The present invention relates to a patient immobilisation device for positioning a patient in the prone position for breast irradiation, comprisinga cephalic module for supporting the head and upper extremities of the patient,a thoracic module for supporting the patient thorax, and shaped to allow at least one breast to extend below the thoracic module, anda caudal module for supporting the pelvis and lower extremities of the patient,wherein the cephalic module is optionally detachable and securable to the thoracic module wherein the thoracic module is optionally detachable and securable to the caudal module.