¹⁸F-EF5 PET-based Imageable Hypoxia Predicts Local Recurrence in Tumors Treated With Highly Conformal Radiation Therapy

Purpose: Tumor hypoxia contributes to radiation resistance. A noninvasive assessment of tumor hypoxia would be valuable for prognostication and possibly selection for hypoxia-targeted therapies. ¹⁸F-pentafluorinated etanidazole (¹⁸F-EF5) is a nitroimidazole derivative that has demonstrated promise as a positron emission tomography (PET) hypoxia imaging agent in preclinical and clinical studies. However, correlation of imageable hypoxia by ¹⁸F-EF5 PET with clinical outcomes after radiation therapy remains limited.

Methods and materials: Our study prospectively enrolled 28 patients undergoing radiation therapy for localized lung or other tumors to receive pretreatment ¹⁸F-EF5 PET imaging. Depending on the level of ¹⁸F-EF5 tumor uptake, patients underwent functional manipulation of tumor oxygenation with either carbogen breathing or oral dichloroacetate followed by repeated ¹⁸F-EF5 PET. The hypoxic subvolume of tumor was defined as the proportion of tumor voxels exhibiting higher ¹⁸F-EF5 uptake than the 95th percentile of ¹⁸F-EF5 uptake in the blood pool. Tumors with a hypoxic subvolume ≥ 10% on baseline ¹⁸F-EF5 PET imaging were classified as hypoxic by imaging. A Cox model was used to assess the correlation between imageable hypoxia and clinical outcomes after treatment.

Results: At baseline, imageable hypoxia was demonstrated in 43% of all patients (12 of 28), including 6 of 16 patients with early-stage non-small cell lung cancer treated with stereotactic ablative radiation therapy and 6 of 12 patients with other cancers. Carbogen breathing was significantly associated with decreased imageable hypoxia, while dichloroacetate did not result in a significant change under our protocol conditions. Tumors with imageable hypoxia had a higher incidence of local recurrence at 12 months (30%) than those without (0%) (P < .01).

Conclusions: Noninvasive hypoxia imaging by ¹⁸F-EF5 PET identified imageable hypoxia in about 40% of tumors in our study population. Local tumor recurrence after highly conformal radiation therapy was higher in tumors with imageable hypoxia.