Nipple adenomas are benign tumors. These tumors most commonly arise in patients in their 40 s and 50 s, but there are reports of nipple adenomas occurring in children in their 10 s and in elderly patients in their 70 s [1]. In Japan, Sakumoto et al. reported 34 cases in 1996 [2], Sugamata et al. reported 31 cases in 2002 [3], and Uchida et al. reported 60 cases in 2010 [4]. The most common reason for detection was a mass or induration (66%), followed by erosions or ulcers (48%) and nipple discharge (35%) [5,6,7]. In reports from Western countries, nipple discharge is common, but in Japan, erosions tend to occur more frequently, sometimes making differential diagnosis from Paget’s disease difficult [2, 17]. The size of nipple adenomas reported in Japan ranged from 0.5 to 24 mm (median, 10 mm). Carter et al. reported nipple adenomas ranging in size from 5 to 35 mm [8], but other than the present case, there are no reports of nipple adenomas larger than 35 mm.

In patients with nipple adenoma, the nipple discharge is often bloody, contributing to the suspicion of malignancy. Many cases of nipple erosions are difficult to visualize and evaluate by ultrasonography and mammography, and cases with smooth margins and a uniform internal mass image [9]. It is important to differentiate erosive lesions from Paget’s disease; thus, it is important to deny the presence of Paget’s cells by performing imprint cytology [10] or skin biopsy [11]. Clinical differentiation is difficult because the symptoms are extremely similar to those of Paget’s disease.

Although this tumor exhibits a variety of histological features, it is possible to differentiate it from Paget’s disease by considering the bilayer structure of the epithelium and myoepithelium, the presence of apocrine-forming cells, and other factors, including the localization of the tumor. If necessary, immunohistochemistry can further improve the accuracy of diagnosis [12]. Frequently used myoepithelial markers are p63, h-caldesmon, calponin 1, α-smooth muscle actin, CK5/6 and CD10. The positivity of at least two markers is sufficient for diagnosis [13, 14].

Although 3.6% of papillary adenomas in the nipple are associated with carcinoma, they are often ectopic [15]. Therefore, progression from this disease is not clear, and there are no reports of metastasis or recurrence after resection [16]. In this case, there was no malignant component despite of large tumor size.

The first choice of treatment is lumpectomy combined with biopsy and treatment because of the difficulty in distinguishing benign from malignant lesions histologically [17]. There have been reports of mastectomies performed because of overdiagnosis, and therefore, an adequate preoperative search is necessary to avoid overtreatment [18].

In the present case, a large tumor occupying the left nipple was clinically suspected to be a malignant lesion, but repeated histological examination led to a diagnosis of nipple adenoma. The tumor in this case was completely resected.