Pleomorphic adenoma (PA) is the most common benign tumor among salivary gland tumors. It most commonly occurs in the parotid gland. It is a tumor containing epithelial, myxoid, chondroid, and other tissues, with obvious pleomorphism and mixed shapes, and so it is called a pleomorphic adenoma or mixed tumor (1, 2). PA occurring in the breast is extremely rare. PA of the breast is classified as an epithelio-myoepithelial tumor, exhibiting unique histological features, including myoepithelial differentiation and a mucinous, cartilaginous-like matrix. Other benign breast tumors include adenomas, fibroepithelial tumors, hamartomas, papillary tumors, and more. Adenomas encompass tubular adenomas, lactating adenocarcinomas, and more. The more common fibroadenomas and phyllodes tumors fall under fibroepithelial tumors. Due to its rarity and tendency to be confused with fibroadenoma, phyllodes tumor, or mucinous carcinoma, PA of the breast can be challenging to distinguish on imaging studies, making preoperative diagnosis difficult. PA of the breast tends to occur in middle-aged and elderly women, with most reported cases occurring in individuals over 40 years of age, with the predilection site being in the subareolar area (3). However, there are few relevant imaging reports (4–8), and preoperative diagnosis is difficult. Therefore, this article aims to report a case of pathologically confirmed PA of the breast, focusing on its detailed imaging features on enhanced CT and MRI and combined with the patient’s history of gastric cancer (GC) to explore its differential diagnosis ideas and clinical management strategies.

The patient, a 66-year-old woman, with no family history of breast tumors, complained of a 2-day-old left breast tumor, about the size of a peanut, with no ulceration, pus, redness, swelling, or pain. Six months ago, the patient underwent a subtotal gastrectomy for gastric adenocarcinoma. Physical examination revealed a palpable mass in the outer lower quadrant of the left breast, approximately 20 to 30 mm in size, hard in consistency, with indistinct boundaries, irregular surface, fixed to underlying tissue, no obvious tenderness, and no local skin flushing or ulceration. No enlarged lymph nodes were palpable in the bilateral axillary and supraclavicular fossa. Ultrasound shows that, as shown in Figure 1, a hypoechoic nodule can be detected at the junction of the lower outer quadrant of the left breast and the areola, with a size of approximately 24 mm × 13 mm, irregular shape, angular edges, uneven internal echo, and no blood flow detected. It is classified as BI-RADS 4a. Contrast-enhanced CT showed an equal-density soft tissue nodule in the outer lower quadrant of the left breast as shown in Figure 2, 5 mm away from the nipple. The enhanced scan showed mild and uniform enhancement in the arterial phase and delayed enhancement in the venous phase and delayed phase, the edge of the lesion was unclear, and no calcification or cystic changes were found in the lesion. In the MRI plain scan + dynamic enhanced image in Figure 3, the lesion is shown to be isointense on the T1WI sequence; T2WI showed high signal with fat suppression, DWI showed high signal, and ADC showed high signal. The ADC value was about 1.5 × 10^-3. There was no obvious diffusion restriction. The dynamic enhanced scan showed obvious uniform enhancement. The enhancement curve was inflow type or plateau type. The edge of the lesion was lobulated and the boundary was clear. The preoperative clinical and radiological preliminary diagnosis was a benign breast tumor, with a high possibility of fibroadenoma. The differential diagnosis also included phyllodes tumors, mucinous carcinomas, and metastases combined with the patient’s history of gastric cancer. Even though breast metastases are rare, they cannot be completely ruled out. Finally, the patient underwent breast lumpectomy, and pathology and immunohistochemistry confirmed pleomorphic adenoma of the breast (Figure 4). Due to the short postoperative period, long-term imaging follow-up data are still being collected for this case.

PA usually occur in the salivary glands and rarely in the breast. PA of the breast was first reported in 1906 (14). PA of the breast are pathologically characterized by a mixture of epithelial/myoepithelial components and myxoid stroma/chondroid components (15). PA of the salivary glands and pleomorphic adenomas of the breast share many of the same pathological and MRI imaging features, such as clear borders, obvious high signal on T2WI, and no obvious diffusion restriction. These features are considered to be related to the rich myxoid stroma (13). This patient showed obvious high signal on T2WI, which is also because the stroma of the lesion is dominated by myxoid and chondroid stroma, and this is also one of the important points of differentiation from fibroadenoma, which is common in the breast. To date, fewer than 100 cases of pleomorphic adenoma of the breast have been reported, with even fewer imaging studies available. Table 1 shows a list of selected cases describing partial imaging features (primarily ultrasound) of pleomorphic adenoma of the breast.

Shuichi Ito et al. (4) once reported a case of MRI manifestations of breast pleomorphic adenoma. It was an oval mass in the left breast with smooth borders. It was composed of solid and cystic components. There was no diffusion restriction. The TIC of the solid part was inflow or plateau type. The difference in this case was that the lesion was smaller and no cystic part was found. This case demonstrates combined features from ultrasound, contrast-enhanced CT, DCE-MRI, and DWI/ADC. Compared to most literature reporting only a single imaging modality, it more comprehensively reflects the imaging–pathology correlation—specifically the high T2 signal and absence of diffusion restriction—resulting from the abundant mucinous/cartilaginous matrix in PA of the breast. Secondly, unlike previous cases, this patient had a history of GC. Therefore, we must differentiate this from breast metastases. Breast metastases are mainly treated with adjuvant therapy, and the treatment effect is poor, the prognosis is poor, and the survival period is short (17). Primary extramammary tumors are mainly gastric cancer and lung cancer in Asia, while melanoma, lung cancer, and ovarian cancer are the main tumors in Europe and America. Leukemia and lymphoma are also common primary tumors (18). MRI appears like benign tumors, with smooth edges, no burrs, and lobulation, but diffusion is often limited, T2WI signal is low, TIC curves are mostly flat or outflow type, often accompanied by axillary lymph node enlargement, and the lymphatic hilum is unclear (10). In this case, the T2WI signal was obviously high and there was no obvious diffusion restriction. The axillary lymph nodes were not enlarged. The imaging findings are inconsistent with metastatic lesions. With advances in genomics technology, next-generation sequencing (NGS) performed on tissue biopsies or liquid biopsies can further aid in differential diagnosis. Pleomorphic adenomas may carry characteristic gene rearrangements (e.g., PLAG1 or HMGA2), while primary breast cancers or metastatic tumors typically exhibit distinct mutation profiles, such as TP53 mutations (common in triple-negative and high-grade cancers) and PIK3CA hotspot mutations (e.g., E542K, E545K, H1047R, present in approximately 30%–40% of HR+/HER2− breast cancers) (19, 20). When tissue sampling is limited, plasma ctDNA analysis can also identify systemic malignancies. Although NGS testing was not performed in this case due to a pathologically confirmed benign diagnosis, recognizing the potential value of these genomic tools is particularly important in patients with a history of other malignancies. This can help guide treatment decisions and avoid overtreatment.

We report a case of PA of the breast with the goal of improving people’s understanding of its imaging characteristics, which is characterized by a significantly high signal on T2WI and no diffusion restriction on MRI. For patients with a history of malignant tumors, recognizing these characteristics can help distinguish them from breast metastases and other breast tumors, providing the correct diagnostic direction for the clinic, thereby avoiding over-treatment and misdiagnosis for patients. However, the final diagnosis depends on pathological confirmation.