FDG PET/CT imaging in the detection of primary tumors in carcinoma of unknown primary sites

Carcinoma of unknown primary sites (CUP) is defined as the presence of histologically proven metastatic disease without evidence of a primary tumor.

The aim of this study was to prospectively assess the utility of integrated Positron Emission Tomography/Computed tomography (PET/CT) with ¹⁸F-Fluro-deoxyglucose (FDG) in depicting the primary lesion in CUP as compared with PET & Contrast Enhanced Computed Tomography (CECT).

45 patients (34 males, age range 27–75 years and 11 females, age range 46–77 years) diagnosed with CUP were subjected to FDG PET/CT with the aim of locating the site of primary tumor. All patients underwent a conventional diagnostic work up that included CECT scan of the relevant parts of the body. The PET/CT images were evaluated separately as only PET images & fused PET/CT images. All potential sites of primary tumor detected by the CECT, PET & PET/CT imaging were subsequently sampled & verified by histopathologic examination as true positive or false positive. Data was compiled and the detection rate and positive predictive value for the detection of the primary tumors were calculated for CECT, PET & PET/CT. The incremental value of PET/CT over CECT and PET alone for the detection of primary tumor was assessed.

CECT detected a possible site of primary tumor in 10 out of 45 cases, of which 9 were true positive. PET/CT depicted the primary tumor in 16 of 45 patients. Of these 14 were true positive and two were false positive. PET alone detected a suspected primary tumor site in 22 of the 45 patients out of which 08 were false positive. In 31 patients (68.9%), the primary tumor site remained occult. PET/CT detected the primary in 55.6% more cases than CECT alone (14 vs 9). The detection rate for CECT, PET and PET/CT were 20%, 31.1% and 31.1% respectively. The positive predictive value of CT, PET & PET/CT were 90%, 63.6% & 87.5% respectively.

Integrated PET/CT has higher detection rate for primary tumor in cases of CUP than CECT. Addition of CT information to PET images of PET/CT increases the diagnostic accuracy. Thus integrated PET/CT appears to be a better tool in detecting the site of primary in cases of CUP, than CECT or PET alone.