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Foreign body reaction mimicking perineal nodular induration

To the Editor, We highlight a case received in consultation of a 56-year-old woman who presented for evaluation of a posterior thigh lesion. Limited patient history was available. Reported physical exam findings included a 1-cm semifirm, solitary nodule with overlying bruising. The well-circumscribed nodule was immobile and painful to palpation. The clinical impression was a dermatofibroma and a 6-mm punch biopsy from the central portion of the lesion was subsequently performed. Histopathologic examination of the biopsy revealed an epidermis with hyperkeratosis and mild acanthosis. In the reticular dermis, there was a well-circumscribed collection of eosinophilic, amorphous material surrounded by dense fibrosis (Fig. 1A). Within the dense fibrous tissue, there was a mixed population of histiocytes and fibroblasts in a background of fibrinous material, collagen bundles, capillaries and degenerated elastic fibers (Figs. 1B,C). A Masson’s trichrome stain and a Mallory’s phosphotungstic acid hematoxylin stain classified the eosinophilic debris as degenerated collagen and fibrin, respectively (Fig. 1D). While the dermal process was suggestive of cyst formation, no epithelial or synovial lining was identified, thus indicating a pseudocyst. These histopathologic findings were consistent with perineal nodular induration, more commonly known as a biker’s nodule. Subsequent clinical follow-up revealed that the patient removed an embedded non-surgical sewing needle from the lesion 2 weeks after the biopsy. The patient, an avid seamstress, was reportedly uncertain of when the needle became embedded in the soft tissue of her thigh. It is postulated that this foreign body resulted in the pseudocystic degenerative changes that morphologically resembled perineal nodular induration. Foreign body reactions are a well-known mimicker of pseudotumors and malignancies, and, as our case shows, may closely resemble perineal nodular induration. Perineal nodular induration represents a soft tissue pseudotumor located in the vicinity of the ischial tuberosities. The entity clinically presents as one or two 2–3 cm perineal nodules that are painful and fixed to underlying soft tissues.1 Perineal nodular induration has been reported predominantly in males, and its location posterior to the scrotum has led some to describe the lesion as a ‘third’ or ‘accessory’ testicle.2,3 The nodule has most frequently been described in cyclists with rare reports of perineal nodular induration in equestrians and lawn mower testers.4,5 The process is believed to result from repetitive microtrauma of soft tissues overlying boney prominences. Examples of this trauma include compression, vibration and friction forces secondary to bicycle and horse saddles. Such traumatic forces lead to collagen degeneration and subsequent pseudocyst formation. The microscopic findings in perineal nodular induration are characterized by a dermal collection of fibrin and degenerated collagen surrounded by dense fibrosis with intermixed fibroblasts, histiocytes, thick collagen bundles and clusters of capillaries. Even though these findings resemble a cyst, the lesion lacks a true lining and is best classified as a pseudocyst. Perineal nodular induration is considered as a pseudotumor and may resemble a number of soft tissue lesions via conventional microscopy. The presence of spindled cells with collagen trapping in a background of histiocytes causes one to consider fibrous histiocytoma, proliferative fasciitis or ischemic fasciitis. Additionally, clusters of capillaries allow one to entertain the possibility of a vascular neoplasm, such as an atypical vascular lesion, Kaposi sarcoma or well-differentiated angiosarcoma. The cyst-like nature of perineal nodular induration brings up the differential of other cutaneous cystic lesions including metaplastic synovial cyst and a ruptured epidermoid