AME of the breast is a rare disorder characterized by the simultaneous proliferation of glandular epithelium and myoepithelium. Characteristically, AMEs tend to exhibit benign clinical behavior, although malignant transformation has been reported in a small number of cases. This transformation is indicated by features such as prominent cytological atypia, an elevated mitotic index, necrosis, and metastasis [7, 8]. Because of the biphasic nature of the tumor, carcinomas may arise from the glandular epithelium, myoepithelium, or both [9–11].
In the present case, the proliferation of myoepithelium was initially more prominent than that of glandular epithelium in the initial surgical specimen. From the time of recurrence, the glandular epithelium was eradicated, and the tumor was mainly myoepithelium and epithelium with squamous metaplasia. Malignant transformation at the time of local recurrence has been reported, but the number of cases is not large [12]. Thus, the proportion of myoepithelium and glandular epithelium proliferation differs from case to case. A case with different proportions between the metastatic site and the primary tumor has also been reported [13].
Mammography findings are a mass without calcification, consistent with previous reports [12]. There were no specific findings on mammography that suggest a diagnosis of AME in previous reports. Macroscopically, AMEs are typically well-circumscribed, unencapsulated, solid tumors that may show focal cystic change. Although there are many reports that do not mention ultrasound examination results, even when they are reported, they are generally hypoechoic, and it seems to be rare for a mass to be a clearly intracystic lesion, as in the present case [13, 14]. Enhanced breast MRI was performed for the first time before the third surgery. In this case, it was useful in diagnosing the spread of the tumor, and it contributed to determining the indication for total mastectomy.
There are no clear treatment guidelines for AME. Partial mastectomy is often performed, but total mastectomy is sometimes performed for large or suspected malignant tumors [6]. Axillary lymph node metastases are rare, and only a few cases of axillary dissection have been reported [15–17]. There may be recurrences even after complete resection with partial mastectomy, as in the present case, and they should be carefully monitored. Ito et al. concluded that, based on previous reports, total mastectomy is often performed for tumors larger than 3 cm [6]. It will become clearer as more cases are reported which patients should undergo total mastectomy, and whether reconstruction is possible after resection. No adjuvant therapy is recommended. Therefore, it was not performed in the present case.