Method for determining a patient specific locally varying margin

Abstract

It is an object of the invention to provide for an improved radiation treatment. According to a first aspect of the invention, this object is achieved by a method for determining a patient specific locally varying margin on a treatment target and/or an organ at risk in order to compensate for local intrafraction motion expected during a radiotherapy fraction to be delivered over a radiotherapy fraction time interval. The patient specific locally varying margin is determined based on a displacement and/or an estimate of the displacement of at least a first location and a second location on the treatment target and/or organ at risk. The method comprises steps of: acquiring a first medical image of the treatment target and/or the organ at risk and acquiring a second medical image of the treatment target and/or the organ at risk, wherein the time between the acquisition of the first medical image and the second medical image is similar to the radiotherapy fraction time interval and determining of positions of the first location and the second location in the first medical image and in the second medical image and using the determined positions for determining the patient specific locally varying margin around the treatment target and/or organ at risk.