The Usefulness of PET/CT In Detecting and Managing Cancers With Unknown Primary Site Depends on Histological Subtype

Introduction: We assessed the role of PET/CT in identifying and managing cancer of unknown primary site (CUP). Methods: : We reviewed 64 patients' PET/CT scans with CUP performed during 2012–2019. Results: : The median age was 65 years. Of 138-FDG-avid lesions, the mean SUVmax was10.6±6.0. Primary tumors (PT) were detected in 28(44%) patients. Detection was positive in only one(10%) patient with squamous cell carcinoma (SCC) histology, compared to 4/14(29%) with poorly differentiated carcinoma, 4/9(44%) with adenocarcinoma, 18/30(60%) of those for whom the origin could be presumed (p= 0.034 for SCC compared to other histologies). The mean age, mean SUVmax, and the distribution of organ involvement were similar between patients with and without discovered PTs, and also between patients with SCC and with the other histologies combined. However, those with SCC were less likely than the others to present with multi-lesion involvement, p<0.001. PET/CT interpretations apparently affected treatment of 8/28(29%) patients with PT detected and in none of the 35 whose PT was not discovered, p<0.001. Conclusion: PET/CT detected PT in almost half of CUP. However, it did not appear beneficial in those with SCC histology. PET/CT showed limited overall value in guiding clinical management but benefited a subgroup with discovered PT.