Bile duct adenoma and small-sized small duct type intrahepatic cholangiocarcinoma show distinct differences in genetic alterations, expression of IMP3 and EZH2 and stromal and inflammatory components.

AIMS: Given that bile duct adenoma was significantly more prevalent in the liver with small duct type intrahepatic cholangiocarcinoma (small duct iCCA), compared to other primary liver carcinomas, we examined the possibility of bile duct adenoma as a precursor of small duct iCCA by analysing genetic alterations and other features in bile duct adenomas. METHODS AND RESULTS: Subjects included 33 bile duct adenomas and 17 small-sized (up to 2 cm in diameter) small duct iCCAs. Genetic alterations were examined by direct sequencing for hot-spot regions and immunohistochemical staining. The expression of p16INK4a , EZH2 and IMP3 and stromal and inflammatory components were also examined. Genetic alterations examined including BRAF were not detected in bile duct adenomas, whereas genetic alterations of p53 (47%), ARID1A (41%), PBRM1 (12%), MTAP (12%), IDH1 (6%), KRAS (6%) and TERT promoter (6%) were detected in 16 small-sized small duct iCCA (94%) (P < 0.01). The expression of IMP3 and EZH2 was not detected in bile duct adenomas, whereas it was detected in most small duct iCCA (94%) (P < 0.01). Immature stroma and neutrophilic infiltration were significantly more prevalent in small duct iCCA, compared to bile duct adenoma (P < 0.01). CONCLUSION: Bile duct adenomas and small-sized small duct iCCAs show distinct differences in genetic alterations, expression of IMP3 and EZH2 and stromal and inflammatory components. There was no evidence suggesting that bile duct adenoma is a precursor of small duct iCCA. Immunohistochemical staining for IMP3, EZH2, p53, ARID1A and MTAP may be useful for differential diagnosis between bile duct adenomas and small duct iCCAs.

Bile duct adenoma may be a precursor lesion of small duct type intrahepatic cholangiocarcinoma.

BACKGROUND/AIMS: Precursor lesions of small duct type intrahepatic cholangiocarcinoma (small duct iCCA) have not been clarified so far. We hypothesised that precursor lesions may be frequently distributed in the background liver of small duct iCCA. METHODS AND RESULTS: We determined by histology the presence of bile duct adenomas and von Meyenburg complexes as candidate precursor lesions in the background liver of small duct iCCA, with other primary liver carcinomas as control. Subjects included 28 patients with small duct iCCA, 29 with large duct iCCAs, 60 with combined hepatocellular-cholangiocarcinoma (Comb) and 40 with hepatocellular carcinoma (HCC). The prevalence of bile duct adenomas in the background liver was significantly higher in small duct iCCA (35.7%) compared to other primary liver carcinomas (Comb, 4.9%; 10%, HCC) (P < 0.01). The prevalence of bile duct adenomas was significantly associated with the presence of von Meyenburg complexes and ductal plate malformation-like patterns in small duct iCCAs and Combs. Von Meyenburg complexes were detected in 11 small duct iCCA (39.3%), five large duct iCCAs (17.2%), 10 Comb (16.4%) and 13 HCC (33.3%), respectively (P > 0.05). Small duct iCCAs showed altered expression of ARID1A (46.4%), p53 (39.3%), PBRM1 (14.3%), IMP3 (85.7%) and EZH2 (82.1%), whereas these markers were negative in bile duct adenomas. CONCLUSION: Bile duct adenomas may be precursor lesions of small duct iCCAs. Alteration of ARID1A, p53 or PBRM1 may be involved in the carcinogenesis of small duct iCCAs.

BRAF V600E mutation in biliary proliferations associated with α1 -antitrypsin deficiency.

