Tonsil is the most frequent primary source of the cancer of unknown primary in the neck by the FDG PET CT

Background: Cancer of unknown primary (CUP) refers to metastatic malignancy without an identified primary source. AimsTo check the difference in the FDG PET/ CT detection rate (DR) for the primary lesions of CUP when presenting with cervical or extra cervical lymph nodes metastasis.

Methods: Retrospectively, 287 cases of CUP that were referred for FDG PET/CT during the last ten years at Osaka University Hospital, Japan. The cases had biopsy-proven malignancy whose primary origin remains unidentified.

Results: Neck lymph nodes ( LN ) metastases were the initial presentation in 23% (67/287) of the CUP cases, while 77 % (220/287) with extra cervical metastases. DR for primary lesion of the CUP cases that presented with neck LN metastases was 39% (26/67). The location of the primary tumor was also in the neck area in 80% (21/26) of the detected primary cancer with cervical metastases presentation. Tonsils were the most frequent primary lesion of CUP in the neck; 52% (11/21), followed the base of the tongue, and larynx, each of them 14% (3/21). While the primary source of 20% (5/26) of the detected primaries that presented with neck LN metastases, were located in extra cervical areas; 3 cases in the thoracic esophagus and one case in each of the stomach and sigmoid colon. 77% (220/287) of the CUP cases presented by extra cervical metastases, DR was 12% (26/220). The common primary sources were: lung 27% (7/26), the stomach 15% (4/26). Collectively, DR of the primary lesions of CUP by FDG PET/CT was 18% (52/287). The primary sources in 40% (21/52) were located in the cervical area, and extra cervical in 60% (31/52).

Conclusions: FDG PET/ CT is more effective in detection of the primary source of the CUP when the presentation is with cervical LN metastasis.Tonsil should be checked carefully in CUP cases presented with cervical LN metastasis.Lung should be checked carefully in CUP cases presented with extra cervical LN metastasis.