Abstract
BACKGROUND: Extramedullary multiple myeloma (EMM) is defined by the infiltration of malignant plasma cells into non-osseous tissues. Its co-occurrence with hilar cholangiocarcinoma remains extraordinarily rare. This diagnostic overlap poses substantial clinical risks, as misclassification of EMM as biliary carcinoma may lead to inappropriate surgical interventions and delayed systemic therapy, ultimately compromising patient survival. Therefore, we report this case of misdiagnosis to help reduce the rate of misdiagnoses in the future. CASE DESCRIPTION: A 72-year-old male with λ multiple myeloma (MM) (type III, Group B) presented with painless jaundice and weight loss. Laboratory findings revealed obstructive jaundice [total bilirubin (TBIL) 312.3 mg/dL, direct bilirubin (DBIL) 161.5 mg/dL] and elevated carbohydrate antigen 19-9 (CA19-9) (130.5 U/mL). Contrast-enhanced computed tomography (CT) demonstrated a 3 cm hilar mass with peripheral ductal dilation, and magnetic resonance cholangiopancreatography (MRCP) showed abrupt biliary stricture. Despite radiographic features mimicking cholangiocarcinoma, considering the patient's history of myeloma, our team selected CT-guided biopsy of the hilar bile duct tumor. Immunohistochemical analysis confirmed syndecan-1 (CD138)(3+)/cluster of differentiation 38 (CD38)(1+)/cluster of differentiation 3 (CD3)(+)/λ-restricted plasma cell infiltration. The patient was diagnosed with EMM complicated with hilar bile duct metastasis. Subsequently, the patient avoided unnecessary surgical procedures and received proper treatment in the hematology department. CONCLUSIONS: This represents the first documented case of EMM metastasizing to the hilar bile duct, underscoring crucial diagnostic considerations. In patients presenting with hepatic hilar masses-particularly those with a history of MM-hematological malignancies must be included in the differential diagnosis. A multidisciplinary diagnostic approach integrating histopathological analysis, serum biomarkers, and advanced imaging modalities can effectively prevent unnecessary surgical interventions while optimizing therapeutic outcomes.