Method at a radiation therapy system

A method of calibrating a positioning system in a radiation therapy system, the system comprising a radiation therapy unit having a fixed radiation focus point;

• a fixation unit for immobilizing a treatment volume in a patient in relation to the fixation unit;
• and
• a positioning system for positioning a treatment volume in a patient in relation to said fixed focus point in the radiation therapy unit, the positioning system comprising a fixed framework for rigid engagement with the radiation therapy unit, a movable carriage for carrying and moving the entire patient, at least one motor for providing movement of the carriage along three substantially orthogonal motional axes, a control system for controlling said motor(s), and at least one engagement point for releasably mounting said fixation unit in fixed engagement with the positioning system;
• the method comprising the steps of:
• a) providing a fixation unit coordinate system defined in relation to the fixation unit, said fixation unit coordinate system being arranged for defining a treatment volume;
• b) determining linearity errors for the motional axes of the carriage and angular offset between said motional axes and said fixation unit coordinate system, whereby the relationships between the axes of said fixation unit coordinate system and the motional axes of the positioning system are determined;
• c) mounting the positioning system in fixed relationship with the radiation therapy unit;
• and
• d) determining the radiation focus point of the radiation therapy unit in relation to the patient positioning system, thereby also determining the relationship between said focus point and said fixation unit coordinate system.