Objective  To evaluate the value of ¹⁸F-FDG positron emission tomography-computed tomography (PET-CT) in detecting the primary cause of normal-sized ovarian cancer syndrome (NOCS).

  Methods  A retrospective study on 17 NOCS female patients, who underwent whole-body PET-CT and pathologic examination at Beijing Army General Hospital during 2005 and 2010, was conducted. The interpretation criteria were as follows.Criterion 1: Ovaries with SUVmax ≥ 3.0 and any abnormal focal lesion with ¹⁸F-FDG uptake greater than the surrounding background activity of the organ being examined were interpreted as possible sites of the malignant disease. High uptake of serous membrane was interpreted as metastasis. Criterion 2: Visible high uptake and abnormal thickening and projections in the stomach wall were interpreted as possible sites of the malignant disease. The image interpretation of ¹⁸F-FDG PET-CT was based on the consensus of two nuclear medicine physicians.

  Results  The result of interpretation criterion 1 showed that PET detected 13 occult primary tumors in 17 NOCS patients (detection rate of 76.5%), including ovarian cancer (n= 9), gastric cancer (n= 2) with ovarian metastasis (Krukenberg tumor), sigmoid colon carcinoma (n= 1), and peritoneal hemangiosarcoma (n= 1). Three patients with false-negative PET findings were diagnosed with ovarian cancer, and one was diagnosed with gastric cancer. The positive predictive value was 100%. The result of interpretation criteria 2 showed that one ovarian cancer with SUVmax of 2.5 was malignant. Moreover, gastric cancer without high uptake showed abnormal thickening of the stomach wall in CT. The detection rate was 88.2%. All 7 patients underwent tumor marker examinations before PET, which showed elevated serum CA125 levels.

  Conclusion  PET-CT can be a valuable tool for identifying a subset of patients with NOCS. Ovarian cancer is the main component of all primary lesions in NOCS. Gastric tumor containing mucus or signet ring cell carcinoma which invades ovaries and causes Krukenberg tumor. PET-CT can simultaneously detect primary gastric cancer and ovarian metastasis. Moreover, SUV does not clearly indicate ovarian and gastric cancers. Thus, visual interpretation is very important. Elevated tumor markers can also be indicators of NOCS.