Abstract
INTRODUCTION: Gallbladder carcinosarcoma is extremely rare, with fewer than 100 cases reported from its first description in 1907 until 2022. Collision carcinoma is a type of synchronous carcinoma in which 2 independently arising tumors come into contact or partially invade each other. CASE PRESENTATION: A man in his 80s was referred to our hospital with the primary complaints of weight loss and decreased appetite. Contrast-enhanced computed tomography revealed a large lobular tumor with heterogeneous enhancement, measuring 66 mm in maximum diameter, located in the fundus of the gallbladder. The mass showed clear signs of liver invasion, raising immediate concerns of malignancy. Magnetic resonance imaging provided additional crucial details. The lesion exhibited low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. Notably, diffusion-weighted imaging demonstrated restricted diffusion, a characteristic often associated with malignant processes. These findings strongly suggested gallbladder cancer with liver invasion. The patient underwent cholecystectomy and hepatectomy involving segments 4, 5, and 8 of the liver. A subsequent pathological examination revealed a complex and unusual tumor composition. The hepatic lesion showed nests of varying sizes with a medullary growth pattern, which is characteristic of intrahepatic cholangiocarcinoma. In contrast, the gallbladder lesion displayed features of adenosquamous carcinoma with a partial sarcomatoid morphology, indicative of gallbladder carcinosarcoma. Intriguingly, the interface between these 2 distinct tumor types exhibited unique characteristics. In some areas, normal hepatocytes were interspersed between the 2 types of tumor cells. Other regions demonstrated an invasive tendency of tumor cells towards each other. This unusual pattern led to the diagnosis of a collision tumor, a rare occurrence in which 2 independent primary malignancies coexist in the same organ or site. CONCLUSIONS: This was an extremely rare case of collision carcinoma involving both intrahepatic cholangiocarcinoma and gallbladder carcinosarcoma. The unique pathological findings and rarity of this tumor combination make this case particularly noteworthy. We present this case to contribute to the limited literature on such rare tumors, aiming to facilitate a better understanding and management of similar cases in the future.