AIMS: Both homozygous and heterozygous α1 -antitrypsin (AAT) deficiency patients are at risk of developing hepatocellular carcinoma (HCC), but also of developing cholangiocarcinoma and combined HCC and cholangiocarcinoma. The aim of our study is to report a series of bile duct adenomas (BDAs) and intrahepatic cholangiocarcinoma (ICCs) in adult AAT deficiency patients, observed in our institution over a 5-year period. Our observational study includes a detailed investigation of their immunohistochemical profile and BRAF V600E mutation status. METHODS AND RESULTS: Eleven biliary lesions from five AAT deficiency patients (six BDAs from three cirrhotic patients with other concurrent liver diseases; three BDAs and two ICCs from two non-cirrhotic patients) were identified between 2010 and 2015 during routine histological investigation. Most BDAs expressed CD56, EpCAM, CD133, and CA19-9, similarly to hepatic progenitor cells (HPCs), and carried the BRAF V600E mutation (87.5%). One ICC showed a similar immunohistochemical profile but no evidence of the BRAF V600E mutation. CONCLUSIONS: Most of the biliary proliferations in AAT deficiency patients have an appearance of BDA with an HPC-related immunohistochemical profile. Their frequent BRAF V600E mutations support their neoplastic nature, but not necessarily their progression to ICC. We believe that this may depend on the patient genotype, or require a different pathway or a second mutational hit for malignant transformation. We postulate that BDA represents a heterogeneous group of biliary lesions, and that those associated with AAT deficiency may constitute a group of their own.

Albumin expression distinguishes bile duct adenomas from metastatic adenocarcinoma.

AIMS: Bile duct adenomas may be difficult to distinguish from metastatic carcinomas, particularly well-differentiated pancreatic ductal adenocarcinoma. Prior studies have evaluated the utility of various immunohistochemical markers, although these markers are notable for low sensitivity and/or specificity. The aim of this study was to investigate the utility of albumin and BRAFV600E expression in distinguishing between metastatic pancreatic adenocarcinoma and bile duct adenoma. METHODS AND RESULTS: We studied 26 bile duct adenomas, three bile duct hamartomas, and 158 pancreatic ductal adenocarcinomas. Branched-chain in-situ hybridization (bISH) for albumin was performed; bISH is based on the branched DNA technology, wherein signal amplification is achieved via a series of sequential steps. Additionally, BRAFV600E immunohistochemistry (IHC) was performed on a subset of cases. Twenty-three of 25 (92%) bile duct adenomas were positive for albumin; 18 (72%) showed diffuse staining, and five showed focal staining (20%), including two challenging examples. Two bile duct hamartomas also stained positively. All pancreatic adenocarcinomas were negative for albumin. Seven of 16 (44%) bile duct adenomas and five of 106 (5%) pancreatic ductal adenocarcinomas were positive for BRAFV600E by IHC. The sensitivity and specificity of expression of albumin, as detected by bISH, for distinguishing bile duct adenomas from metastatic pancreatic adenocarcinomas were 92% and 100%, respectively; the sensitivity and specificity of BRAFV600E IHC for distinguishing bile duct adenomas from metastatic pancreatic adenocarcinomas were 43.8% and 95.3%, respectively. CONCLUSIONS: Diagnostically challenging examples of bile duct adenoma may be distinguished from metastatic pancreatic adenocarcinoma by the use of albumin bISH.

BRAF V600E mutational status in bile duct adenomas and hamartomas.

AIMS: Bile duct adenomas (BDA) and bile duct hamartomas (BDH) are benign bile duct lesions considered neoplastic or secondary to ductal plate malformation, respectively. We have reported previously a high prevalence of BRAF V600E mutations detected by allele-specific polymerase chain reaction assay in BDA, and suggested that BDA may be precursors to a subset of intrahepatic cholangiocarcinomas harbouring V600E mutations. The aim of the present study was to assess the existence of BRAF V600E mutations, using immunohistochemical methods, in additional BDA as well as in BDH. METHODS AND RESULTS: Fifteen BDA and 35 BDH were retrieved from the archives of the pathology departments of two French university hospitals. All cases were reviewed by two pathologists specialized in liver diseases. BRAF V600E mutational status was investigated by immunohistochemistry. Mutated BRAF mutant protein was detected in 53% of the BDA and in none of the cases of BDH. CONCLUSION: Our findings suggest that BDA and BDH are different processes, and that BDA represent true benign neoplasms. They also support the hypothesis that mutated BDA might precede the development of the subset of intrahepatic cholangiocarcinomas harbouring BRAF V600E mutations.

