Fig. 2

An 80-year-old man, who was incidentally found to have a 47 × 38 × 38-mm large mass in the S5 liver, was pathologically diagnosed with intrahepatic cholangiocarcinoma (ICC). The patient had a liver reserve classified as Child–Pugh A. The tumor showed hypointensity in the hepatobiliary phase of gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid-enhanced magnetic resonance imaging (MRI) before treatment (a). Subsequent imaging studies, such as computed tomography (CT), MRI, and fluorodeoxyglucose positron emission tomography-CT, revealed a diagnosis of T1aN0M0 stage IA cholangiocarcinoma (Union for International Cancer Control 8th edition). The isodose lines are displayed on the images obtained from the planning CT (b). This lesion was treated with proton beam therapy with respiratory synchronization and without the use of fiducial markers. No combination chemotherapy was administered. The proton therapy plan was prepared with two beams of 10° and 260°, and a dose of 83.6 relative biological effectiveness/22 fractions was prescribed. At 1 year post-irradiation, the lesion size had reduced (c). Subsequently, a small right-sided pleural effusion appeared, with a gradual increase at the 3-year post-irradiation follow-up examination (d). The patient complained of shortness of breath and was medically treated, and the dyspnea was judged to be grade 2 according to the Common Terminology Criteria for Adverse Events version 5. Five years after the completion of irradiation, the lesion is under control, and recurrence has not been observed. The patient’s pleural effusion has decreased as the patient continues to be followed up