Dependence of FDG uptake on tumor microenvironment

FDG--a marker of tumour hypoxia? A comparison with [18F]fluoromisonidazole and pO2-polarography in metastatic head and neck cancer

PURPOSE

Experimental data suggest that the accumulation of [(18)F]fluorodeoxyglucose (FDG) in malignant tumours is related to regional hypoxia. The aim of this study was to evaluate the clinical potential of FDG positron emission tomography (PET) to assess tumour hypoxia in comparison with [(18)F]fluoromisonidazole (FMISO) PET and pO(2)-polarography.

METHODS

Twenty-four patients with head and neck malignancies underwent FDG PET, FMISO PET, and pO(2)-polarography within 1 week. Parameters of pO(2)-polarography were the relative frequency of pO(2) readings <or=2.5 mmHg, <or=5 mmHg and <or=10 mmHg, respectively, as well as the mean and median pO(2).

RESULTS

We observed a moderate correlation of the maximum standardised uptake value (SUV) of FDG with the tumour to blood ratio of FMISO at 2 h (R=0.53, p<0.05). However, SUV of FDG was similar in hypoxic and normoxic tumours as defined by pO(2)-polarography (6.9+/-3.2 vs 6.2+/-3.0, NS), and the FDG uptake was not correlated with the results of pO(2)-polarography. The retention of FMISO was significantly higher in hypoxic tumours than in normoxic tumours (tumour to muscle ratio at 2 h: 1.8+/-0.4 vs 1.4+/-0.1, p<0.05), and the FMISO tumour to muscle ratio showed a strong correlation with the frequency of pO(2) readings <or=5 mmHg (R=0.80, p<0.001).

CONCLUSION

These results support the hypothesis that tumour hypoxia has an effect on glucose metabolism. However, other factors affecting FDG uptake may be more predominant in chronic hypoxia, and thus FDG PET cannot reliably differentiate hypoxic from normoxic tumours.

Effects of hyperoxygenation on FDG-uptake in head-and-neck cancer

PURPOSE

Tumor hyperoxygenation results in high response rates to ARCON (accelerated radiotherapy with carbogen and nicotinamide). The effect of hyperoxygenation on tumor metabolism using [(18)F]fluorodeoxyglucose (FDG) positron emission tomography (PET) was investigated.

METHODS

Within one week, FDG-PET was performed without and with hyperoxygenation by carbogen breathing and/or nicotinamide administration in 22 patients, eligible for ARCON for head-and-neck cancer. Maximum standardized uptake values (SUV(max)) in both scans and the relative change were calculated in the primary tumor and in normal muscle.

RESULTS

Alteration of the tumor oxygenation state induced profound, but variable, metabolic changes (median DeltaSUV(max) -4%; range -61% to +30%). Metabolism in normal muscle was not affected. In three patients who did not achieve local tumor control, the SUV(max) after hyperoxygenation differed less than 5% change as compared to baseline, whereas 13 of the 16 patients with local tumor control showed a larger difference (p<0.05).

CONCLUSION

Given the heterogeneous response pattern of nicotinamide and carbogen on FDG-uptake in head-and-neck carcinoma, the prognostic significance of semiquantitative FDG-PET before and after hyperoxygenation remains uncertain and requires confirmation in larger clinical studies before introducing the procedure as a predictive tool for oxygenation modifying treatments.

[18F]fluoro-deoxy-glucose positron emission tomography ([18F]FDG-PET) voxel intensity-based intensity-modulated radiation therapy (IMRT) for head and neck cancer

BACKGROUND AND PURPOSE

Focused dose escalation may improve local control in head and neck cancer. Planning results of [(18)F]fluoro-deoxy-glucose positron emission tomography ([(18)F]FDG-PET) voxel intensity-based intensity-modulated radiation therapy (IMRT) were compared with those of PET contour-based IMRT.

PATIENTS AND METHODS

PET contour-based IMRT aims to deliver a homogeneous boost dose to a PET-based subvolume of the planning target volume (PTV), called PTV(PET). The present PET voxel intensity-based planning study aims to prescribe the boost dose directly as a function of PET voxel intensity values, while leaving the dose distribution outside the PTV unchanged. Two escalation steps (2.5 and 3 Gy/fraction) were performed for 15 patients.

RESULTS

PTV(PET) was irradiated with a homogeneous dose in the contour-based approach. In the voxel intensity-based approach, one or more sharp dose peaks were created inside the PTV, following the distribution of PET voxel intensity values.

CONCLUSIONS

While PET voxel intensity-based IMRT had a large effect on the dose distribution within the PTV, only small effects were observed on the dose distribution outside this PTV and on the dose delivered to the organs at risk. Therefore both methods are alternatives for boosting subvolumes inside a selected PTV.

The influence of tumor oxygenation on (18)F-FDG (fluorine-18 deoxyglucose) uptake: a mouse study using positron emission tomography (PET)

Background

This study investigated whether changing a tumor's oxygenation would alter tumor metabolism, and thus uptake of ¹⁸F-FDG (fluorine-18 deoxyglucose), a marker for glucose metabolism using positron emission tomography (PET).

Results

Tumor-bearing mice (squamous cell carcinoma) maintained at 37°C were studied while breathing either normal air or carbogen (95% O₂, 5% CO₂), known to significantly oxygenate tumors. Tumor activity was measured within an automatically determined volume of interest (VOI). Activity was corrected for the arterial input function as estimated from image and blood-derived data. Tumor FDG uptake was initially evaluated for tumor-bearing animals breathing only air (2 animals) or only carbogen (2 animals). Subsequently, 5 animals were studied using two sequential ¹⁸F-FDG injections administered to the same tumor-bearing mouse, 60 min apart; the first injection on one gas (air or carbogen) and the second on the other gas. When examining the entire tumor VOI, there was no significant difference of ¹⁸F-FDG uptake between mice breathing either air or carbogen (i.e. air/carbogen ratio near unity). However, when only the highest ¹⁸F-FDG uptake regions of the tumor were considered (small VOIs), there was a modest (21%), but significant increase in the air/carbogen ratio suggesting that in these potentially most hypoxic regions of the tumor, ¹⁸F-FDG uptake and hence glucose metabolism, may be reduced by increasing tumor oxygenation.

Conclusion

Tumor ¹⁸F-FDG uptake may be reduced by increases in tumor oxygenation and thus may provide a means to further enhance ¹⁸F-FDG functional imaging.