Bile duct adenoma and von Meyenburg complex-like duct arising in hepatitis and cirrhosis: pathogenesis and histological characteristics.

Morphologic features and neoplastic potentials of bile duct adenoma (BDA) and von Meyenburg complex (VMC)-like duct arising in chronic liver disease were unknown. Thirty-five BDAs and 12 VMC-like duct lesions were observed in 39 cases with chronic liver disease. BDAs were divided into the EMA-cytoplasmic type (n = 14) and EMA-luminal type (n = 21). EMA-cytoplasmic BDA composed of a proliferation of cuboidal to low-columnar cells forming an open lumen with NCAM(+)/MUC6(-), resembling an interlobular bile duct. EMA-luminal BDA showed uniform cuboidal cells with narrow lumen, and NCAM(++)/MUC6(++), resembling a ductular reaction. VMC-like duct showed positive MUC1 expression and negative MUC6. The expression of S100P, glucose transporter-1 (GLUT-1) and insulin-like growth factor II mRNA-binding protein 3 (IMP-3) were not detected in three lesions. p16 expression was higher than those of the ductular reaction, and the Ki67 and p53 indexes were very low (<1.0%). Large-sized EMA-luminal BDA shows sclerotic stroma. We classified small nodular lesions of ductal or ductular cells in chronic hepatitis and cirrhosis into the following groups: BDA, interlobular bile duct type; BDA, ductular/peribiliary gland type; and VMC-like duct. They may be reactive proliferation rather than neoplastic lesions.

Ultrasonography of intrahepatic bile duct adenoma with renal cell carcinoma: correlation with pathology.

Intrahepatic bile duct adenoma (BDA) is a relatively rare benign tumor. Most cases are incidentally discovered during surgery or autopsy. We report here the co-existence of renal cell carcinoma and BDA mimicking metastasis in a 30-year-old female. An isoechoic nodule with a hypoechoic rim sized 10 × 9 mm was observed by ultrasonography in S2 of the liver. On contrast-enhanced ultrasonography (CEUS), the mass was enhanced in the early vascular phase and a defect with a clear border appeared in the post-vascular phase. We present the ultrasonography findings of BDA, including those yielded by CEUS using Sonazoid, along with the gross and microscopic pathological correlation.

NRAS-mutated oncocytic benign liver lesion in an organ donor: Pitfalls and troubles in frozen section diagnosis and risk assessment.

BACKGROUND: In the transplant setting, the definition of the risk of neoplastic transmission from donor to recipient often requires intraoperative pathological evaluation on frozen sections. Although most lesions can be easily classified into acceptable or unacceptable risk according to the Italian National Guidelines, there are cases in which unusual histologic features cannot be further investigated because of the lack of ancillary techniques on frozen sections. CASE PRESENTATION: Here we present a case of a liver lesion in a 51-year-old male donor, subjected to histopathological on-call examination. The frozen sections showed a well-demarcated lesion consisting of epithelioid cells disposed in laminar structures and intermingled with a dense lymphocytic population: this led to organ discard with interruption of the donation process. The definitive histological analysis required an extensive immunohistochemical (IHC) investigation: the final diagnosis was "bile duct adenoma with oncocytic features", eventually confirmed by a strongly positive anti-mitochondrial IHC. Finally, an NGS panel analysis was performed, which revealed NRAS mutation. DISCUSSION: To the best of our knowledge, this is the first case of oncocytic bile duct adenoma confirmed by anti-mitochondrial IHC and with NRAS mutation. The most challenging aspect of this case was represented by the transplant setting. In fact, the oncocytic features and the dense lymphocytic infiltrate represented concomitant unusual histological features that led to the halt of the organ donation procedures.

