Purpose: Recent clinical investigations have shown a strong correlation between pretreatment tumor hypoxia and poor response to radiotherapy. These observations raise questions about standard assumptions of tumor reoxygenation during radiotherapy, which has been poorly studied in human cancers. Positron emission tomography (PET) imaging of [F-18]fluoromisonidazole (FMISO) uptake allows noninvasive assessment of tumor hypoxia, and is amenable for repeated studies during fractionated radiotherapy to systematically evaluate changes in tumor oxygenation.
Methods and materials: Seven patients with locally advanced nonsmall cell lung cancers underwent sequential [F-18]FMISO PET imaging while receiving primary radiotherapy. Computed tomograms were used to calculate tumor volumes, define tumor extent for PET image analysis, and assist in PET image registration between serial studies. Fractional hypoxic volume (FHV) was calculated for each study as the percentage of pixels within the analyzed imaged tumor volume with a tumor:blood [F-18]FMISO ratio > or = 1.4 by 120 min after injection. Serial FHVs were compared for each patient.
Results: Pretreatment FHVs ranged from 20-84% (median 58%). Subsequent FHVs varied from 8-79% (median 29%) at midtreatment, and ranged from 3-65% (median 22%) by the end of radiotherapy. One patient had essentially no detectable residual tumor hypoxia by the end of radiation, while two others showed no apparent decrease in serial FHVs. There was no correlation between tumor size and pretreatment FHV.
Conclusions: Although there is a general tendency toward improved oxygenation in human tumors during fractionated radiotherapy, these changes are unpredictable and may be insufficient in extent and timing to overcome the negative effects of existing pretreatment hypoxia. Selection of patients for clinical trials addressing radioresistant hypoxic cancers can be appropriately achieved through single pretreatment evaluations of tumor hypoxia.