document: 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/sub wedge / -LRB- x , y -RRB- , describing the transmission of a wedged photon beam through a patient . This in an extension of the method that we have published for open -LRB- nonwedged -RRB- fields -LSB- Med . Phys . 25 , 830-840 -LRB- 1998 -RRB- -RSB- . Transmission functions for open fields are being used in our clinic for prediction of portal dose images -LRB- PDI , i.e. , a dose distribution behind the patient in a plane normal to the beam axis -RRB- , which are compared with PDIs measured with an electronic portal imaging device -LRB- EPID -RRB- . 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 -LRB- the already available -RRB- 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 -LRB- 6 and 23 MV -RRB- good agreement -LRB- ~ 1 % -RRB- has been found between calculated and measured transmissions for a slab and a thorax phantom
doc_bio_tags: B I I I O B I I O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I I O O O B I O O O O O O O O O O O O O O O O O O O O O B I
extractive_keyphrases: portal dose image prediction dosimetric treatment verification radiotherapy wedged photon beam planning ct scan irradiation geometry high energy photon beams thorax phantom open beams 23 mv
abstractive_keyphrases: transmission dosimetry wedged beams algorithm two-dimensional function low energy photon beams slab phantom in vivo dosimetry electronic portal imaging devices fluoroscopic ccd camera pencil beam algorithm cadplan planning system virtual wedges 6 mv