document: A three-source model for the calculation of head scatter factors Accurate determination of the head scatter factor S/sub c / is an important issue , especially for intensity modulated radiation therapy , where the segmented fields are often very irregular and much less than the collimator jaw settings . In this work , we report an S/sub c / calculation algorithm for symmetric , asymmetric , and irregular open fields shaped by the tertiary collimator -LRB- a multileaf collimator or blocks -RRB- at different source-to-chamber distance . The algorithm was based on a three-source model , in which the photon radiation to the point of calculation was treated as if it originated from three effective sources : one source for the primary photons from the target and two extra-focal photon sources for the scattered photons from the primary collimator and the flattening filter , respectively . The field mapping method proposed by Kim et al. -LSB- Phys . Med . Biol . 43 , 1593-1604 -LRB- 1998 -RRB- -RSB- was extended to two extra-focal source planes and the scatter contributions were integrated over the projected areas -LRB- determined by the detector 's eye view -RRB- in the three source planes considering the source intensity distributions . The algorithm was implemented using Microsoft Visual C/C + + in the MS Windows environment . The only input data required were head scatter factors for symmetric square fields , which are normally acquired during machine commissioning . A large number of different fields were used to evaluate the algorithm and the results were compared with measurements . We found that most of the calculated S/sub c / 's agreed with the measured values to within 0.4 % . The algorithm can also be easily applied to deal with irregular fields shaped by a multileaf collimator that replaces the upper or lower collimator jaws
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extractive_keyphrases: head scatter factors intensity modulated radiation therapy segmented fields collimator jaw settings calculation algorithm symmetric fields asymmetric tertiary collimator multileaf collimator blocks photon radiation target scattered photons primary collimator flattening filter field mapping method source intensity distributions ms windows environment input data machine commissioning lower collimator jaws
abstractive_keyphrases: three-source model irregular open fields source-to-chamber distance extra-focal photon sources extra-focal source planes symmetric square fields upper collimator jaws