Portal dose image prediction for dosimetric treatment verification in radiotherapy. II. An algorithm for wedged beams

A method is presented for calculation of a two-dimensional function, T(wedge)(x,y), describing the transmission of a wedged photon beam through a patient. This in an extension of the method that we have published for open (nonwedged) fields [Med. Phys. 25, 830-840 (1998)]. Transmission functions for open fields are being used in our clinic for prediction of portal dose images (PDI, i.e., a dose distribution behind the patient in a plane normal to the beam axis), which are compared with PDIs measured with an electronic portal imaging device (EPID). The calculations are based on the planning CT scan of the patient and on the irradiation geometry as determined in the treatment planning process. Input data for the developed algorithm for wedged beams are derived from (the already available) measured input data set for transmission prediction in open beams, which is extended with only a limited set of measurements in the wedged beam. The method has been tested for a PDI plane at 160 cm from the focus, in agreement with the applied focus-to-detector distance of our fluoroscopic EPIDs. For low and high energy photon beams (6 and 23 MV) good agreement (approximately 1%) has been found between calculated and measured transmissions for a slab and a thorax phantom.