The invention relates to a method of determining geometrical information about a medical treatment arrangement (1) comprising a rotatable treatment radiation source unit (2), wherein the method comprises:• attaching a phantom (M1) to a patient support (10) of the medical treatment arrangement (1),• attaching a calibration module (M2) to the rotatable treatment radiation source unit (2), so that the calibration module (M2) rotates together with the rotatable treatment radiation source unit (2) when the rotatable treatment radiation source unit (2) is rotated,• obtaining for each of a plurality of rotational positions of the rotatable treatment radiation source unit (2) at least one projection image of the phantom (M1) and of the calibration module (M2) by using an image detector (6) of the medical treatment arrangement (1), while at least a part of the calibration module (M2) is positioned in a radiation propagation zone between the rotatable treatment radiation source unit (2) and the image detector (6),• evaluating images obtained for the plurality of rotational positions with respect to coordinates of the calibration module (M2) in a coordinate system of the phantom (M1), thereby obtaining an evaluation result and• determining geometrical information about the medical treatment arrangement (1) from the evaluation result.
