Quantitative MRI to Characterize Hypoxic Tumors in Comparison to FMISO PET/CT for Radiotherapy in Oropharynx Cancers

Simple Summary The definition of tumor hypoxia is important in oncology because this characteristic is linked to a poor prognosis but remains debated because there are no reference modalities. In this context, we compared PET hypoxia (FMISO) and MRI data before surgery to determine the hypoxic volume at which to increase the radiotherapy dose in head and neck cancers. To our knowledge, this is the first study showing the value of combining tumor volumes obtained via PET and MRI to define the hypoxic lesion subvolume. The quantitative MRI parameters ADC, T1 mapping, and T2 mapping showed differences between hypoxic and normoxic volumes. Intratumoral hypoxia is associated with a poor prognosis and poor response to treatment in head and neck cancers. Its identification would allow for increasing the radiation dose to hypoxic tumor subvolumes. 18F-FMISO PET imaging is the gold standard; however, quantitative multiparametric MRI could show the presence of intratumoral hypoxia. Thus, 16 patients were prospectively included and underwent 18F-FDG PET/CT, 18F-FMISO PET/CT, and multiparametric quantitative MRI (DCE, diffusion and relaxometry T1 and T2 techniques) in the same position before treatment. PET and MRI sub-volumes were segmented and classified as hypoxic or non-hypoxic volumes to compare quantitative MRI parameters between normoxic and hypoxic volumes. In total, 13 patients had hypoxic lesions. The Dice, Jaccard, and overlap fraction similarity indices were 0.43, 0.28, and 0.71, respectively, between the FDG PET and MRI-measured lesion volumes, showing that the FDG PET tumor volume is partially contained within the MRI tumor volume. The results showed significant differences in the parameters of SUV in FDG and FMISO PET between patients with and without measurable hypoxic lesions. The quantitative MRI parameters of ADC, T1 max mapping and T2 max mapping were different between hypoxic and normoxic subvolumes. Quantitative MRI, based on free water diffusion and T1 and T2 mapping, seems to be able to identify intra-tumoral hypoxic sub-volumes for additional radiotherapy doses.

Automated summary

This study demonstrates the value of combining PET and MRI to define hypoxic subvolumes and determine the radiotherapy dose in head and neck cancers. Hypoxic subvolumes are associated with poor prognosis and response to treatment. Quantitative MRI parameters show differences between hypoxic and normoxic volumes, suggesting the ability to identify hypoxic subvolumes for additional radiotherapy doses.