Encapsulated papillary carcinoma (EPC) is considered rare breast cancer since it refers to only 1–2% in women (1). Additionally, the prognosis is excellent in the absence of invasiveness (2). The commonest population group regarding ethnicity, are Caucasian women from 55 to 67 years old that are in the post-menopausal stage (3, 4). It is usually diagnosed at an average age of 67 years (5, 6). EPCs with invasion appear at an average age of 59.3 years, which is considered relatively young (1). Male patients cover about 2–7% of EPC cases (1, 7).

Upon histological observation, EPC seems like a sole lesion within a cyst (8). Morphologically, the existing papillary architecture is a mesh where branches of fibrovascular cores are lined with neoplastic epithelial cells (8). Peripherally, EPC is characterized by a capsule composed of fibers of varying thicknesses (8) (Figure 1). Inside this capsule, luminal epithelial cells proliferate in combination with thin fibrovascular cores (Figure 2). A myoepithelial layer is absent both in papillary structures and in the capsule (9).

Sometimes, ductal carcinoma in situ (DCIS) and/or invasive ductal carcinoma can be associated with EPC (10). When the stroma is infiltrated by abnormal neoplastic cells crossing the fibrous capsule, invasion is present (8). DCIS in the nearby breast tissue has been recorded in 28.6–70% of cases (1, 8).

There are characteristics, however, which justify the noninvasive character of the tumor, such as very good prognosis, very small number of metastasis incidents, limited lymphovascular invasion, and relapse (11). Nevertheless, there are some cases of invasive EPC that cross out of the fibrous capsule and show lymph node (LN) metastasis, such as EPC with invasion (11). In addition, these EPCs are devoid of myoepithelial cells around the capsule, indicating characteristics of invasive ductal carcinoma. This causes a diagnostic challenge regarding invasion (11).

In the scientific community, it is widely believed that EPC is a DCIS variation, as it exhibits the following characteristics. Initially, there is a fibrous capsule, there is no stromal reaction, and there is a slowly progressing clinical pattern (12). Conversely, there is another view supporting that a tumor is a slowly-developing form of invasive carcinoma with expansive potential (12). This view is justified by the lack of myoepithelial cells that do not support the idea of an in situ carcinoma (12). Moreover, the possibility of finding LN metastasis supports the latter opinion further (12).

The medical opinion that prevails is to classify it as DCIS (1).

The usual classification of EPC is a noninvasive type of breast cancer at a rate of about 60% (1). Additionally, almost 40% of the cases are classified as a varied subgroup of low-grade DCIS (1). As to the EPC classification of one of the two groups, opinion is still divided.

The WHO Classification of Tumors of the Breast (2012) defines EPC as with or without invasion. Due to the slow-growing nature of EPC, WHO suggests considering it like a DCIS.

Having examined the overall molecular modifications, it was implied that an EPC shares the characteristic features of DCIS to a greater extent than those of invasive carcinoma (12).

At times, EPC has been linked with cancer due to the presence of invasive ductal carcinoma on the condition that an invasive component is present outside the fibrous capsule (9). In fact, biological characteristics of both DCIS and invasive carcinoma are present, with the latter being predominant (13). This case is defined as EPC with invasion. On rare occasions, both EPC and EPC with invasion can result in axillary metastasis. Concerning receptors, EPC retains a diffusing pattern of hormonal receptors. Progesterone and estrogen receptors (PRs, ERs) are categorized as positive, while human epidermal growth factor receptor-2 neu (HER2neu) is classified as negative in most of the cases (1, 8, 9). In 2018, a clinicopathologic study on 49 cases of patients with EPC and an average diagnostic age of 68.5 years performed by Li et al. revealed that 95.9% of tumors were ER- and PR- positive and that only 8 cases exhibited HER/2 1+ immunoreactivity (13).

Identification of histologic characteristics of these tumors on core needle biopsy is critically important (20). Performing core needle biopsy can indicate the nature of papillary lesions as benign or malignant, but it cannot distinguish between invasiveness and non-invasiveness of a tumor (4). In order to define the tumor morphologically, nearby in situ and invasive diseases may be located peripherally in EPC, and they are likely to escape sampling in core needle biopsy. To make diagnosis more efficient, it is essential to include the mass wall where a lack of myoepithelial markers will be observed (10).

A physician has the option of surgery to excise such lesions without performing a biopsy pre-operatively (1). However, a biopsy is obligatory post-operatively in papillary lesions to expose the risk of peripheral lesion-malignancy (1). This diagnostic path is challenging, and only the existence of resection specimens can provide a definitive answer (20).

Controversy still clouds therapeutic recommendations for EPC (1). When an actual invasion is not present, EPC is evaluated, classified, and managed as an in situ disease (8). On the other hand, when an invasion is present, classification and management are decided according to invasive features (8). The common treatment choice is complete surgical resection (1, 7). An investigative adjunct can be sentinel lymph node biopsy, which is recommended due to minimal LN metastasis (7, 8). Pure EPC cases, cases with DCIS association, and cases with invasion are all possible candidates for recurrence (7).

According to specific selective criteria, hormonal therapy, chemotherapy, and radiotherapy may ensue (21, 22). Specifically, adjuvant radiotherapy is a treatment option in cases where DCIS and/or invasion is involved. Additionally, if invasive tumors are histologically aggressive, the treatment choice is adjuvant chemotherapy (1, 8). Finally, adjuvant hormonal therapy is an option for patients who cannot undergo surgery, patients whose tumors appear repeatedly, or patients whose age is lower than 50 years old (7, 8, 22).

The clinical path of EPC has an excellent prognosis. It is recommended that meticulous investigation should be performed regarding high nuclear grade and invasion, as the stage of the tumor and selected treatment will be decided based on these elements (8). On the whole, the behavior of EPC is very good, with scarce local relapse, few distant metastases, or death owing to breast cancer (8). Specifically, this good prognosis results from the slow-growth nature of the tumor, with 10-year survival reaching about 100% and 10-year disease-free survival reaching 91% (1, 8).