Magnetic Resonance Imaging Findings of an Intrahepatic Bile Duct Adenoma: A Case Report.

Bile duct adenoma (BDA) is a benign tumor that arises from the epithelium of the intrahepatic bile ducts. Herein, we present a case and discuss the characteristic magnetic resonance imaging (MRI) features of intrahepatic BDA by radiologic-pathologic correlation. A 41-year-old male visited our hospital. He was incidentally shown to have a liver-occupying lesion during a routine medical examination. MRI revealed a 16 mm × 17 mm × 18 mm circular hepatic mass occupying segment 2 of the liver. It showed low signal intensity on T1-weighted images (T1WI) and high signal intensity on T2-weighted images (T2WI). Diffusion-weighted imaging (DWI) MRI showed a ring of high intensity. Gadolinium ethoxybenzyl diethylenetriaminepentaacetic (Gd-DTPA) dynamic enhanced scanning showed a prolonged "ring enhancement" pattern. It showed a ring of high intensity in the hepatobiliary specific period and low signal peripheral and central of the tumor. The pathology result of the surgical resection showed a diagnosis of intrahepatic BDA. Postoperatively, the patient is currently under outpatient observation for seven months with no apparent recurrence. Intrahepatic BDA can be characterized as a small circular lesion located in the liver. MRI and pathologic features are well characterized in this tumor. MRI enhancement plays an important role in the diagnosis and evaluation of BDA.

Intrahepatic Cholangiocarcinoma Coexisting With Multiple Bile Duct Adenoma Treated as Liver Metastasis from a Pancreatic Neuroendocrine Tumor.

BACKGROUND: Bile duct adenomas (BDA) may be precursor lesions of small duct-type, including mass-forming type intrahepatic cholangiocarcinoma (ICC). CASE REPORT: A 68-year-old woman was transferred to our facility for the treatment of a liver tumor, possibly metastasized from a pancreatic neuroendocrine tumor. Finally, two liver tumors were resected and histopathologically diagnosed as "BDA" and "ICC with a BDA-like component". In the BDA-like component, the MUC6 positive rate was notably lower and the Ki-67 positive rate was higher than the other BDAs and ICC component, respectively. The doubling time of the tumor volume in BDA was very long but was shortened (1,510 and 719 days). Distinct enlargement of the tumor and appearance of enhancement through diagnostic imaging was useful in diagnosing the transformation from a BDA to an ICC. CONCLUSION: An "adenoma-carcinoma sequence" may exist in the transformation process from a BDA to an ICC.

Imaging features of intrahepatic bile duct adenoma in MRI.

BACKGROUND: Intrahepatic bile duct adenoma (BDA) is a rare benign epithelial hepatic tumor, and is very easy to miss or misdiagnose. This study was to investigate the magnetic resonance imaging (MRI) findings of intrahepatic BDA and to improve the imaging understanding of the disease. METHODS: The clinical and MRI imaging data of 25 cases of intrahepatic BDA confirmed by operation and pathology were analyzed retrospectively from the aspects of tumor location, size and shape, signal characteristics and enhancement mode. RESULTS: Among the 25 patients, 24 cases (96%) were single lesions, only 1 case had multiple lesions (3 lesions), a total of 27 lesions. Twelve lesions were located in the left lobe of the liver, 15 lesions were located in the right lobe of the liver, and the lesions were located under or near the hepatic capsule. The lesions were round and the size was 4 to 20 mm, with an average of 10.2 mm. In T1 weighted image (T1WI) sequence, all lesions were low or slightly low signal. In T2 weighted image (T2WI) sequence, 20 lesions were high or slightly high signal (74.1%), 4 were central equal signal peripheral ring high signal, and 3 were equal signal. In diffusion-weighted imaging (DWI) sequence, 23 lesions were high or slightly high signals (85.2%), and 4 were ring high signals. Nineteen lesions (70.4%) showed marked enhancement in the arterial phase, and the portal vein and lag phase continued to strengthen with a slightly higher or equal signal, 6 lesions (22.2%) showed mild or obvious peripheral ring enhancement in the arterial phase, and the portal vein and lag phase continued to strengthen with a slightly higher signal around the circumference and a slightly lower signal at the center. Eight cases were examined by gadolinium ethoxybenzyl diethylenetriaminepentaacetic acid (Gd-EOB-DTPA)-enhanced MRI, 7 cases were low signal in hepatobiliary phase, and 1 case was Peripheral low signal. CONCLUSIONS: The MRI findings of intrahepatic BDA have certain characteristics. Careful observation and analysis are helpful to the understanding and diagnosis of the disease.

