PTs are rare fibroepithelial breast tumors whose behavior can range from a benign recurring fibroadenoma to metastatic neoplasms. The median age of presentation is 40–50 years old and older patients tend to present with higher grade disease.[1] In contrast, Fibroadenomas (FA) are the most common benign breast tumor and growth can fluctuate with hormonal changes. They are common in women under 35 years old and, unlike PT, are not usually associated with an increased risk of malignancy. Surgery is curative for most benign, borderline, and malignant PTs without uniformly poor histologic features. [4]
PTs often present grossly as a unilateral, well-defined, mobile and painless breast mass that are smooth and multinodular. The can vary widely in size, but the average diameter is 4–7 cm. On physical exam, PTs rarely present with nipple retraction, ulceration, or chest wall fixation. PT growth rate is also highly variable, and metastatic involvement is uncommon. Imaging alone is not enough to confirm a PT diagnosis as they will present nearly indistinguishable from FAs, both reading as smooth, poly-lobulated masses on both ultrasound and mammography [1, 3].
While gross features such as large size and rapid growth rate can separate a PT from FA, biopsies are diagnostic. Core needle biopsies (CNB) are preferable to fine needle aspirations (FNA) as the latter has a higher false negative rate. In cases of high clinical suspicion for PT, excisional biopsies are considered even if CNB pathology shows a cellular FA. Microscopically, PTs are often described as having a leaf-like architecture. Histological features such as increased mitotic activity, stromal cellularity, overgrowth, fragmentation, and tumor borders distinguish a PT from a fibroadenoma. [5]Based on histology, PTs can be categorized as benign, borderline malignant, or malignant based on stromal cellularity and cellular atypia, mitotic activity, tumor margins, and stromal growth. Benign PTs are marked by increased stromal cellularity, mild to moderate cellular atypia, and circumscribed tumor margins with fewer than 4 mitoses per 10 hp field. Borderline PTs, like in our patient, have a greater degree of stromal cellularity and atypia, 4–9 mitoses per 10 hp fields, and microscopic infiltrative borders. Both benign and borderline PTs do not have stromal overgrowth. Malignant PTs have markedly increased stromal cellularity and atypia, along with a high mitotic rate (greater than 10 mitoses per 10 hp fields), diffusely infiltrative borders, and the presence of stromal overgrowth, which portends greater metastatic potential. [5]
The breast mass in this patient had been stable in size for approximately 12 months before she began to notice rapid growth. Her initial mammogram and biopsy results revealed a palpable, well circumscribed, oval hypoechoic mass in her left breast. Her follow-up US-guided biopsy revealed benign breast tissue with features of fibroadenoma and usual duct hyperplasia. The clinical and gross features of the breast mass was consistent with a fibroadenoma, and based on the cellular features of her biopsy, a diagnosis of benign FA was made.
18 months after the patient’s initial FA diagnosis, she presented after noticing 6 months of rapid growth in her left breast. The biopsy revealed a fibroepithelial tumor with stromal expansion, mild stromal cell atypia, scattered mitotic figures, and cells that suggested rhabdomyoblastic differentiation. Along with characteristic rapid growth, the histological features were consistent with a borderline PT (bPT) and an IDC and Ductal Carcinoma in situ (DCIS) were incidentally found within the bPT specimen.
At around 1% incidence, PT is a very rare breast neoplasm. Even more infrequent is the case of an IDC within a PT, accounting for around 1–2% of PT cases. In these rare cases, the encapsulating PT is typically classified as malignant or benign. [1] Amongst types of PT, the most infrequent is a borderline PT (bPT). To our knowledge, there are only 5 other cases of bPT encapsulating an IDC in current literature. However, none of these cases displayed the same combination of concomitant breast carcinomas as our patient’s IDC and DCIS. The only identifying and unifying factor among these case reports was that each woman endorsed rapid, noticeable growth of the lesion within several months, prompting the patients to seek evaluation. There were no signs and symptoms that suggested malignancy. The patient’s breast mass did not present as an immobile, hard mass with irregular borders or have other classic features of a cancerous lesion, rather, it closely resembled that of a PT or FA.
Current literature demonstrates that IDC displays the same biologic and clinical behavior whether it is found inside or outside of a fibroadenomatous tumor, although this continues to be an evolving subject of research. [3]While our patient did not have evidence of lymphatic spread, there are 2 published case reports of patients with bPT encapsulating IDC who had metastases, requiring adjuvant chemotherapy and radiation.[6] [7]
Notable to this case is the patient’s initial CNB of her left breast that showed a fibroadenoma (FA), another fibroepithelial neoplasm (FEN). Rare cases have been reported of FA transformation to a borderline or malignant PT over time, which may also be the case in our patient. [8, 9]Review of the initial CNB was firm in the diagnosis of a FA without indication of PT histological features. To our knowledge, there has only been 1 other case report published displaying a CNB-proven FA transformation to a PT with concomitant breast carcinoma.The authors of the case describe a 53-year-old female with a twice CNB-proven FA that progressed to bPT with components of tubular carcinoma and lobular carcinoma in situ. Similar to our patient, all sentinel lymph nodes were negative for malignancy. [10]However, it is impossible to determine whether our patient’s initial presentation was a FA with true transformation to encapsulating bPT or a failure of CNB to capture a bPT histology. It is exceedingly difficult to differentiate FA from PT with a traditional CNB, as accurate diagnosis requires both epithelial and stromal components.[11] Moreover, no single feature available on CNB can fully differentiate FA from PT.
This study reports a rare case of a bPT-encapsulated IDC and DCIS that initially presented as a FA. The lack of specificity in a traditional CNB to differentiate a bPT from a FA makes it challenging to discern the true course of this patient’s disease. [11]Fortunately, the rate of growth of the patient’s mass prompted her to seek treatment early and, to our knowledge, she has not had any complications or required further medical intervention to this date. There are currently no specific treatment guidelines for encapsulated IDC and DCIS. While these cases are exceedingly rare, breast surgeons should be aware of the possible PT disease course and how to counsel their patient if an encapsulated breast carcinoma is found.