An Atypical Papillary Lesion (APL) of the breast is a specific finding in breast pathology. Classified as a high-risk benign lesion, APL is not cancer but indicates an increased potential for malignancy developing in the future. The term applies to growths found within the milk ducts of the breast tissue. Recognizing an APL signals a need for further evaluation and intervention to determine the nature of the lesion and manage the associated risk.
Defining Atypical Papillary Lesion
A papillary lesion describes a growth pattern in the breast ducts resembling small, finger-like projections or fronds. These growths, known as papillomas, have a central core of connective tissue and blood vessels, covered by epithelial and myoepithelial cells, which line the duct. Benign intraductal papillomas are common and generally non-threatening, especially when occurring as a single lesion near the nipple.
The term “atypical” elevates the concern. Atypical Papillary Lesion (APL) refers to a papilloma containing cells with irregular or abnormal characteristics, known as atypia. Microscopically, these atypical cells exhibit features of low-grade ductal neoplasia, meaning they have begun to proliferate but do not yet meet the full criteria for cancer.
Pathologists subclassify APL based on the extent of abnormal cellular growth. If the area of atypia is limited to less than three millimeters, it is diagnosed as a papilloma with Atypical Ductal Hyperplasia (ADH). If the atypical proliferation extends three millimeters or more, it is classified as a papilloma with Ductal Carcinoma In Situ (DCIS), a non-invasive form of breast cancer.
A benign papilloma is distinguished by the presence of two distinct cell layers—epithelial and myoepithelial cells. APLs, conversely, often show a reduction or absence of the myoepithelial layer in the area of atypia. The epithelial cells themselves appear less varied and more uniform than in a purely benign lesion, justifying the need for subsequent surgical removal.
How APL is Identified and Diagnosed
APLs are often discovered incidentally during routine breast screening. When symptoms occur, the most common presentation is a spontaneous, watery, or bloody nipple discharge, particularly with central papillomas located near the nipple. Peripheral papillomas, located farther from the nipple and often multiple, are more likely to be found as an occult mass or microcalcifications on imaging.
The diagnostic process begins with imaging, typically a mammogram or ultrasound, which may reveal a solid mass or microcalcifications. Imaging alone cannot definitively distinguish a benign papilloma from an APL or a papillary carcinoma due to overlapping appearances. Therefore, a tissue sample is required for a conclusive diagnosis.
The definitive diagnosis of APL is achieved through a percutaneous needle biopsy, such as a core needle biopsy (CNB) or a vacuum-assisted biopsy (VAB). These image-guided procedures extract small pieces of the lesion for microscopic examination. Vacuum-assisted biopsy is often preferred because it removes a larger volume of tissue, reducing the chance of underestimating the lesion’s severity.
The pathologist confirms the presence of cellular atypia within the papillary structure. A diagnosis of APL on a core biopsy is a high-risk finding necessitating further management. Since the initial biopsy may not capture the entire lesion, the most concerning part may remain, justifying a follow-up procedure.
Understanding the Risk and Progression
An APL diagnosis is classified as a high-risk finding, justifying an aggressive management approach. The primary concern is that the APL diagnosed on the needle biopsy may be concealing a more serious malignancy, such as DCIS or invasive cancer, that was missed in the small sample. This statistical likelihood is known as the “upgrade rate.”
Studies show that the risk of upgrading to a malignancy upon surgical excision is significant, often ranging from 22% to over 37%. Most malignancies found upon subsequent surgery are Ductal Carcinoma In Situ (DCIS), which is non-invasive cancer confined to the milk duct. A smaller percentage may upgrade to an invasive carcinoma.
The biological risk stems from APL’s connection to the pathway of cancer development. Atypical cells in the papilloma are considered a precursor to malignancy, possessing some but not all cancer features. The presence of atypia indicates a proliferative process with a greater likelihood of progressing to a fully malignant state compared to a purely benign papilloma.
The upgrade risk is influenced by specific characteristics. Factors that increase the probability of finding cancer include older age (over 50 or 55 years) and a larger lesion size (greater than two centimeters). The high and variable upgrade rate strongly indicates the need for complete removal of the lesion.
Management and Follow-Up Strategies
Following an APL diagnosis, the standard recommendation is surgical excision, which serves both diagnostic and therapeutic purposes. Excision ensures the entire lesion is removed, allowing the pathologist to examine all tissue for hidden malignancy. This step is necessary due to the high risk of the lesion being upgraded to cancer, particularly DCIS.
The surgical procedure is usually a lumpectomy or excisional biopsy. The surgeon removes the lesion along with a small surrounding margin of normal breast tissue. The goal is to achieve “clear margins,” meaning atypical cells do not extend to the edges of the removed tissue block. If cancer is found upon excision, clear margins confirm the malignancy has been fully removed, often completing treatment for non-invasive disease.
Close surveillance may be considered in rare circumstances, such as when a patient has significant health issues making surgery undesirable. It may also be an option if the initial vacuum-assisted biopsy is deemed highly successful in removing the entire lesion. This approach is less common and requires careful discussion with a multidisciplinary team.
For all patients who have had APL, long-term follow-up is recommended. This involves increased breast cancer surveillance, typically including annual mammograms and more frequent clinical breast exams. Due to the increased lifetime risk associated with APL, some patients may be candidates for risk-reducing medications, such as selective estrogen receptor modulators.