Background:A proliferating trichilemmal cyst (PTC) is an uncommon, rapidly-reproducing cutaneous epithelial cyst, differentiating from the isthmic portion of the outer hair root sheath. It is usually described as a benign tumor, but malignant transformation has been reported and is then denominated as a malignant proliferating trichilemmal tumor. Ki67 immunostaining has been used as a methodology for the evaluation of tumor grade in other tumors, due to its distinctive reaction patterns which exclusively involve proliferating cells.Objectives:(1) To report the incidence of cases of PTCs in a General Hospital during a 12 years period. (2) To determine the expression of ki67 using immunohistochemical staining. (3) To correlate ki67 reaction patterns with clinical prognosis.Materials and Methods:The dermatology department's files during a period of 12 years were reviewed; cases with a diagnosis of PTC were selected, and ki67 immunostaining was done when enough biological material was available.Results:A total of 15 cases with a diagnosis of PTC were identified. In 12 cases, ki67 immunostaining was carried out. In 9 of the 12 cases, ki67 was observed in the basal cells of the cystic epithelium, one case was moderately positive in palisading epithelial cells; in the other two cases ki67 immunostaining was negative. Clinical follow-up was done and revealed that no patient had local recurrence in 5 years after surgical removal of PTC. We therefore consider this immunostaining technique is probably correlated with low recurrence potential.

A 19‐year‐old female presented at our hospital with a 1‐year history of a 5‐cm indurated nontender lesion on the left shoulder. Two months later, a similar lesion appeared in the left mammary region, accompanied by necrosis and fever. At admission to our department, she presented disseminated lesions in the left submandibular region, the sternal area and the posterior part of the upper and lower extremities. The lesions were indurated erythematous‐violaceous plaques affecting the adipose tissue, some with ulcerations and necrotic crusting (Fig. 1). The first histopathological report was thrombosis of the capillary vessels and focal vasculitis (Fig. 2). 1 Ulceration and necrotic crusting lesion in the left mammary region 2 Histopathology showing dilation of thrombosed vessels with fibrin deposits (hematoxylin and eosin, magnification ×244) A biopsy from a lesion on the left thigh showed dilated vessels with large atypical intravascular cells with hyperchromatic nuclei and numerous atypical mytoses in the dermis and subcutaneous tissue. Some of these cells were immersed in fibrin thrombi or in the perivascular connective tissue (Fig. 3). An immunohistochemical study was positive for the common leukocytic antigen and for the B‐cell phenotype. The diagnosis was large B‐cell lymphoma. The patient persisted with a high fever, dry cough, pericardial effusion complicated by an abscess and left pleural effusion. She died before systemic workings for lymphoma could be carried out, and before chemotherapy could be attempted. 3 Histopathology showing atypical intravascular lymphocytes (hematoxylin and eosin, magnification ×366)