Cholangiocarcinoma (CCA) is a heterogeneous malignancy with a broad morphologic spectrum, from overtly malignant tumours to deceptively bland lesions closely mimicking benign biliary proliferations. This may result in diagnostic difficulty, particularly on biopsies. CCAs harbour a wide spectrum of genetic alterations, including mutations (IDH1/2), amplifications (ERBB2) and fusions (FGFR2), several of which are actionable, making molecular profiling a key component of clinical management. We report a case illustrating this pitfall and highlighting the decisive role of molecular analysis. A 41-year-old man was incidentally found to have a 15-mm hepatic nodule. Magnetic resonance imaging confirmed a solitary lesion in a steatotic liver. Liver function tests were normal, and positron emission tomography showed no abnormal FDG uptake. Because imaging findings were inconclusive, a biopsy was performed. Histologically, the biopsy showed background steatohepatitis with stage F2 fibrosis. The lesion consisted of a well-circumscribed tubular proliferation embedded in abundant collagenous stroma and surrounded by a prominent lymphoid infiltrate. The cells displayed absent to mild cytologic atypia, rare hyperchromatic nuclei and prominent mucin secretion (Figure 1). Immunohistochemistry supported a biliary phenotype (CK7 and CK19 positive) and excluded metastatic carcinoma. p53 staining was wild-type, BAP1 expression was retained, and the Ki-67 proliferation index was low (~10%). Overall, the lesion was considered indeterminate, with concern for very well-differentiated intrahepatic CCA (iCCA). Staging revealed no extrahepatic disease, and an atypical liver resection was performed. Gross examination revealed a well-defined, white nodule measuring 14 mm. Microscopically, the lesion was sharply demarcated and associated with a prominent peripheral lymphoid infiltrate. It was composed of small, regular mucin-producing tubules embedded in fibro-hyaline stroma, without cytologic atypia, mitoses, vascular or perineural invasion (Figure 1). These features closely resembled a bile duct adenoma (BDA), and no unequivocal features of malignancy were identified. Surgical margins were negative, and no adjuvant therapy was initially planned. Given the unusual presentation in a young patient and the diagnostic uncertainty, comprehensive molecular profiling was performed. DNA sequencing revealed no pathogenic alterations. However, RNA fusion analysis identified a novel HIP1R-ALK fusion. The in-frame fusion involved canonical ALK exon 20 and preserved the tyrosine kinase domain while truncating regulatory regions, consistent with constitutive oncogenic activation. Diffuse and strong ALK protein overexpression (3+) was confirmed by immunohistochemistry (Figure 2). Based on these findings, the diagnosis was revised to well-differentiated ALK-rearranged iCCA, a rare molecular subtype. The patient remains alive and disease-free 1 year after surgery. This case illustrates a challenging scenario in liver pathology: the distinction between benign biliary proliferations and well-differentiated iCCA. Entities such as BDA, ductular reaction, von Meyenburg complexes and biliary adenofibroma may closely mimic carcinoma,1 particularly when cytologic atypia, infiltrative growth and vascular or perineural invasion are absent. In addition, cholangiolocarcinoma, a subtype of small duct iCCA, represents an important differential diagnosis, as it is characterized by bland, ductular or cord-like proliferations resembling reactive bile ductules and frequently arises in the setting of chronic liver disease. However, cholangiolocarcinoma typically exhibits an infiltrative growth pattern rather than a sharply demarcated, adenoma-like architecture. Conversely, well-differentiated iCCA may show bland morphology and low proliferative activity, rendering morphology and immunohistochemistry alone insufficient and highlighting the value of molecular analysis. ICCAs sometimes harbour actionable genetic alterations, including FGFR2 fusions and IDH1 mutations. In contrast, ALK rearrangements are exceedingly rare, with only six cases reported to date. These cases span a wide age range (26–76 years), predominantly involve intrahepatic tumours (4/6), and show variable differentiation and fusion partners (EML4, STRN and AMBRA1). Importantly, three patients achieved partial responses to ALK inhibitors, underscoring the therapeutic relevance of detecting such alterations.2-5 In the present case, identification of a novel HIP1R-ALK fusion in a biliary lesion with adenoma-like morphology provides strong evidence of a true neoplastic process driven by an oncogenic event. Together with prior reports of BRAF-mutated BDAs6 and progression to invasive carcinoma, this finding supports the concept that BDAs may represent early forms of iCCA. While routine molecular testing of all BDAs is not justified, our case suggests that molecular analysis should be considered in adenoma-like biliary lesions showing diagnostic uncertainty or occurring in unusual clinical settings. In this context, ALK immunohistochemistry and possibly BRAFV600E immunohistochemistry should be evaluated as readily accessible ancillary tools in routine clinical practice. In conclusion, this case is notable for its deceptively benign morphology and unexpected HIP1R-ALK fusion. It emphasizes the importance of integrating morphology with molecular profiling in diagnostically challenging biliary lesions and expands the molecular spectrum of ALK-rearranged iCCA, with potential therapeutic implications. ACB, MD, RG and JS performed the histopathological analyses. ACB, HR and JS interpreted molecular analyses. FM was responsible for the clinical care of the patient and performed surgery. ACB and JS wrote the paper. None declared. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.
Brunac et al. (Wed,) studied this question.