Use of H(2) (15)O-PET for investigating perfusion changes in pelvic tumors due to regional hyperthermia

An increase in tumor oxygenation and perfusion due to hyperthermia has been reported for experimental tumors. The present study was performed to investigate this hypothesis in patients who underwent regional hyperthermia.

Twenty-seven patients with primary or recurrent pelvic tumors were included in this study. Prior to and up to 1 h after regional hyperthermia, perfusion and partition coefficient were quantitatively determined by utilizing H(2) (15)O-PET. First pass PET images were fused with the segmented common iliac artery from separately acquired CT scan. The arterial input function was extracted from the common iliac arteries using the dynamic PET images and the fused CT. The fused images were also used to extract tumor activity-time curves. Perfusion was calculated from the total tumor curves with correction for arterial spill-over. Changes in perfusion and partition coefficient were analyzed and correlated with various treatment parameters.

Heating under hyperthermia conditions significantly increased the partition coefficient for pelvic tumors (P = 0.005). The increase correlated with the duration of hyperthermia and was found in patients treated for more than 1 h and persisted for more than 1 h after the end. Significant changes in perfusion were not observed. Perfusion had recurred to initial values 20 min after heating.

The increase in partition coefficient reflects an increased diffusion distance of radio-labeled water. Therefore water diffusion is increased due to hyperthermia. Analogous to water diffusion, the diffusion of inert gases is also facilitated, improving the oxygenation of hypoxic tumor cells. Our results suggest that tumor oxygenation can probably be enhanced by regional hyperthermia for a period of more than 1 h after heating, provided hyperthermia is applied for at least 60 min. The effect was observed to be reversible within one week.