Dual time point F18 FDG PET Scan in differentiating malignancy from Koch's infection.

An increasing number of reports and studies in world literature document the fact that while malignant cells continue to take up 18 F-FDG over many hours, the benign\infections or inflammatory lesions have peak uptakes within a short period of time and then show evidence of wash out or decreasing accumulation. Hence, dual – time point PET scans with semi-quantitative SUV evaluations are being increasingly employed to try and differentiate benign from malignant tissue. This prospective study was undertaken to prove that SUV's in malignancies increase over time and to study if a dual time point PET scan is helpful to differentiate malignant from benign Koch's lesions.

30 patients with proven primary malignancy, 10 patients with active Koch's infection and 2 patients with acute infections were examined using early and delayed F 18 FDG PET scans. The results were evaluated and comparisons drawn between the SUV values at different times in the same patient and the general trends of SUV change in various pathologies.

All the lesions of proven primary malignancy patients, showed a rising trend of SUV at 3 hours compared to 1-hour values (avg rise of 52%). The active Koch's lesions also showed rising trend of SUV values over the same time frames (avg 32%). No significant difference between the Retention Index (RI) of F18 FDG – SUV was seen between the malignant and Koch's lesions while the acute infectious lesions showed a decreasing trend in their SUV's.

Dual time point PET imaging with semi quantitative SUV evaluation is not a reliable method for the differentiation between malignant lesions and Koch's infectious foci as both show increased RI values over time. However, a decreasing trend in SUV uptake values over time is consistent with benign lesions.