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

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.