The initial staging of locally advanced prostate cancer is made with Computed Tomography scan (CT), Magnetic Resonance Imaging (MRI), and bone scan (BS).

For this type of cancer, reference treatment is radiotherapy combined with hormone therapy.

The added value of MRI in the delineation of volumes for radiotherapy is known, especially for the definition of extra-prostatic extensions and prostatic apex. However, its regular use is difficult. Indeed, acquisition of Magnetic Resonance Imaging parameters for diagnostic are not adapted to be fused with the planning Computed Tomography.

The literature shows that Positron Emission Tomography-Computed Tomography with Fluorocholine is better in terms of diagnostic performance compared to bone scan for bone metastases and to pelvic Magnetic Resonance Imaging for nodal extension.

It would therefore improve staging for these patients with a high risk of locoregional and metastatic invasion even if its use is currently not recommended in the initial staging.

Thanks to Magnetic Resonance Imaging acquisition parameters adapted to Radiotherapy and additional functional information, an acquisition in tri-modality Positron Emission Tomography/computed Tomography/ Magnetic Resonance Imaging could have an impact on the volumes definition for radiotherapy or even on the therapeutic strategy.

The aim of the study is to evaluate the volume modifications obtained on the trimodality evaluation, compared to the standard initial staging (geometric comparison).

In addition, it would be a preliminary study for a project using Prostate-specific membrane antigen (PSMA) in trimodality, and / or for a therapeutic interventional study.

Prostate cancer (Pca) is the first human cancer in terms of incidence and the third leading cause of tumor death (56,000 new cases/year in France in 2012 and nearly 9,000 deaths). According to the French Association of Urology 2013, an individual screening based on rectal examination and prostatic specific antigen (PSA) is recommended for men over 50. Ultrasound (US) guided biopsy allows the histological diagnosis and the Gleason score evaluation. For high-risk of prostate Cancer, initial staging is based on pelvic Magnetic Resonance Imaging, abdominal pelvic computed tomography and bone scan.

External radiotherapy combined with prolonged hormone therapy is the reference treatment for these cancers.

Radiotherapy requires a planning Computed Tomography. Magnetic Resonance Imaging is known to be better for definition of extra-prostatic extensions and prostatic apex. However, its regular use is difficult for the radiotherapy planning.

Moreover, Positron Emission Tomography-Computed Tomography with Fluorocholine detects earlier nodal and bone metastasis.

Hypothesis: A single planning acquisition in trimodality in radiotherapy position should improve simultaneously the initial staging and the volume delineation for radiotherapy.

Main objective: Main objective is to compare volumes delineation (prostatic target and organ at risk) thanks to geometric index (Jaccard, Dice and overlap), got by trimodality and by standard initial staging and planning Computed Tomography.

Expected results: Thanks to literature data, it can be expected that prostatic delineation target volume will significantly decrease with Magnetic Resonance Imaging.

Furthermore, thanks to Positron Emission Tomography-Computed Tomography with Fluorocholine, radiotherapy planning would be different, detecting 14% of nodal extension and 7% of bone metastasis.

Finally, with trimodality the investigators expect a smaller prostate volume and a higher detection of metastatic lymph node or bone metastasis.

Then, the investigators expect an improvement of the management of patients by:

• Detecting more nodal or bone metastasis and avoiding a useless local treatment by radiotherapy
• Improving prostatic target volume delineation and allowing a better protection of organ at risk.

Inclusion Criteria:

• More than 18 years
• prostate cancer : High risk (according to D'Amico Classification)
• and/or Node positive on MRI
• T2c-T3 on MRI
• with staging assessment
• will receive radiotherapy

Exclusion Criteria:

• second cancer
• contraindication to MRI
• participation in an another therapeutic trial
• patients under guardianship or curatorship