Proliferating Trichilemmal Cysts: A Clinicopathological Study of 14 Cases.

Sir,

Proliferating trichilemmal (pilar) cysts (PTCs) are a rare dermal neoplasm, usually benign, but with locally invasive and even metastatic potential. First described by Wilson-Jones in 1966, it consists of a scalp lesion with similar pathologic features to squamous cell carcinoma (SCC) in an elderly patient.[12] Subsequently, these tumors were given multiple denominations, such as invasive pilomatrixoma, trichoclamidocarcinoma, subepidermal acanthoma, and giant hair matrix tumor, among others.[2] This makes it difficult to determine its true incidence. They may arise de novo or in a preexisting cystic lesion, and based on few case reports, it is three to four times more common in women, usually seen in elderly patients, and located in up to 85% of cases on the scalp.[123]

The objective of this study was to evaluate the clinical and histopathological characteristics of patients with PTC in a group of latin patients. We performed a retrospective, observational study in the Dermatology and Pathology departments of a Chilean university hospital. Using the pathology department database on all skin biopsies with a pathological diagnosis of PTC between 1995 and 2018, the biopsy slides were reviewed again by a dermatopathologist to confirm the diagnoses. Sex, age, clinical diagnosis, location, and histopathological diagnosis were analyzed.

A total of 14 patients were included [Table 1]; 9 (64.3%) were female, with an average age of 56 years (±20.9, range: 16–85 years). The clinical diagnoses were nonspecific tumor = 6 (43%), epidermal cyst = 3 (21.4%), trichilemmal cyst = 3 (21.4%), pilomatrixoma = 1 (7.1%), and atypical nevus = 1 (7.1%). The tumors were located on the scalp in 11 cases (78.7%), hand in 1 case (7.1%), elbow in 1 case (7.1%), and thigh in 1 case (7.1%) [Figure 1]. Eleven (78.7%) lesions corresponded to PTC [Figure 2a], 2 (14.2%) to hybrid proliferating cysts with epidermoid and trichilemmal components, the latter being predominant, and 1 (7.1%) to a PTC with moderately differentiated intracystic SCC [Figure 2b and c].

Table 1

Clinical and histopathological characteristics of patients presenting with proliferating trichilemmal cysts

Number	Age	Sex	Clinical diagnosis	Location	Histopathological diagnosis
1	85	Female	Nonspecific tumor	Scalp	PTC
2	47	Female	Atypical nevus	Scalp	PTC
3	45	Male	Epidermal cyst	Scalp	PTC
4	47	Female	Pilomatrixoma	Thigh	PTC
5	86	Female	Nonspecific tumor	Scalp	PTC
6	45	Male	Nonspecific tumor	Scalp	PTC
7	64	Female	Nonspecific tumor	Hand	PTC
8	21	Female	Epidermal cyst	Scalp	PTC
9	76	Female	Nonspecific tumor	Scalp	PTC with moderately differentiated intracystic SCC
10	67	Male	Nonspecific tumor	Elbow	PTC
11	56	Male	Epidermal cyst	Scalp	PTC
12	16	Female	Trichilemmal cyst	Scalp	PTC
13	64	Male	Trichilemmal cyst	Scalp	Hybrid proliferating cyst with epidermoid and trichilemmal components
14	65	Female	Trichilemmal cyst	Scalp	Hybrid proliferating cyst with epidermoid and trichilemmal components

PTC – Proliferating trichilemmal cyst; SCC – Squamous cell carcinoma

Figure 1

(a) Proliferating trichilemmal cyst on the scalp (arrow) with multiple trichilemmal cysts. (b) Proliferating trichilemmal cyst in elbow

Figure 2

(a) Cyst with hyperkeratosis and squamous proliferation (H and E, ×20). (b) Proliferating trichilemmal cyst with squamous cell carcinoma (H and E, ×40). (c) Malignant squamous proliferation with marked cellular pleomorphism, hyperchromasia, and mitotic figures (H and E, ×400)

The present study is the first report on this rare entity in Latin patients and showed a younger age of presentation compared to previously reported series, with a female predominance and a higher incidence on the scalp, which correlates with features described on the available medical literature.[13] It also highlights the presence of two proliferating cysts of the trichilemmal–epidermal hybrid type, which, according to the available evidence, there are no reported cases of proliferative epithelial cysts with both components.[4] Consequently, when evaluating these lesions, it is necessary to distinguish if there are areas with a granular layer, which defines the epidermoid component and therefore the diagnosis of hybrid proliferative cysts. Furthermore, we report the case of an intracystic SCC, since the malignant transformation arising within a cutaneous cyst is a very rare but described phenomenon.[35]

Further studies with larger sample size, possibly multicenter, and more extensive clinical evaluation will help to better understand the clinical behavior of this pathology, in specific populations.

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Conflicts of interest

There are no conflicts of interest.