly woman might be confused with SCC. We report this case of a low-grade proliferating pilar tumour on the left malar area due to its rarity and atypical appearance. We present this case to raise awareness of this disease entity and the need to differentiate these tumours from SCCs in the head region.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.