Hypoxic resistance to radiation therapy has been known for over a century. However, effective image-guided approaches to target resistant hypoxic tumor regions have been lacking. A recent publication of the results from a Phase II clinical trial in France indicates that positron emission tomography (PET) using 18F-fluoro- misonidazole (FMISO) to define region-of-interest (ROI) for hypoxic tumor targeting was shown to fail in improving tumor control and treatment outcome. The long-term objective of this proposed research project is to develop novel integrated multi-modality imaging approaches to effectively guide radiation delivery for significantly improving therapy precision and treatment outcome of resistant hypoxic tumor regions. Our recent work in animal studies using electron paramagnetic resonance (EPR) images (EPRI) of absolute pO2 has demonstrated that targeting the hypoxic regions of tumors with extra radiation dose (i.e., boost in dose painting) increases tumor cure. This involved novel 3D rapidly printed radiation blocks and conformal animal radiation. We showed improved tumor cure by comparing uniform radiation delivery of the dose to all tumors sufficient to cure 15% of tumors (determined in separate experiments) and then randomized to receive extra doses of radiation to either (1) all hypoxic tumor volumes determined by the EPR pO2 image (pO2 < 10 torr) or (2) equal volume dose boosts to better-oxygenated tumor. The results showed that treatment (1) offered a significantly better outcome, including sparing critical organs from damage caused by high dose radiation. This demonstrates that EPR pO2 images have the potential to guide improved radiation therapy of hypoxic tumors. Unfortunately, EPR images are currently not available for routine uses in clinical practice. We hypothesize that using EPRI pO2 images as the gold standard, novel quantitative hypoxia parametric imaging methodologies based on PET-FMISO data can be established, incorporating consideration and correlation of data from other clinically available hypoxia-related imaging methods such as dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) and Iodopamidol diamagnetic chemical exchange saturation transfer (Idia-CEST or ICEST) pH MRI. These clinically available MRI studies will sharpen the hypoxic tumor region definition for more effective radiation boost delivery in improving treatment outcomes. We will initially pursue the following specific aims in animal studies: (1) Implementing and validating novel quantitative multi-modality PET/MR/EPR imaging methodologies; (2) Developing statistical methodologies for deriving modified parametric images by integrating multi-modality PET and MRI data in order to emulate EPR images; (3) Employing the established methods developed in Aim (2) for validation in delivering improved precision radiotherapy of hypoxic tumors to achieve better treatment outcomes using multi-modality parametric imaging.
Recent electron paramagnetic resonance (EPR) images (EPRI) of absolute pO2 in murine tumors have shown for the first time significantly improved tumor cure when hypoxic regions were specifically targeted for dose boosts when compared to dose boosts to equal volumes of well oxygenated tumor. For translation to routine clinical use of this radiotherapy strategy, we will use preclinical EPR pO2 images as a ground truth, to develop novel methods to derive a signature from a suite of magnetic resonance (MR) images of several tissue physiologic factors, available for human use, to sharpen the definition of tumor hypoxia delineated by positron emission tomography (PET) images using a hypoxia radiotracer, 18F-fuoronitroimidazole (FMISO). The resulting modified PET-FMISO images can more accurately represent and emulate EPRI pO2 images, which can be used in turn in image-guided radiation therapy of hypoxia tumors with improved delivery and effectiveness.