Utility of DNA flow cytometry in distinguishing between malignant and benign intrahepatic biliary lesions.

The distinction between well-differentiated intrahepatic cholangiocarcinoma (iCCA) from its morphological mimics such as bile duct adenoma (BDA) and hamartoma (BDH) can be challenging, particularly in small biopsies. Although a few cases of BDA and BDH have been reported to undergo malignant transformation into iCCA, their neoplastic versus benign nature remains debated. DNA flow cytometry was performed on 47 formalin-fixed paraffin-embedded samples of iCCA, 14 BDA, and 18 BDH. Aneuploidy was detected in 22 iCCA (47%) but in none of the 32 BDA and BDH samples. Among the 34 iCCA patients who underwent complete resection and were followed up to tumor recurrence, tumor-related death, or at least for 1 year, the overall recurrence or death rates (regardless of flow cytometric results) were 18, 56, and 71% within 1, 3, and 5 years, respectively. The 1-, 3-, and 5-year recurrence or death rates in 18 iCCA patients with aneuploidy were 28, 66, and 66%, respectively, whereas 16 iCCA patients in the setting of normal DNA content had 1-, 3-, and 5-year rates of 6, 44, and 72%, respectively. Although aneuploid tumors were associated with worse outcomes during the first 3 years, this difference was not statistically significant (hazard ratio = 1.4, p = 0.473) in the present sample size. In conclusion, the frequency of aneuploidy was significantly higher in iCCA (47%) than in its benign morphological mimics (0%), suggesting that it may potentially serve as a diagnostic marker of malignancy in challenging situations. Our findings also suggest that most BDAs and BDHs, if not all, are benign entities and may not represent precursor lesions to iCCAs that often harbor aneuploidy. Although a larger cohort will be necessary to further determine the prognostic significance of aneuploidy in iCCA patients after resection, the patients with aneuploid tumors may have a higher risk for tumor progression, especially during the first 3 years.

Bile duct adenoma: imaging features and radiologic-pathologic correlation.

PURPOSE: This study aimed to reveal characteristic imaging features of bile duct adenoma (BDA) by radiologic-pathologic correlation. MATERIALS AND METHODS: We retrospectively analyzed pathological and imaging findings of seven patients with BDA. RESULTS: The median maximum diameter of BDA was 5.5 mm. Six lesions had hemispheric morphology. Seven lesions were located in the liver subcapsular region, and proliferation of bile ductules without atypia and fibrous stroma was observed. Two lesions had different microscopic findings. In both lesions, proliferation of bile ductules without atypia was observed in the margin. In one lesion, the percentage of fibrosis and hyalinization was higher at the center than at the margin. In the other lesion, inflammatory cell infiltration was observed in the center. On contrast-enhanced imaging, BDAs showed hypervascularity in the early phase and prolonged enhancement in the delayed phase. On contrast-enhanced multidetector computed tomography during hepatic arteriography, two lesions showed ring-like enhancement in the first phase and prolonged enhancement in the second phase. These were the different histopathologic features of BDAs between the margin and center. CONCLUSION: Bile duct adenoma can be characterized as a small semicircular lesion located in the liver subcapsular region, which show hypervascularity in the early phase with prolonged enhancement.

