This is a dual arm, single centre, investigator initiated study to investigate the use of FAZA-PET in combination with MRI. FAZA is an investigational radiotracer used in PET scans. FAZA PET/MRI will be used to measure hypoxia in sarcoma tissues and will occur for:

Arm A: before neo-adjuvant radiation/chemotherapy treatment; Arm B: before surgery (optional).

After the FAZA PET/MRI scan, patients will be followed up via telephone, 48 hours after the scan, to see if there are any side effects due to FAZA.

Up to 50 patients enrolled in Arm B will receive pimonidazole approximately 16-20 hours before surgery.

Mutliple studies show that hypoxia in sarcoma has been consistently associated with poor treatment outcomes. Subsequently, there has been a study that shows inhibition of hypoxia-induced transcription factor (HIF-1 alpha), with radiation, was effective in disease treatment in pre-clinical models. This shows how important hypoxia was in playing a role in overall disease outcome and survival.

As a means of examining tumor features, such as hypoxia, hybrid PET/MR imaging combines the advantages of MRI's soft-tissue and contrast resolution with PET's functional metabolic capabilities. There have been few reports of using 18F-FDG in diagnostic PET/MRI. with the use of 18F-FAZA in this study, parameters such as diffusion, micro-perfusion, and contrast enhancement (as a surrogate marker for viable tumour tissue) can be simultaneously compared with the FAZA-derived pattern, in a single procedure.

The study is being conducted to measure the volume of hypoxic tumor in high-risk STS patients, using FAZA-PET/MRI, before and after neoadjuvant radiotherapy/chemotherapy.

Inclusion Criteria:

1. Age ≥ 18 years
2. Patients with either: high-risk extremity STS (g2-3, > 10 cm largest dimension, and leiomyosarcoma, synovial sarcoma, malignant peripheral nerve sheath tumour, or undifferentiated pleomorphic sarcoma), or: high-risk RPS (g2-3, and leiomyosarcoma or dedifferentiated liposarcoma)
3. Intention to treat using radiotherapy/chemotherapy/surgery according to the current treatment policies of the Sarcoma Site Group of PM
4. A negative serum pregnancy test within the two-week interval immediately prior to PET-MR imaging, for women of child-bearing age
5. Ability to provide written informed consent to participate in the study

Exclusion Criteria:

1. Previous radiotherapy to intended treatment volumes.
2. Previous systemic therapy
3. Active malignancy other than sarcoma
4. Unable to remain supine for at least 60 minutes
5. Pregnancy or breast feeding
6. Age less than 18 years old
7. Failure to provide written informed consent
8. Contraindication for MR as per current institutional guidelines.