Reproducibility of intratumor distribution of (18)F-fluoromisonidazole in head and neck cancer.

PURPOSE: Hypoxia is one of the main causes of the failure to achieve local control using radiotherapy. This is due to the increased radioresistance of hypoxic cells. (18)F-fluoromisonidazole ((18)F-FMISO) positron emission tomography (PET) is a noninvasive imaging technique that can assist in the identification of intratumor regions of hypoxia. The aim of this study was to evaluate the reproducibility of (18)F-FMISO intratumor distribution using two pretreatment PET scans.

METHODS AND MATERIALS: We enrolled 20 head and neck cancer patients in this study. Of these, 6 were excluded from the analysis for technical reasons. All patients underwent an (18)F-fluorodeoxyglucose study, followed by two (18)F-FMISO studies 3 days apart. The hypoxic volumes were delineated according to a tumor/blood ratio >or=1.2. The (18)F-FMISO tracer distributions from the two (18)F-FMISO studies were co-registered on a voxel-by-voxel basis using the computed tomography images from the PET/computed tomography examinations. A correlation between the (18)F-FMISO intensities of the corresponding spatial voxels was derived.

RESULTS: A voxel-by-voxel analysis of the (18)F-FMISO distributions in the entire tumor volume showed a strong correlation in 71% of the patients. Restraining the correlation to putatively hypoxic zones reduced the number of patients exhibiting a strong correlation to 46%.

CONCLUSION: Variability in spatial uptake can occur between repeat (18)F-FMISO PET scans in patients with head and neck cancer. Blood data for one patient was not available. Of 13 patients, 6 had well-correlated intratumor distributions of (18)F-FMISO-suggestive of chronic hypoxia. More work is required to identify the underlying causes of changes in intratumor distribution before single-time-point (18)F-FMISO PET images can be used as the basis of hypoxia-targeting intensity-modulated radiotherapy.