This study's purpose is the comparison of the automatically segmented 3D model to the reference manual segmentation, based on the Dice precision index. It is implemented by making parents' patients, surgeons and surgical helpers answer specific questions comparing 3D images to usual 2D images of the patient's tumor.

The investigator will inform the child and his parents during the pre-surgical consultation, and will collect their non-opposition to be included in the study at the latest on the day of the MRI examination or CT scan. Patients requiring emergency imaging will not be included.

Pelvic tumors will benefit from MRI imaging and renal tumors from MRI and/or CT scan.

A CT scan will be carried out for retroperitoneal tumors other than renal tumors (mainly neuroblastomas).

All of the above examinations are carried out as part of the usual treatment, in the month preceding the surgery.

Pre-operatively After the imaging has been carried out, the 2D images will be presented to the patient and his family before surgery as it is done routinely. The 3D image will be showed afterwards. The family will have the opportunity to ask questions which the surgeon will answer as usually done.

The specific questionnaire will be completed by the patient and his family at the end of the consultation and given to the research team.

Once the patient is included and the examinations have been carried out, the operating surgeons and their assistants (help No. 1 and 2) will look at the 2D images, followed by the 3D images secondly a few days before surgery. They will complete the specific questionnaire and give it to the research team.

Once the patient is included and the imaging examinations have been carried out, surgeons external to the service will be contacted by the research team to organize a remote review session. The 2D images will be presented to them first, followed by the 3D images on the visualization software, via a remote communication system with screen sharing. They will complete the specific questionnaire independently and a copy of the questionnaires will then be sent to the research team by email and the originals sent by post.

Intraoperatively:

The 3D model of the patient will be displayed in the operating room, and/or integrated into the robot's display console (for robot-assisted surgeries) during the surgical procedure. At the end of the surgery, the operating surgeon and his assistants will complete (independently) a questionnaire on the consistency of the 2D and 3D images with the anatomy identified during the procedure and on the help or not provided by the 3D model.

Inclusion Criteria:

Children:

• Children between 3 months and less than 18 years old
• Children with a pelvic tumor requiring an MRI for a possible surgical intervention
• Children with a retroperitoneal tumor requiring a CT scan or MRI with a view to surgical intervention
• Children with no contraindication for a CT scan and/or 3T MRI
• Children whose parents do not object to their participation in the study

Other participants:

• Operating surgeon agreeing to participate in the study
• Caregiver agreeing to participate in the study
• External surgeon agreeing to participate in the study

Exclusion Criteria:

Children :

• Contraindication to MRI: metallic ocular foreign body, pacemaker, mechanical heart valve, old vascular clips on cerebral aneurysm
• Need for an MRI under general anaesthesia
• Contraindication for a CT scan with injection: renal failure, allergy to iodinated contrast products
• Patients having participated in a therapeutic clinical trial involving a new molecule within 30 days before inclusion
• Emergency situation

Children with a pelvic or retroperitoneal tumor requiring an MRI or CT scan for a possible surgical intervention coming at the hospital for a consultation as part of their standard care.