Potential impact of [ 18 F]3'-deoxy-3'-fluorothymidine versus [ 18 F]fluoro-2-deoxy-d-glucose in positron emission tomography for colorectal cancer

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Abstract
Fluorine-18 labelled fluoro-2-deoxy-d-glucose (18FDG) positron emission tomography (PET) imaging demonstrates the increased glucose consumption of malignant cells, but problems with specificity have led to the development of new PET tracers. [18F]3'-deoxy-3'-fluorothymidine (18FLT) is a new tracer which images cellular proliferation by entering the salvage pathway of DNA synthesis. In this study we compared the cellular uptake of 18FLT and 18FDG in patients with colorectal cancer (CRC). Seventeen patients with 50 primary or metastatic CRC lesions were prospectively recruited. Lesions were initially identified using computed tomography. Patients underwent both 18FDG and 18FLT scanning. Semi-quantitative analysis of tracer uptake was carried out using standardised uptake values. All the primary tumours (n=6) were visualised by both tracers, with 18FDG showing on average twice the uptake of 18FLT. Similar uptake of both tracers was seen in lung and peritoneal lesions, with 18FLT imaging five of the six lung lesions and all of the peritoneal lesions. Of the 32 colorectal liver metastases, 11 (34%) were seen as avid for 18FLT, compared with 31 (97%) for 18FDG. No correlation was seen between the uptake of the two tracers (R 2=0.03). 18FLT shows a high sensitivity in the detection of extrahepatic disease but poor sensitivity for the imaging of colorectal liver metastases, making it unlikely to have a role as a diagnostic tracer in CRC. We have demonstrated that 18FDG and 18FLT image two distinct processes. The prognostic implications of the uptake of 18FLT need to be assessed in terms of response to chemoradiotherapy and survival.