The present invention relates to a data processing method performed by a computer (7) for determining the position of a soft tissue body part within a patients body, which is to be treated by radiotherapy with a treatment beam arrangement of at least one position of a treatment beam issued by a treatment device (1), the data processing method comprising the following steps: a. acquiring CT-image data comprising information about the position of the body part within a coordinate system assigned to a transportable CT-device (3), wherein the patients body is positioned relative to the treatment device (1), and wherein the CT-device (3) is configured to be positioned relative to the patients body and/or relative to the treatment device (1 ) b. acquiring first transformation data comprising information about a first transformation between the coordinate system assigned to the CT- device (3) and a coordinate system assigned to the treatment device (1) c. determining, based on the CT-image data and the first transformation data, position data comprising information about the position of the body part within the coordinate system assigned to the treatment device (1 ), The present invention further relates to a corresponding computer (7) and system for determining the position of a soft tissue body part within a patients body.La présente invention concerne un procédé de traitement de données effectuée par un ordinateur (7) pour déterminer la position dune partie corporelle de tissu mou dans le corps dun patient, qui est destiné à être traitée par radiothérapie avec un agencement de faisceau de traitement dau moins une position dun faisceau de traitement émis par un dispositif de traitement (1), le procédé de traitement de données comprenant les étapes suivantes : a. acquisition des données dimage TDM comprenant des informations sur la position de la partie corporelle dans un système de coordonnées assigné à un dispositif TDM transportable (3), le corps du patient étant posit
