A Hybrid Skin Cyst at an Unusual Location: A Case Report

Abstract

Hybrid cysts originate from more than two components of the pilosebaceous unit. The pathogenesis of this cystic lesion remains unclear. Most of the investigated cases have scalp and face predilection. The lesion is predominantly observed in females. We present a case of a 69-year-old woman with a painless cutaneous nodule measuring 0.7 cm on the right calf. After a surgical excision, the histological investigation showed that the normal epidermis was elevated by a deeply encapsulated intradermal cyst, which was lined by two distinct histological types of epithelium with an abrupt transition between them. One-half of the cystic surface consisted of a keratinizing squamous layer with a granular layer and keratin lamellae, while the other half was lined by epithelium showing pilomatricial differentiation. Finally, an extremely rare diagnosis was set of a hybrid cyst.

Keywords: benign cutaneous cyst; epidermal cyst; hybrid cyst; pilomatricoma; trichilemmal cyst.

Figure 1. The cyst is composed of two distinct components: the epidermal component on the right side (red) and the pilomatricial component on the left side (blue) of the picture. Hematoxylin-eosin-saffron, 50x.

Figure 2. Epidermal component of the cyst. The lining contains a granular layer (red) and lamellated keratin in the lumen (blue). Hematoxylin-eosin-saffron, 100x.

Figure 3. Pilomatricial component of the hybrid cyst. (A) Hematoxylin-eosin-saffron, 50x. A nest of small basaloid cells (red) and eosinophilic ghost cells (blue) with a relatively gradual transition are observed. (B) Hematoxylin-eosin-saffron, 400x. The transition between the two types of cells is seen.

Figure 4. Granulomatous inflammation with multinucleated giant cells in contact with ghost cells and calcifications (arrow). Hematoxylin-eosin-saffron, 100x.