BilIN is diagnosed based on histological observation which indicates a microscopic precancerous stage in the biliary tract, corresponding to carcinoma in situ. This is accepted as a precursor lesion of CCA, especially large duct type, which is known to have a poor prognosis [11]. Early precursors of CCA also encompass intraductal papillary neoplasms of the bile ducts, biliary mucinous cystic neoplasms, bile duct adenoma, and von Meyenburg complexes [12].
Due to the lack of distinct mass formation, BilIN cannot be definitively identified using imaging. Although it is typically incidentally encountered proximal to invasive carcinoma, the prevalence of isolated BilIN, unaccompanied by invasive carcinoma, remains uncertain.
These lesions may develop as a consequence of chronic inflammation of the bile ducts. Numerous biliary epithelial cell-associated molecular pathways have been identified. For example, Kirsten rat sarcoma 2 viral oncogene homolog gene expression has been observed in approximately 40% of BilIN cases, and tumor protein 53 (TP53) mutations have also been reported, particularly in advanced stages [13].
Histologically, BilIN is confirmed in the biliary epithelium adjacent to the invasive adenocarcinoma. Grossly, it often presents with unclear features such as mucosal thickening, granularity, and subtle alterations in mucosal color. However, as the BilIN grade increases, changes within epithelial cells become more pronounced [12].
Low-grade BilIN (classified as BilIN grades I and II) exhibits mild cytological atypia characterized by hyperchromatic nuclei, prominent nucleoli, a slight increase in the nucleus-to-cytoplasm ratio, and minor nuclear pseudostratification, while retaining nuclear polarity. In contrast, high-grade BilIN (BilIN grade III) displays severe nuclear atypia, irregular and pleomorphic nuclei, several mitotic figures, and complete loss of nuclear polarization with complex stratification. Additionally, high-grade BilIN may exhibit features such as pseudopapillary eosinophilic epithelium or micropapillary structures [14]. The pathological findings in our patients are consistent with those described above (Fig. 1).
Immunohistochemistry can be applied to differentiate between low- and high-grade BilINs. Molecules such as p21, p53, cyclin D1, S100P, MUC1, and MUC5AC are increased in BilIN [15]. However, distinguishing preinvasive lesions from invasive lesions is challenging. Currently, three markers, S100, p53, and p16, are used for this purpose [14]. In the present study, increased expression of p53, Ki67, and MUC1 were observed in all BilIN specimens (Fig. 2). In cases of high-grade BilIN, p53, and Ki67 expression was relatively more prominent. These differences can also be indicative of more active carcinogenesis. No significant differences in S100 expression were observed. This is different from previous reports describing increased S100 expression in BilIN, which further increases within increasing BilIN grade [15]. Distinguishing preinvasive lesions from invasive carcinomas remains difficult. Zen et al. further confirmed the interobserver agreement in distinguishing between BilIN and reactive lesions, demonstrating a moderate diagnostic sensitivity of this method [16].
In a 1995 study that established CC as a risk factor for CCA, Stain et al. reported the presence of tumors in 26% of patients with CC [17]. However, the histology of these tumors was diverse, with adenocarcinoma being the most common, followed by adenosquamous carcinoma, squamous cell carcinoma, and rhabdomyosarcoma. The incidence of these tumors was observed to increase with patient age.
In a retrospective study conducted by Katabi et al., BilIN was identified in 28.5% of patients (age range, 11–67 years) who underwent surgery for CCs at the Memorial Sloan Kettering Cancer Center [6]. Herein, we noted the presence of BilIN in a CC specimen obtained from a 7-year-old girl in 2022. Prior to this patient, the youngest patient reported with a single case of BilIN was a 21-year-old female in whom high-grade BilIN was confirmed as a Type IA CC. Based on the 2006 report by Tanaka et al., the youngest patient diagnosed with CCA within a CC was 11, and the age of our aforementioned 7-year-old patient, an awareness of the clinical significance of BilIN in younger patients is necessary [10].
As of the writing of this study, there have been no reports of progression of residual bile duct cancer to CCA in any of the patients enrolled in this study. However, given that the oldest patient was 17, long-term follow-up is necessary for further assessment. Considering the nature of BilIN, which is not identified in imaging, further research is needed to determine which tests should be performed during the follow-up of patients for early detection of carcinogenesis. In addition, well-designed prospective studies, including those with data from complete resection of lesions and potential histopathological differential diagnosis of BiIIN, are needed to further investigate this disease. Because, this study was a retrospective, single-center study based on incidental diagnostic results.
The clinical significance and treatment standards of BilIN have not been fully established, even in the adult population; therefore, conducting a study to determine the clinical significance of the lesion through a large-scale analysis, including a review of histopathological examinations, may be helpful. This is particularly meaningful considering the relatively short history of successful CC excisions in children, and the increased risk of carcinogenesis in older patients.
Currently, little is known about the clinical implications of BilIN other than that it is a precancerous lesion [7, 8, 19]. Because of the possibility of tumorigenesis in residual BilIN, complete resection of high-grade BilIN is indicated during CCA excision in the adult population. Thus, when high-grade BilIN is confirmed in a pediatric CC specimen, surgeons must assess margin involvement and consider the necessity of additional resection. As no established consensus on treatment guidelines for BilIN exists, developing a strategy for detecting future carcinogenesis is also crucial [20].
To gain a better understanding of the clinical implications of BilIN in pediatric patients, future large-scale prospective studies are essential to identify risk factors and establish long-term follow-up guidelines. This will help verify the clinical significance of BilIN and guide its management in children.
To the best of our knowledge, this is the first report that elucidated BilIN in a pediatric population, both clinically and pathologically. Our findings suggest that the possibility of BilIN should not be overlooked in children, particularly in those with risk factors such as preoperative chronic pancreatitis. Surveillance of CCA should not be omitted in this population. Moreover, our findings promote the need for early and complete resection of CCs which may be important for preventing neoplastic changes in the biliary system. Furthermore, in patients at high risk for BilIN, verifying the presence of BilIN at the resection margins is essential to ensure complete resection.