Repeated Positron Emission Tomography-Computed Tomography and Perfusion-Computed Tomography Imaging in Rectal Cancer: Fluorodeoxyglucose Uptake Corresponds With Tumor Perfusion

Purpose The purpose of this study was to analyze both the intratumoral fluorodeoxyglucose (FDG) uptake and perfusion within rectal tumors before and after hypofractionated radiotherapy. Methods and Materials Rectal cancer patients, referred for preoperative hypofractionated radiotherapy (RT), underwent FDG–positron emission tomography (PET)–computed tomography (CT) and perfusion-CT (pCT) imaging before the start of hypofractionated RT and at the day of the last RT fraction. The pCT-images were analyzed using the extended Kety model, quantifying tumor perfusion with the pharmacokinetic parameters K trans , v e , and v p . The mean and maximum FDG uptake based on the standardized uptake value (SUV) and transfer constant ( K trans ) within the tumor were correlated. Also, the tumor was subdivided into eight subregions and for each subregion the mean and maximum SUVs and K trans values were assessed and correlated. Furthermore, the mean FDG uptake in voxels presenting with the lowest 25% of perfusion was compared with the FDG uptake in the voxels with the 25% highest perfusion. Results The mean and maximum K trans values were positively correlated with the corresponding SUVs ( ρ = 0.596, p = 0.001 and ρ = 0.779, p K trans values and SUVs within the subregions (mean, ρ = 0.413, p ρ = 0.540, p p = 0.017). During hypofractionated radiotherapy, stable mean ( p = 0.379) and maximum ( p = 0.280) FDG uptake levels were found, whereas the mean ( p = 0.040) and maximum ( p = 0.003) K trans values were found to significantly increase. Conclusion Highly perfused rectal tumors presented with higher FDG-uptake levels compared with relatively low perfused tumors. Also, intratumor regions with a high FDG uptake demonstrated with higher levels of perfusion than regions with a relatively low FDG-uptake. Early after hypofractionated RT, stable FDG uptake levels were found, whereas tumor perfusion was found to significantly increase.