¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography in carcinoma of unknown primary: A subgroup-specific analysis based on clinical presentation

The aim of this study was to explore the clinical efficacy of ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) in tumor detection in patients with proven or suspected carcinoma of unknown primary origin (CUP) and making a subgroup-specific analysis. This was a retrospective, cross-sectional survey of patients with CUP syndrome who were referred for ¹⁸F-FDG PET-CT studies over a 2-year period. FDG-PET-CT scans were performed in compliance with the standard whole-body protocol, i.e., at least 6 h of fasting and were carried out with injected FDG radioactivity dose between 259 MBq and 370 MBq. The time from FDG injection to PET data acquisition was between 60 and 90 min. PET/CT scanning was acquired from the skull base to the upper third of the thighs. Nonenhanced, low-dose attenuation correction CT (110/70 kV/mAs) was performed for all patients. Twenty-one patients of clinically designated with CUP (male:female = 7:14; age range: 42–70 years; mean age: 57.95 years) fulfilling the inclusion criteria were enrolled in this analysis. The patients were subdivided into two groups: A - Those with histopathological proof (n = 12); B - Those with clinical/tumor markers/radiological suspicion of malignancy (n = 9). Among the first group, the sites of metastases in decreasing order of frequency were lymph nodes (n = 9/20; 75%), brain (n = 2; 16.67%), and liver (n = 1; 8.33%). In group B, six patients (66.7%) presented with hypodense/enhancing lesions in the brain and three (33.3%) had altered marrow signal intensity of spine. Overall, hypermetabolic lesions on FDG-PET/CT indicating the primary tumor sites were identified in 14 patients (66.7%). Twelve out of 14 primary sites were subsequently proven by histopathology, whereas two patients with biopsy-proven metastatic lesions in brain, with suspicious primary site in lung had been corroborated by FDG-PET/CT revealing multiple other metastatic sites, were not biopsied and were subsequently enrolled for palliative chemotherapy. When the results were examined individually in each of the Group A and Group B, the primary tumor detection rate was 58.3% and 77.7%, respectively. The identified primary tumor sites were lung 9/14 (64.4%), uterus/cervi 2/14 (14.3%), breast 1/14 (7.1%), esophagus 1/14 (7.1%), and aryepiglottic fold 1/14 (7.1%). In conclusion, FDG-PET/CT is not only helpful in histologically proven cases of CUP (irrespective of the metastatic sites), this modality also demonstrates high tumor detection rate in patients with clinical/radiological suspicion of malignancy. Being a whole body technique, it can additionally aid in disease staging in these patients which could be potentially helpful in their clinical management.

Keywords

Adenocarcinoma - carcinoma of unknown primary - FDG-PET/CT - squamous cell carcinoma