Metastatic ovarian cancer is generally the usual suspect when isolated peritoneal carcinomatosis is found on CT.  However, in this male patient, other entities such as metastasis from a gastrointestinal malignancy, or a primary tumor of the peritoneum were considered.   

Peritoneal lymphomatosis is the most likely diagnosis in this case because of the intense degree of FDG uptake and the presence of mediastinal adenopathy.  Isolated peritoneal lymphomatosis is a rare manifestation of non-Hodgkin lymphoma, but is an uncommon finding when other sites of disease are present. 

The differential diagnosis of peritoneal carcinomatosis can be divided into two broad categories: metastatic disease and primary tumor of the peritoneum.  While metastatic disease is by far more common (generally from the gastrointestinal tract and ovaries), primary peritoneal neoplasm should be considered when a known primary is not clearly evident.

Primary peritoneal neoplasm includes malignant tumors such as malignant mesothelioma, papillary serous carcinoma, desmoplastic small round cell tumor, lymphoproliferative disorders (peritoneal lymphomatosis and leukemic infiltration), as well as mesenchymal tumors (liposarcoma, angiosarcoma, malignant fibrous histiocytoma).

Considering the hypermetabolic nature of the findings, a primary benign peritoneal tumor is unlikely in this case.  This includes cystic mesothelioma and benign mesenchymal tumors (lymphangioma, hemangioma, leiomyomatosis peritonealis).