Pilomatrixoma (calcifying epithelioma of Malherbe) was first described by Malherbe and Chenatais in 1880 as a sebaceous gland origin, but in 1961, Forbis and Helwig proved that it originates from the hair matrix and was considered as a hamartoma. 1234567891011 Pilomatrixoma is a benign adnexal tumor of hair matrix cells. 1234567891011121314151617 The exact prevalence is unknown, but it accounts for nearly less than 1% of all benign skin tumors. 13 The most common sites are the head, neck, and upper extremities. It is less common in the trunk and lower extremities and very rarely reported in the breast. It can occur at any age but is more common in children and young adults, and again after the age of 60, with a higher incidence in women. 123456789101112131415 The etiology of pilomatrixoma is unclear, but it may be related to the inflammatory process as a result of repeated traumas. 13810 To the best of our knowledge, there are only a few reported cases in the literature for breast pilomatrixoma. 1234567891011121314151617 Herein, we report the case of a 47-year-old woman presenting with a right breast mass diagnosed as breast pilomatrixoma, followed by a brief discussion of its histopathological and imaging characteristic.

A 47-year-old woman was referred to the surgery ward with a complaint of a palpable mass in the right breast from 1 year ago. On physical examination, there was a painless and firm lump in the right breast. No skin abnormalities were observed. Mammography revealed benign calcification in the right breast (BIRADs=0) and ultrasound studies showed a lobulated hypoechoic area measuring 37x8 mm at 12 o'clock and 14.7x5 mm at 10 o'clock in the left breast and 10.9x6.8 mm at 10 o'clock in the right breast, which looked like several lumps in the vicinity of each other, primarily indicative of benign breast lesions (BIRADs3). There was a well-defined hypoechoic area measuring 21x15 mm in the right axillary area (Figure 1).

The patient underwent surgery with informed consent and the sample was sent for a frozen section. The received specimen consisted of yellow-colored tissue measuring 8x5x2cm with the overlying skin tissue that measured 6x3cm. On the section, there was a well-circumscribed gray-colored lesion measuring 2.5cm in diameter with bony consistency. Microscopic examination revealed a tumoral lesion composed of nests of dark round cells, ghost cells admixed with multinucleated giant cells, and bone trabeculae (Figures 2a, 2b, 2c,2d). The permanent section showed a sclerotic nodule. In terms of histology, the tumor was completely compatible with pilomatrixoma and only immunohistochemistry (IHC) was used to confirm the tumor and to rule out breast metaplastic carcinoma as well as round cell neoplasms. The basaloid cells were positive for AE1/AE3, Beta-catenin, and BCL2 and negative for CD45, Mart1 and CD117 (Figure 3a, 3b). The surgeon also dissected the right axillary lymph nodes suspicious of malignancy, which were negative for malignancy and showed reactive lymphoid follicular hyperplasia. Based on the H&E and IHC studies, pilomatrixoma of the breast tissue was confirmed.

Pilomatrixoma of the breast is very rare, originating from the hair matrix cells, and comprises 1% of all benign skin tumors. 1234567891011121314151617 It could be misdiagnosed as a malignant lesion clinically, so radiological imaging and histopathological findings should be evaluated together. 1 They are usually asymptomatic, have a firm consistency, and are revealed as solitary, slow-growing, non-tender lesions measuring 3mm to 30mm in diameter. Sasaki et al. reported a parotid pilomatrixoma with dimensions of 8x5cm. 18 The overlying skin can be normal or inflamed. 1234568 A pathogonomic sign of pilomatrixoma is the stretching of the skin over the tumor. It is called a "tent sign" and another characteristic feature is the blue-red discoloration of the overlying skin. 13 Tokur et al. reported a breast pilomatrixoma in a 42-year-old male that was palpable firm and painless mass in the retroareolar area with no skin abnormalities and axillary lymphadenopathy. 1 Our case was a 47-year-old woman, who was presented with a palpable, painless, and firm mass without skin abnormality and with axillary lymphadenopathy.

Clinical types of pilomatrixoma include ulcerative lesions, multiple eruptive and familial cases, superficial pilomatrixoma presenting as a cutaneous horn or bullous lesion, and giant pilomatrixoma. Multiple lesions account for 2-10% of cases and are associated with Gardner syndrome, Turner syndrome, Myotonic dystrophy, and sarcoidosis. 2315 Lesions larger than 5 cm are named "giant" pilomatrixoma and can cause pain and tenderness. The tumor may be misdiagnosed clinically with hemangiomas or neurofibromas. 2 Imaging findings of the breast pilomatrixoma are similar to malignant lesions, so the diagnosis is confirmed by histopathology. On mammography, they are presented as amorphous and well-defined lesions with irregular coarse calcifications that can increase in size over time. They are usually diagnosed as BIRADs III to IV. In sonography, they are well-defined, hyperecho or isoecho lesions with a hypoechoic rim and posterior acoustic shadowing. 125 There are many differential diagnoses for breast pilomatrixoma and it is difficult to distinguish them from each other, which include seborrheic keratosis, inclusion cyst, epidermoid cyst, calcified giant hemangioma, foreign body reaction, sarcoma, squamous and basal cell carcinoma, calcified fibroadenoma, fibrocystic changes, fat necrosis, ductal hyperplasia, adenosis, apocrine metaplasia, papillomas, lobular neoplasia, metaplastic carcinoma and invasive ductal carcinoma. 1237810121315 The basaloid cells of pilomatrixoma have a positive reaction for pancytokeratin, Beta catenin, BCL2, CDX2, ER, PR and a negative reaction for Mart1, and SOX10 in immunohistochemical studies. Complications of pilomatrixoma are rare. However, sometimes they can get too big, and very rarely become malignant. Malignant transformation of pilomatrixoma should be distinguished from proliferating pilomatrixoma which is a benign tumor with lobular proliferation of 4 Pilomatrixoma of the breast was reported mainly in males and it is very rare in females. 1 Spontaneous regression of the breast pilomatrixoma is rare and surgical excision is the treatment of choice, and in cases where mammographic or ultrasound findings are benign, conservative treatment is suggested. 131415 In our case, surgical excision was performed.

Recurrence after surgical treatment is rare, with a rate of 2-6%, and is usually the result of incomplete excision. Recurrence should make the surgeon suspicious of malignant pilomatrixoma. 2915 CONCLUSION Pilomatrixoma of the breast is a very rare breast lesion and can be misdiagnosed as a breast neoplasm. It should be considered in the differential diagnosis of the breast masses. Breast pilomatrixoma cannot be reliably diagnosed by mammographic and sonographic features alone and clinical examination, radiological features, and histopathological findings should be evaluated together to avoid more invasive surgeries and unnecessary removal of axillary lymph nodes.