Epidermal Inclusion Cyst Embedded in a Recurrent Benign Fibrous Histiocytoma

Abstract

Epidermal infundibular cysts have been known to arise from implantation of epidermis in the dermis or subcutaneous tissues. The present case illustrates the theory of implantation in an epidermal inclusion cyst in a previously operated case of recurrent benign fibrous histiocytoma.

What was known?

Epidermal cysts may arise by true inclusion of surface epidermis into the dermis.

Introduction

Epidermal inclusion cysts are the commonest type of epithelial cysts of the skin. These cysts are lined by stratified squamous epithelium that is identical to that seen in the normal interfollicular or infundibular epidermis. These cyst may arise from implantation of epidermis in the dermis or subcutaneous tissues.[1,2] Implantation may occur during development as in the extremely rare midline cysts of the scalp, that are present at birth.[2,3] However, the implantation can occur due to trauma and as a result of surgery.[1] I present a case of recurrence of benign fibrous histiocytoma following incomplete excision with implantation of an epidermal inclusion cyst. The case is a rare incidence of the two unrelated conditions occurring together in one lesion and also illustrates the implantation theory of epidermal inclusion cyst.

Case Report

A 36-year-old man presented with recurrence of a soft tissue nodule on the leg 2 years after surgery for a similar nodule, which was reported as a benign fibrous histiocytoma on histopathological examination, though it was not available for review. The histopathology revealed a keratinous cyst lined by stratified squamous lining with few keratohyaline granules in the superficial luminal layers and mimicking the interfollicular portion of epidermis [Figure 1]. The lumen showed flakes of anucleated keratin. The cyst was surrounded by cellular nodule, comprising of crisscross and vaguely storiform cells with ill-defined cytoplasm and oval to spindle nuclei. The cells showed engulfed hemosiderin and scattered inflammatory cells with occasional multinucleated giant cell [Figure 2]. Areas of hemorrhage and foreign body giant cells with cholesterol and foreign body (possibly sutures from previous surgery) clefts were also seen. The cyst did not show an epidermal connection on multiple serial sections.

Figure 1.

Epidermal inclusion cyst with surrounding cellular nodule (H and E, ×40)

Figure 2.

Interspersed between the spindled fibroblast are histiocytes and giant cells containing hemosiderin (H and E, ×100)

Discussion

Epidermal cysts are solitary, slowly growing cysts with predilection for the trunk and face. These cysts are lined by squamous epithelium, which resembles the infundibulum of the hair follicle and by this virtue is also called an infundibular cyst.[2] Infundibulum of the hair follicle is lined by epithelium, which is virtually indistinguishable from the surface interfollicular epidermis.[4]

Epidermal cysts are associated with Gardner's syndrome, have been reported with mycosis fungoides, severe solar elastosis (Favre-Racouchot syndrome), focal basal epidermal proliferation, and seborrheic keratosis-like changes. Also, some cases have shown to be HPV-related. Cases of unequivocal squamous cell carcinoma has been reported in epidermal cyst.[2] Although the cyst has many histological variations and has been seen in contiguity of an adjacent malignant melanoma, the cyst has never been reported to occur within a benign fibrous histiocytoma previously.[2,5]

These cysts may form as a result of an inclusion of the surface epidermis in the dermis congenitally (i.e., a dermoid cyst) or as a result of trauma (i.e., an implantation dermoid) or surgery as in the present case.[2] In 1884, Schweninger was the first to produce experimentally epidermal inclusion cysts by burying bit of skin in the subcutaneous tissues of dogs.[1] Trauma causing such inclusions could occur from both blunt and sharp objects. The occurrence on sole and foot has been reported as a confirmation of the theory of impingement and implantation of epidermis by minimal and blunt trauma.[1,6] Minimal access surgeries are also known to have greater risk of causing epidermal inclusion cysts by virtue of the technique, which uses small incisions and rapidly rotating instruments.[7] So much so that, epidermal inclusion cyst is a known complication of abdominoplasty.[8] However, the occurrence of such an epidermal inclusions has not been previously documented in a operated case of fibrous histiocytoma.

Fibrous histiocytoma is known to engulf adnexal structures in the dermis including hair follicles and also are known to induce follicular germ cell proliferations on the overlying epidermis.[9] Such engulfed hair even if dilated would retain its ostia, isthmus, sebaceous follicle, and hair bulb, which were not seen in the present case. There was no epidermal connection seen. Moreover, a dilated infundibulum, which is obstructed like a closed comedo or a comedonal cyst, is also a small epidermal cyst.

The presented case is not only a rare development of epidermal inclusion cyst within a benign fibrous histiocytoma but also exemplifies the implantation theory of such inclusion cysts following surgery.

What is new?

The presented case is a rare development of epidermal inclusion cyst within a benign fibrous histiocytoma and also exemplifies the implantation theory of such inclusion cysts following surgery.