The study aims at quantifying the registration error obtained in routine patients in different parts of the prostate and for operators with different experience in order to assess the average precision of the elastic MR-ultrasound fusion obtained by Koelis™ system.

Patients referred for placement of intraprostatic fiducials before radiotherapy for prostate cancer will be prospectively offered to enter the study.

The fiducials will be placed under transrectal ultrasound guidance, in the prostate apex, midgland and base, according to our routine procedure. A 3D Ultrasound acquisition of the prostate will be obtained at the end of the placement. As per our routine procedure, patients will undergo unenhanced prostate MRI to control the position of the fiducials. An elastic fusion of the MR images and the 3D ultrasound acquisitions will be retrospectively performed by operators of varying experience using the Koelis system. The fiducials (visible on MR and ultrasound images) will be used to quantify the registration error.

Inclusion Criteria:

• Age ≥ 18 years
• Patient requiring placement of intraprostatic fiducials
• No contra-indication for fiducials placement
• No contra -indication to MRI
• No contra -indication for intravenous injection of gadolinium chelates
• Patient affiliated to the social security system
• Patients who received oral and written information

Exclusion Criteria:

• Patient deprived of liberty as a result of a judicial or administrative decision.
• Patient under guardianship or curatorship.
• Any event preventing or prematurely interrupting the collection of assessment criteria would result in the premature exit of the subject.

Patients with prostate cancer referred for placement of intraprostatic fiducials before prostate radiotherapy