Detection of unknown occult primary tumors using positron emission tomography.

BACKGROUND: The potential of positron emission tomography (PET) with 18F-fluoro-2-deoxy-D-glucose (FDG) to detect primary tumors after unsuccessful conventional diagnostic workup was assessed in patients with metastatic disease from an unknown primary tumor.

METHODS: Twenty-nine patients with various histologic types of metastases from an unknown primary site were studied after unsuccessful conventional diagnostic workup. The patients received 370 megabecquerels (MBq) (10 millicuries) FDG intravenously and whole body scans were made after 30 minutes after injection onward.

RESULTS: All but one known metastatic tumor sites were visualized. Additional metastases were discovered in five patients. With FDG-PET the primary tumor was identified in 7 patients (24%): in 2 patients with carcinoma of the nasopharynx, in 1 patient with plasmacytoma of the base of the tongue, in 1 patient with carcinoma of the lung, in 1 patient with carcinoma of the colon, and in 2 patients with breast carcinoma. FDG-PET did not identify a primary tumor in the remaining 22 patients (76%). Despite a negative PET study, the primary lesion was identified in a later phase in 3 of these patients (14%). Survival was not altered by discovery of the primary tumor.

CONCLUSIONS: A previously unknown primary tumor was able to be identified with FDG-PET in 7 of 29 patients after an unsuccessful conventional diagnostic workup. However, the clinical relevance of PET information in this setting is limited.