A method for estimating organ coordinates of at least a part of an organ which may move and/or change shape due to breathing in an associated patient, the method comprising: scanning the thorax of the patient to obtain CT three-dimensional data of internal structures of the thorax of said associated patient at a first point in time with a CT-scanner, wherein during this scanning step the abdomen is not imaged together with the thorax, obtaining a biomechanical model of said associated patient, said biomechanical model being based on the CT three-dimensional data of internal structures of the thorax and on surface coordinates of at least a part of the surface of the thorax and/or at least a part of the surface of the abdomen of said associated patient at the first point in time, using a surface determining unit comprising any one of a surface scanner, a fiber optic shape sensing garment, and/ or a plurality of cameras enabling photogrammetry to obtain surface coordinates indicative of a position and shape at a second point in time of at least a part of the surface of the thorax of said associated patient, and at least a part of the surface of the abdomen of said associated patient so as to enable estimating an external abdominal volume, obtaining the organ coordinates, by inputting said surface coordinates in said biomechanical model, and outputting from said biomechanical model the organ coordinates, wherein said biomechanical model models at least a part of an internal abdominal volume, and tissue of the chest wall of said patient, as incompressible. scanning the thorax of the patient to obtain CT three-dimensional data of internal structures of the thorax of said associated patient at a first point in time with a CT-scanner, wherein during this scanning step the abdomen is not imaged together with the thorax, obtaining a biomechanical model of said associated patient, said biomechanical model being based on the CT three-dimensional data of internal structures of the thorax and on surface coordinates of at least a part of the surface of the thorax and/or at least a part of the surface of the abdomen of said associated patient at the first point in time, using a surface determining unit comprising any one of a surface scanner, a fiber optic shape sensing garment, and/ or a plurality of cameras enabling photogrammetry to obtain surface coordinates indicative of a position and shape at a second point in time of at least a part of the surface of the thorax of said associated patient, and at least a part of the surface of the abdomen of said associated patient so as to enable estimating an external abdominal volume, obtaining the organ coordinates, by inputting said surface coordinates in said biomechanical model, and outputting from said biomechanical model the organ coordinates, inputting said surface coordinates in said biomechanical model, and outputting from said biomechanical model the organ coordinates, wherein said biomechanical model models at least a part of an internal abdominal volume, and tissue of the chest wall of said patient, as incompressible. at least a part of an internal abdominal volume, and tissue of the chest wall of said patient, as incompressible.