The patient was an 84-year-old man with a chief complaint of exertional dyspnea. Chest X-radiographies showed a right pleural effusion. Chest computed tomography also revealed a right pleural effusion, with a nodular shadow in the anterior mediastinum and a 40-mm×30-mm mass shadow at the cardiophrenic angle. A maximum standardized uptake value of 8 to 10 at both lesions was shown by an ¹⁸F-fluorodeoxyglucose-positron emission tomography (FDG-PET). A thoracoscopy was performed under local anesthesia to investigate the imaging findings. A tumor was evident in the right cardiophrenic angle, with the surface covered in fine blood vessels and adipose tissue on the outer border. A biopsy was attempted, but the tumor was hard and could not be sampled using regular biopsy forceps. Marberger biopsy forceps were therefore used to successfully sample tissue. Pathological findings showed a diffuse proliferation of small atypical lymphocytes that were positive for CD20 on immunostaining, and extranodal marginal-zone, mucosa-associated lymphoid tissue (MALT) lymphoma was made. After this we will need to consider various biopsy methods.