In radiotherapy, Magnetic Resonance Imaging (MRI) is used as a complement to the CT scanner because it provides better tissue contrast and therefore more precise delineations without the need for additional irradiation. However, MRI does not allow the definition of the electronic densities of the tissues necessary for dose calculation.

In this work, we sought to measure the feasibility of a method for generating CT-synthetic images from MRI in terms of dosimetric and geometrical precision for the purpose of MRI workflow alone (see diagram). The cerebral sphere and the prostate are the two tumor locations considered. All patients will have a planning CT (reference) and an MRI in the treatment position. The contours of the structures will be contoured by a radiotherapist on both the MRI and the reference CT. Synthetic CTs will be generated from the MRI with the method of automatic assignment of densities in five classes. The volumes bypassed and the dosimetries performed will be compared. A study of the quality of the images generated from MRI for dose calculation and images for the verification of per-treatment positioning will be performed.

This is a prospective single-center dosimetry study that exhaustively included brain tumor and prostate cancer patients treated at the ICO Site d'Angers by external radiotherapy and prior to which they were given an MRI and a simulation CT in treatment position over a 12-month period.

For patients who agree to participate in the study, CT-synthetic images will be generated from the MRI images by automatic assignment of densities (5 classes).

Reference CT images from CT and synthetic CT images from MRI will be available for comparison for all eligible patients.

Inclusion Criteria:

1. Age ≥ 18 years old
2. Male or Female with a (primary) brain tumour or Male with cancer and localized (non-metastatic) prostate cancer
3. Patient having benefited from a planning MRI scan in treatment position performed according to the new acquisition modalities (allowing the generation of CT-synthetics by automatic assignment of densities) performed since January 2020 and a planning scanner in treatment position prior to external radiotherapy.
4. Good general condition (SP < 3)

Exclusion Criteria:

1. Excluded forms of the disease,
2. MRI-specific contraindications (claustrophobia, foreign bodies)
3. Obesity preventing the placement of equipment (contention/antennae)
4. Poor general condition (SP > or = 2) preventing the patient from standing on the table -

Patient with prostate or brain cancer receiving an MRI in addition to the simulated CT scan prior to the start of their external radiotherapy treatment