Biliary Intraepithelial Neoplasia (BilIN): Understanding Your Pathology Report

By Jason Wasserman MD PhD FRCPC
April 23, 2026

Biliary intraepithelial neoplasia (BilIN) is a precancerous change in the thin layer of cells that line the gallbladder and bile ducts. In BilIN, the abnormal cells are confined to the inner surface of the duct or gallbladder and have not invaded deeper tissues. Because BilIN can only be seen under the microscope, it is not visible on imaging scans and is almost always discovered by chance — usually when the gallbladder or a portion of the bile duct has been removed for another reason, such as gallstones or a suspicious mass.

This article will help you understand the findings in your pathology report — what each term means and why it matters for your care.

What causes biliary intraepithelial neoplasia?

BilIN is believed to develop after years of irritation or inflammation of the gallbladder or bile ducts. Chronic injury damages the cells lining these structures, and over time, mutations (genetic changes) accumulate. In BilIN, mutations in the KRAS gene are often among the earliest changes, while mutations in the TP53 gene tend to occur later as the lesion progresses toward invasive cancer.

Known risk factors include:

• Gallstones — Long-standing gallstones cause chronic irritation of the gallbladder wall and are the most common condition associated with BilIN found on routine cholecystectomy.
• Primary sclerosing cholangitis — A chronic disease that causes scarring and inflammation of the bile ducts and is associated with an increased risk of both BilIN and cholangiocarcinoma.
• Choledochal cysts — Fluid-filled sacs that form in the bile ducts, usually present from birth.
• Anomalous pancreaticobiliary junction — A structural abnormality in which the pancreatic duct and bile duct join before entering the intestine, allowing pancreatic fluid to reflux into the bile ducts and gallbladder.
• Liver fluke infections — Parasitic infections with Opisthorchis viverrini or Clonorchis sinensis, most common in parts of Southeast Asia, that chronically injure the bile ducts.
• Familial adenomatous polyposis (FAP) — A rare inherited syndrome that increases the risk of growths throughout the digestive tract, including the biliary system.

What are the symptoms of biliary intraepithelial neoplasia?

BilIN itself does not cause symptoms. When symptoms are present, they come from the underlying condition that prompted surgery — typically gallstones (causing right upper abdominal pain, nausea, or vomiting), cholangitis (causing fever, jaundice, or abdominal pain), or another bile duct abnormality. BilIN is then discovered incidentally when the removed tissue is examined under the microscope.

How is the diagnosis made?

The diagnosis of BilIN can only be made by a pathologist examining tissue under the microscope. The tissue is obtained when the gallbladder or a portion of the bile duct is surgically removed, most often during cholecystectomy (gallbladder removal) for gallstones, or when a section of the bile duct is removed as part of surgery for another condition.

Under the microscope, BilIN is composed of abnormal epithelial cells (the cells that line the gallbladder or bile ducts). These cells may look crowded, enlarged, or darker than normal, and they may form flat layers or papillary (finger-like) projections. The pathologist assesses how abnormal the cells look and how much of the duct lining is involved, and uses these features to assign a grade.

BilIN is one of several precursor lesions that can give rise to cancer of the gallbladder or bile ducts. Others include intraductal papillary neoplasm of the bile duct (IPNB) and intracholecystic papillary neoplasm of the gallbladder (ICPN), both of which form visible growths that can be seen on imaging or by the naked eye. BilIN, in contrast, is microscopic only.

How is BilIN graded?

BilIN is classified into two grades based on how abnormal the cells appear under the microscope. The grade is the most important piece of information in the pathology report, because it determines what happens next.

• Low-grade BilIN — The cells show only mild abnormalities. Their nuclei (the control centers of the cells) may look slightly darker or irregular, but the overall organization of the lining is preserved. Low-grade BilIN has little to no immediate risk of becoming cancer and, on its own, usually has no clinical significance.
• High-grade BilIN — The cells are much more abnormal. They lose their normal organization (called loss of polarity), their nuclei become markedly irregular and dark, and dividing cells (mitotic figures) are more easily found. High-grade BilIN is equivalent to carcinoma in situ, meaning the cells are cancer-like but still confined to the inner lining. If left untreated, high-grade BilIN can progress to invasive cancer.

