Stereotactic radiotherapy enables bone metastases to be treated with highly precise irradiation beams, enabling small targets to be irradiated. Planning requires the use of cross-sectional imaging such as computed tomography (CT) or magnetic resonance imaging (MRI). Bone scintigraphy with Tc99m-labelled biphosphonates (T1/2=6h) is indicated in the extension assessment of prostate cancers. Thanks to the advent of single-photon emission computed tomography (SPECT) based on CZT detectors; whole-body SPECT is now compatible with clinical constraints. We propose to study the value of using whole-body SPECT for planning stereotactic radiotherapy of metastatic prostate cancer. This is a prospective, single-center, non-randomized study involving 30 patients. Patients will benefit from routine examinations (CT scan, MRI) including a bone SPECT/CT in treatment position performed on the VERITON-CT (Spectrum Dynamics, Haifa, Israel). Planning based on whole-body SPECT images will be carried out remotely from the patient's point of care. The examination will be interpreted by the nuclear physician, and planning will be based on the dosimetric CT scanner, as in standard management. Virtual replanning will be carried out at distance from treatment, incorporating SPECT imaging. Treatment plans with and without SPECT will be compared quantitatively and qualitatively. The feasibility of SPECT imaging in the treatment position will be assessed, enabling lesion location imaging and dosimetric scanning to be performed in a single examination. All therapeutic management and clinical follow-up will be carried out as part of routine care.

This is a prospective, single-center, non-randomized study. After a decision in a multidisciplinary consultation meeting, the patient will be offered to join the study. All therapeutic care and clinical follow-up is carried out as part of routine care.

Patients will benefit from routine examinations (CT scan, MRI) including a SPECT/CT scan of the bone in the treatment position. Three hours after injection of 9 MBq/kg of 99mTc-HDP, CT imaging followed by whole-body SPECT/CT will be performed on the VERITON-CT (Spectrum Dynamics, Haifa, Israel). In order to make the images in the treatment position, the molding of the BodyFIX (Elekta) compression system that is used to reposition the patient between sessions will be made on the examination bed of the VERITON. It is a mattress that stiffens due to air vacuums. It will be placed on an external radiotherapy tray as for the dosimetry scanner. If it is not possible to take the images under these conditions, the patient will benefit from an examination in the standard position.

The images will then be interpreted by a nuclear physician who will identify the targets and define the contours from the SPECT/CT data using the segmentation tools available in the Syngo.via visualization software (Siemens Healthineers, Erlangen, Germany). The images and contours will be anonymized in order to allow blind virtual replanning, at least 6 months before the planning. Non-anonymized images alone will be transmitted as a standard examination, but cannot be incorporated into the treatment plan, as is currently the case.

The patient will benefit from a standard dosimetry scanner for treatment planning. The molding of the mattress made during the SPECT/CT scan of the bone will be reused for the dosimetric scanner. The therapeutic procedure will then follow the local protocol and in line with national recommendations. Whole-body SPECT/CT imaging will then be performed at 3 and 6 months on the conventional examination bed. The targets identified during the reference whole-body SPECT/CT will be reused in order to measure the evolution of the SUV quantification.

Inclusion Criteria:

• Age ≥ 18 years,
• Good general health WHO ≤ 1
• Informed and signed consent prior to any specific study procedure.
• Patient affiliated to the social security system
• Bone metastases from prostate cancer
• Indication for bone stereotactic radiotherapy

Exclusion Criteria:

• Presence of active cancer in the previous three years
• Protected adults (under guardianship or curatorship)
• Unable to undergo medical monitoring for geographical, social or psychological reasons
• Unable to decubitus (orthopnea, etc.),
• Hypersensitivity to HDP or to one of the excipients of the radiopharmaceutical.
• History of radiotherapy of the volume to be treated by stereotaxy.