SkIndia Quiz 9: Solitary cystic nodule with a horn on the thigh of elderly male

A 50-year-old male presented with an asymptomatic swelling on left thigh. Swelling increased slowly to the present size over a period of 4 years. Also he noticed development of a verrucous growth on swelling since last 1 year [Figure 1]. There was no any other complaint. His cutaneous and systemic examination revealed no any other abnormality. A wide surgical excision was done and specimen was sent for histopathology. Histopathologic findings are shown in Figures 2-5.

Figure 1.

Solitary skin-colored nodule over the thigh with an overlying cutaneous horn

Figure 2.

Upper part of dermal cyst showing epidermoid keratinization with granular layer (H and E, ×40)

Figure 5.

A basaloid island with calcification (H and E, ×100)

ANSWER

Hybrid follicular cyst (HFC) (infundibular and pilomatrical) with a cutaneous horn.

Hematoxylin- and eosin-stained sections showed an intradermal tumor with cystic and solid areas in the lower dermis. Cyst wall showed infundibular lining with thick granular layer in upper part [Figure 2] whereas in lower part a sharp transition in the cyst wall from infundibular to pilomatrical differentiation. Also, sharp delimitation in the cyst contents between laminated keratin in infundibular cyst and shadow cells in pilomatricoma was seen [Figure 3]. A pilomatrical differentiation consisting of basaloid matrical-type cells with luminal shadow cell keratinization and calcification was noted in lowermost part [Figures 4 and 5]. However, no connection between overlying horn and deeply located dermal cyst was found even on step sections. On clinicopathological correlation, a diagnosis of HFC showing a combination of epidermoid cyst and pilomatricoma with cutaneous horn was made.

Figure 3.

Sharp transition from epidermoid to pilomatrical keratinization in the cyst wall (H and E, ×100)

Figure 4.

lower part of cyst showing pilomatrical differentiation (short arrow) with basaloid island and area of transition in cyst wall (long arrow) (H and E, ×40)

DISCUSSION

Hybrid cysts develop from more than two components of the pilosebaceous unit. It was first described by McGavran et al. in 1966 as a cystic tumor that was a combination of infundibular and trichilemmal cysts.[1]

Brownstein et al. in 1983 reported seven cases of hybrid cyst, in which the upper part near the epidermis was infundibular cyst and the deep part was trichilemmal cyst.[2]

The concept of hybrid cyst was expanded to include lesions with a cystic structure that had more than two components of the pilosebaceous unit, including an epidermal cyst, trichilemmal cyst, pilomatricoma, eruptive vellus hair cyst, steatocystoma, and apocrine hidrocystoma.[3,4]

Different parts of the pilosebaceous unit can give rise to a variety of cutaneous cysts, such as epidermal cysts, trichilemmal cysts, steatocystomas, or pilomatricomas.[3] An epidermal cyst originates from the follicular infundibulum and contains stratified squamous epithelium with granular layer and lamellar keratinized material. A trichilemmal cyst shows keratinization similar to the isthmus of the outer root sheath of the hair follicle. A pilomatricoma is a benign tumor that is derived from hair matrix cells and composed of basophilic cells and shadow cells.[5]

The most common site for HFC is face, scalp,[1] and upper extremities.[3] Lower extremity appears a rare site for follicular hybrid cysts (FHC). According to a review of 15 cases of FHC in Japan, the most frequent histological type was the combination of an epidermal cyst with a trichilemmal cyst.[1] In a series reported by Kim et al., all cases had combination of epidermoid cyst and pilomatricoma.[3] In our case also, combination of epidermoid cyst and pilomatricoma was seen with clear areas of transition in the cyst wall.

The pathogenesis of the hybrid cyst has not been fully understood. These lesions may represent a transitional stage from an epidermal cyst to a pilomatricoma or traumatic inoculation of the epidermis by a pilomatricoma might explain progression to an epidermal cyst.[6] The granulomatous reaction that alters the sequence of keratinization might result in the formation of shadow cells. However, the transition between a pilomatricoma and an epidermal cyst, in the same cyst, might suggest a common histogenesis. Alternatively, FHC might originate from multipotential stem cells.[3,6,7]

FHC can occur in single or multiple lesions. It has been reported that pilomatricoma-like changes may be a feature of the epidermal cysts associated with Gardner's syndrome. However, in our case and also in series of Kim et al.,[3] there was no any evidence of Gardner's syndrome.

A mass of keratinized tissue protruding from skin surface with length more than the diameter is called cutaneous horn. It can arise over variety of benign conditions like seborrheic keratosis, verrucae, epidermoid or trichilemmal cysts, and malignant conditions like actinic keratosis, trichilemmal carcinoma, etc. FHC associated with overlying horn is rare. A case of superficial pilomatricoma arising within epidermis and presenting with a horn (pilomatricomal horn) is reported.[8] In our case, no connection of overlying horn with cyst was found.

Hybrid cyst is not so common, however, is an interesting pathological phenomenon. Studying this disease is useful in understanding the pathogenesis of pilosebaceous tumors.[1,3]