Pathologic stage

Because BilIN has not invaded beyond the inner lining, it is not staged in the usual way. Low-grade BilIN is not assigned a cancer stage because it is considered a precancerous change rather than cancer. High-grade BilIN is classified as Tis (carcinoma in situ), meaning that the abnormal cells are confined to the lining and have not spread into deeper tissues. A finding of Tis alone does not mean that cancer is present.

Surgical margins

A margin is the cut edge of the tissue that the surgeon removed. When BilIN is found in a gallbladder or bile duct specimen, the pathologist examines the margins to determine whether any abnormal cells reach the cut edge.

• Negative margin — No abnormal cells are seen at the cut edge. This suggests that the BilIN was completely removed.
• Positive margin — Abnormal cells are present at the cut edge. When this involves high-grade BilIN, it may prompt additional surgery or closer follow-up, because abnormal cells may remain in the body.

Margin status is most important when high-grade BilIN involves a bile duct that has been cut through, because any remaining abnormal cells continue to carry a risk of progression to cancer.

What is the prognosis?

The outlook depends mainly on whether the BilIN is low-grade or high-grade, where it was found, and whether it was completely removed.

• Low-grade BilIN — Usually of no clinical consequence. No specific treatment or follow-up is needed beyond care for the condition that prompted surgery.
• High-grade BilIN in the gallbladder — Most cases are cured by removal of the gallbladder (cholecystectomy). A small number of patients may later develop cancer elsewhere in the biliary tract, a phenomenon called a field effect, in which the entire lining of the biliary system carries an increased risk of developing cancer.
• High-grade BilIN in the bile ducts — Carries a higher risk of progression to invasive cancer than gallbladder BilIN, because complete removal is often harder to achieve. Patients with bile duct BilIN usually need closer follow-up.
• High-grade BilIN in the setting of primary sclerosing cholangitis — Carries a higher risk of future cancer elsewhere in the bile ducts because of the underlying field effect.

Other features that may raise the risk of later cancer include extensive (widespread) high-grade BilIN, involvement of Rokitansky–Aschoff sinuses (small pockets that extend into the gallbladder wall), and positive surgical margins.

What happens after the diagnosis?

For low-grade BilIN, no specific treatment or additional follow-up is usually needed beyond routine care for the condition that led to surgery. For high-grade BilIN, management depends on where it was found and whether it was completely removed. When high-grade BilIN is limited to a gallbladder that has already been removed, no further surgery is usually required, but your care team may recommend imaging at intervals to watch for new lesions.

When high-grade BilIN is found in a bile duct specimen — or when margins are positive — additional surgery may be considered to remove any remaining abnormal tissue. Patients with primary sclerosing cholangitis, choledochal cysts, or other underlying biliary conditions are often followed with periodic imaging (MRI with MRCP) and blood tests, including the tumor marker CA 19-9, to watch for progression. Treatment of the underlying condition — such as managing PSC or monitoring choledochal cysts — continues to be important, because ongoing injury to the bile ducts drives further risk.

Questions to ask your doctor

• Was the BilIN in my sample low grade or high grade?
• Where was the BilIN found — in the gallbladder, a bile duct, or both?
• If high grade, was it completely removed?
• Were the surgical margins negative or positive for abnormal cells?
• Was the BilIN limited in extent, or did it involve a large area?
• Were Rokitansky–Aschoff sinuses or deeper gallbladder layers involved?
• Do I have an underlying condition — such as primary sclerosing cholangitis, a choledochal cyst, or familial adenomatous polyposis — that raises my risk of future cancer?
• Do I need any additional surgery?
• What follow-up imaging or blood tests will I need, and how often?
• Are there symptoms I should watch for that might suggest a new problem in the biliary system?
• Should any of my family members be screened, particularly if an inherited syndrome is a possibility?