Combined use of preoperative 18F FDG-PET imaging and intraoperative gamma probe detection for accurate assessment of tumor recurrence in patients with colorectal cancer.

BACKGROUND: The purpose of this study was to combine intraoperative gamma probe (GP) detection with preoperative fluorine 18-fluoro-2-deoxy-glucose positron emission tomography (18F FDG-PET) imaging in order to improve detection of tumor recurrence in colorectal cancer (CRC) patients. METHODS: Twenty-one patients (12 females, 9 males) with a mean age of 54 years (range 31-78) were enrolled. Patients were suspected to have recurrent CRC by elevated CEA (n = 11), suspicious CT findings (n = 1), and clinically suspicious findings (n = 9). Preoperative FDG-PET scan and intraoperative GP study were performed in all patients. Mean time interval between preoperative FDG-PET scan and surgery was 16 days (range 1-41 days) in 19 patients. For intraoperative GP studies, 19 patients were injected with a dose of 10-15 mCi 18F FDG at approximately 30 minutes before the planned surgery time. In two patients, the intraoperative GP study was performed immediately after preoperative FDG-PET scan. RESULTS: Preoperative FDG-PET and intraoperative GP detected 48 and 45 lesions, respectively. A total of 50 presumed site of recurrent disease from 20 patients were resected. Thirty-seven of 50 presumed sites of recurrent disease were histological-proven tumor positive and 13 of 50 presumed sites of recurrent disease were histological-proven tumor negative. When correlated with final histopathology, the number of true positive lesions and false positive lesions by preoperative FDG-PET and intraoperative GP were 31/9 and 35/8, respectively. Both preoperative FDG-PET and intraoperative GP were true positive in 29 lesions. Intraoperative GP detected additional small lesions in the omentum and pelvis which were not seen on preoperative FDG-PET scan. FDG-PET scan demonstrated additional liver metastases which were not detected by intraoperative GP. Preoperative FDG-PET detected distant metastasis in the lung in one patient. The estimated radiation dose received by a surgeon during a single 18F FDG GP surgery was below the occupational limit. CONCLUSION: The combined use of preoperative FDG-PET and intraoperative GP is potentially helpful to the surgeon as a roadmap for accurately locating and determining the extent of tumor recurrence in patients with CRC. While intraoperative GP appears to be more sensitive in detecting the extent of abdominal and pelvic recurrence, preoperative FDG-PET appears to be more sensitive in detecting liver metastases. FDG-PET is also a valuable method in detecting distant metastases.

18F protection issues: human and gamma-camera considerations.

OBJECTIVE: Two main issues in protecting radiation workers and the general public from (18)F radiation-distance from and lead shielding for an (18)F source-were investigated. We also examined the effect of an (18)F source on the counting rate of a neighboring gamma-camera. METHODS: The dose rates of an (18)F vial and a water-filled cylinder were measured using an ionization chamber at different distances with or without lead shielding. In addition, the counting rates of gamma-cameras in the presence of the (18)F cylinder were measured with different detector orientations, distances, and energy windows. RESULTS: The dose rate of a point or an extended source in air was proportional to the inverse square of the distance from the source. At 2 m, the dose rate for a 370-MBq (18)F source was less than 20 micro Gy in any single hour, which is the limit for unrestricted areas. The dose rate with 0.318-cm-thick lead shielding decreased to about 60%, and that with 5.08-cm-thick lead shielding decreased to about 4%; these rates were higher than those estimated using the narrow-beam attenuation formula. The scattered photons and characteristic x-rays from the lead brick and surrounding structures may have contributed to this result. The decrease in dose rate resulting from a 33% increase in distance was similar to the effect from shielding the source with 0.318-cm-thick lead. At 3 m from a 185-MBq (18)F source, the counting rate in the (99m)Tc window of an Orbiter camera was about 120,000/min when the detector faced the source. This rate was comparable to that of a typical (99m)Tc clinical study ( approximately 200,000/min). Only when the distance was increased to 11 m and the detector did not face the source did the counting rate decrease to the background level (3,234/min). The counting rate also depended on the energy window of the gamma-camera. On a Vertex camera, the counting rate of (18)F in the (99m)Tc window versus that in the (201)Tl (or (67)Ga) window was 1:1.7 (or 1:2.7). CONCLUSION: (18)F dose rate can be significantly reduced with distance. Lead shielding is not as effective as was predicted. (18)F sources should be kept substantial distances away from gamma-cameras to avoid contamination of image quality.