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Background
Cholangiocarcinoma (CCA) is the second most common hepatobiliary malignancy after hepatocellular carcinoma (HCC). CCA demonstrates a wide variability in pathologic subtype, genetics, and growth patterns. Imaging plays a vital role in CCA diagnosis, staging, presurgical evaluation and treatment follow up. Familiarity with various CCA subtypes, imaging features and staging is essential for accurate treatment.

Educational Goals / Teaching Points
The aim of this exhibit is to give an overview of CCA, premalignant lesions and current classification systems and highlight the role and accuracy of various imaging modalities in managing CCA. Using a case-based approach, we aim to provide a guide for interpreting both initial and follow-up imaging of CCA. We identify common challenges encountered with each modality and provide tips and methods to overcome them.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
CCA is a heterogeneous disease that can be classified based on anatomic distribution and morphological growth patterns. Intrahepatic CCA is an adenocarcinoma histologically, which can also be subdivided, based on the size of the duct involved and degree of differentiation. Staging is based on anatomic subtype and using TNM system from the American Joint Committee on Cancer (AJCC) and Union for International Cancer Control (UICC). CT and MRI are primarily imaging modalities for evaluation of CCA, with typical imaging features of most CCAs demonstrating progressive enhancement, peripheral ductal dilatation, rounded margins, capsular retraction, and areas of low T2 signal intensity within the lesion. Standard-of-care CT provides accurate evaluation of the tumor, extent of local invasion of adjacent structure, the presence of metastasis and serves as a presurgical assessment tool. Comprehensive MRI /MRCP provides superior evaluation of the degree of duct involvement particularly for perihilar CCA. DWI has high accuracy in detecting small lesions. PET/CT or PET/MRI can aid in detecting small lymph nodes and distant metastasis. Many pathologies may mimic CCA on imaging, including inflammatory conditions (primary sclerosing cholangitis, inflammatory cholangiopathies, or pseudotumor) or tumors (gallbladder carcinoma, hepatocellular cholangiocarcinoma biphenotypic tumor, metastases, lymphoma). MRI can serve as a problem-solving tool for characterization. Challenges in evaluating extent of CCA on MRI post biliary stenting include overestimating tumor burden by confusing postintervention inflammatory reaction with tumor spread. Utilizing nodular tumor morphology on T2WI, DWI and postcontrast imaging can help differentiate tumor from inflammatory reaction. Treatment options include resection with systemic and radiation therapy depending on the disease extent. Treatment affects tumor imaging characteristics and alters enhancement patterns.

Conclusion
Diagnosis and staging CCA can be a challenge due to its heterogenous appearance on imaging. Improving our understanding of this disease process, treatment, and unique imaging features can aid in diagnosis and provide clinicians with critical information on disease progression and prognosis.