Intrahepatic bile duct adenoma in a patient with gastric cancer.

We report a case of intrahepatic bile duct adenoma (BDA) detected during laparoscopic distal gastrectomy for gastric cancer. A 70-year-old man was referred to our hospital for the treatment of gastric cancer. Esophagogastroduodenoscopy revealed an irregular, nodular, and elevated lesion on the greater curvature side of the middle third of the stomach. Abdominal contrast-enhanced computed tomography showed wall thickening with homogeneous enhancement in the middle part of the stomach, and no lesions in the liver. The patient underwent laparoscopic distal gastrectomy with regional lymphadenectomy, and during the operation a small whitish nodule was observed on the lateral segment of the liver surface. The lesion was excised by partial resection of the liver for the purpose of both histological diagnosis and treatment. Pathological examination of the liver lesion revealed no structural or cellular atypia, no stromal invasion, and immunohistochemical positivity for CK7 and CK19, but negativity for p53. The final diagnosis was well-differentiated adenocarcinoma invading the gastric serosal layer without lymph node metastasis, and intrahepatic BDA measuring 0.4 × 0.3 cm. Following surgery, the patient remained symptom-free without evidence of recurrence for 5 months. To the best of our knowledge, this is the first case of BDA with gastric cancer. Because it is difficult to distinguish BDA from other liver tumors including metastatic cancer due to its characteristically small size and lack of specific morphological features on standard imaging, surgical resection should be considered as the most suitable approach for both accurate diagnosis and treatment.

A case of biliary adenofibroma of the liver with malignant transformation: a morphomolecular case report and review of the literature.

BACKGROUND: Biliary adenofibroma is an exceptionally rare benign liver tumor with the potential for malignant transformation. In literature, only 21 cases have been described. CLINICAL PRESENTATION: In a healthy 63-year-old woman, a partly solid, partly cystic mass in the left lobe of the liver during a routine ultrasound examination was found. The computed tomography (CT) scan of the abdomen showed a 6.3 × 5.0-cm multilobulated cystic, partly hypervascularized mass in the liver segment IVa, with extension into segments II and IVb. There was no evidence of lymph node or distant metastases. Extirpation of the tumor was indicated by the multidisciplinary tumorboard. Microscopic examination showed a biphasic composed tumor with tubules embedded in fibrous stroma. In addition, there were also areas with pseudopapillary projections, as well as parts with focal cribriform-like growth pattern, which have been indicated as a possible sign of malignant transformation. Additionally, we found two different polymorphisms in the encoded TP53 und KIT in both distinct morphology tumor areas by molecular analysis, which ensured a tumor in malignant transformation. The patient has been alive for 24 months after R0 resection without tumor recurrence. Further investigation of more cases of this rare entity is necessary to proof molecular genesis. CONCLUSIONS: We report a rare case of a biliary adenofibroma with transition to an intrahepatic cholangiocellular carcinoma and present a brief literature review.

Pediatric intrahepatic bile duct adenoma - rare liver tumor: A case report.

BACKGROUND: Intrahepatic bile duct adenoma (BDA) is one of the rarest of the rare benign tumors of the liver in the pediatric age group. It arises from the epithelial lining of intrahepatic bile ducts. The overall incidence of BDA is 1.3% of all primary benign liver tumors. Few case reports of this rare tumor occurring in adult population are present in the literature and to date, only one pediatric case has been reported worldwide. CASE SUMMARY: An 18-month-old male child presented with chief complaints of mass per abdomen for 8 mo. Computerized tomography abdomen revealed a well-defined exophytic solid tumor arising from the posteroinferior margin of the right lobe of the liver with heterogenous enhancement and cystic changes, suggesting a suspicion of hepatoblastoma. Non-anatomical liver resection was done and a tumor of 10 cm × 9.5 cm was excised. Histopathology of the specimen was conclusive with the diagnosis of intrahepatic bile duct adenoma, which was further supported by immunohistochemistry panel testing. The post-operative period was uneventful. On follow-up, the child was doing well and had no post-operative complication. CONCLUSION: Intrahepatic bile duct adenoma in children is very rare and to date only one case has been reported. This is the second pediatric case of intrahepatic bile duct adenoma in the world. Additionally this is the first ever case of such a large tumor presenting in a child.

