Calculation of head scatter factors at isocenter or at center of field for any arbitrary jaw setting.
The purpose of this work is to calculate the head scatter factors for any arbitrary jaw setting by using two different semi-empirical methods. The head scatter factor at the center of field (COF) for any arbitrary jaw setting can be defined as H(COF)(X1,X2,Y1,Y2,r)=DairCOF(XI1,X2,Y1,Y2,r)/ [Dair(5,5,5,5,0)*OAR(r)], where X1, X2, Y1, and Y2 are the jaw positions; r is the distance between COF and isocenter (IC); OAR(r) is the Off-Axis-Ratio; DairCOF(X1,X2,Y1,Y2,r) is the dose in air measured at COF; Dair(5,5,5,5,0) is the dose in air measured at IC for the 10 x 10 cm2 field. In certain clinical situations, doses are prescribed at IC instead of COF for asymmetric fields. In these cases, head scatter factors should be determined at IC. It is found that the head scatter factors at IC for asymmetric fields [H(IC)(X1,X2,Y1,Y2)] are lower than H(COF)(X1,X2,Y1,Y2,r) for the same jaw setting by up to 4%. The values of H(IC)(X1,X2,Y1,Y2) and H(COF)(X1,X2,Y1,Y2,r) for a variety of jaw settings were measured using a miniphantom of 3-cm diameter for a 6- and a 18-MV photon beams. An equivalent square formula, derived presently at the source plane for any jaw setting, was used to calculate H(COF)(X1,X2,Y1,Y2,r). The calculation and the measurement agree within +/-1% (+/-0.5% for most clinical situations). To calculate H(IC)(X1,X2,Y1,Y2), we have generalized the Day's "quarter-field" method, i.e., H(IC)(X1,X2,Y1,Y2) = [H(X1,X1,Y1,Y1) + H(X1,X1,Y2,Y2) + H(X2,X2,Y1,Y1) + H(X2,X2,Y2,Y2)]/4. We found that the calculation and the measurement agree within +/-0.8% for the beams studied.
Shih
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Department of Medical Physics, Rush-Presbyterian--St. Luke's Medical Center, Chicago, Illinois 60612, USA. rshih@therad.rpslmc.edu
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D002138:Calibration; D006257:Head; D006801:Humans; D007568:Jaw; D019047:Phantoms, Imaging; D017785:Photons; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012542:Scattering, Radiation