Abstract: Malignant tumor may be detected using various diagnostic methods. Different techniques can reveal various results for the same lesion; thus, clinicians must seriously consider the approach selected. This study introduces a case that was the subject of a multidisciplinary comprehensive discussion at the Tianjin Medical University Cancer Institute and Hospital. A patient presented with malignant ascites. Positron emission tomography-computed tomography examination revealed more thyroid, parotid gland, and umbilical lesions, which were all considered malignant. B-mode ultrasound was used to assess the thyroid and parotid benign lesions, as well as the umbilical lesions for metastatic tumor. The primary lesions were not definite although the umbilical lesions were metastatic adenocarcinoma, as determined by aspiration biopsy. The multidisciplinary consultation committee determined that the patient had peritoneal carcinoma through analysis of radiologic, pathologic, and immunohistochemical characteristics. This case presentation suggests that the pathological approach remains the gold standard for malignant tumor diagnosis. Reasonable selection of an immunohistochemical item to determine the source of the carcinoma is considerably significant.