Histopathology of a benign bile duct lesion in the liver: Morphologic mimicker or precursor of intrahepatic cholangiocarcinoma.

A bile duct lesion originating from intrahepatic bile ducts is generally regarded as an incidental pathologic finding in liver specimens. However, a recent study on the molecular classification of intrahepatic cholangiocarcinoma has focused on the heterogeneity of this carcinoma and has suggested that the cells of different origins present in the biliary tree may have a major role in the mechanism of oncogenesis. In this review, benign intrahepatic bile duct lesions-regarded in the past as reactive changes or remnant developmental anomalies and now noted to have potential for developing precursor lesions of intrahepatic cholangiocarcinoma-are discussed by focusing on the histopathologic features and its implications in clinical practice.

Resolution of Bile Duct Adenoma over Follow-up Period; A Case Report.

Bile duct adenoma (BDA) is a rare neoplasm of bile ducts with various clinical manifestations and imaging appearances. A few cases of BDA and their predisposing factors have been described. We report a 35-year-old woman with right upper quadrant pain who consumed oral contraceptive pills. Ultrasound study revealed three hypoechoic subcapsular liver masses; two of them were hypodense in computed tomography. Fine needle biopsy of the largest mass showed bile duct adenoma. Liver masses disappeared after discontinuing the pills over a 2-year follow-up. BDAs can manifest in imaging. Although previous studies have not reported tumor resolution over a follow-up period, we suggest paying more attention to predisposing factors in order to give an opportunity for tumor resolution by risk factor elimination.

Proliferative index facilitates distinction between benign biliary lesions and intrahepatic cholangiocarcinoma.

Differentiation between benign and malignant lesions of the hepatic biliary tree may pose a diagnostic problem because well-differentiated intrahepatic cholangiocarcinoma may mimic biliary hamartoma, bile duct adenoma, or parenchymal extinction. We evaluated Ki-67 proliferative index and p53 status by immunohistochemical staining to aid in exclusion of cholangiocarcinoma. Fourteen biliary hamartomas, 21 bile duct adenomas, and 11 livers with parenchymal extinction were compared with 26 intrahepatic cholangiocarcinomas (16 well-differentiated and 10 moderately or poorly differentiated tumors). We found an increased proliferative index in intrahepatic cholangiocarcinomas compared with benign biliary lesions (average 23.0% in cholangiocarcinoma versus 1.4% in all benign biliary lesions, n = 26 versus n = 46, P < .001). No difference in average proliferative index was observed between well-differentiated and moderately/poorly differentiated cholangiocarcinomas (average 22.7% versus 23.3%, n = 16 versus n = 10, P = .92). Average proliferation indices of benign biliary lesions were uniformly low (biliary hamartoma, 1.2%; bile duct adenoma, 2%; parenchymal extinction, 0.5%). Most cholangiocarcinomas (23/26; 88.5%), but none of the benign lesions (0/46; 0%), had proliferative indices greater than 10%. Strong nuclear p53 immunohistochemical staining was only seen in cholangiocarcinomas (9/26; 34.6%) and not in benign biliary lesions (0/46; 0%), although many of the benign lesions showed weak to moderate staining. Immunohistochemical staining for Ki-67 facilitates distinction between benign and malignant lesions of the intrahepatic biliary tree, whereas p53 immunohistochemical staining is less helpful.