Calculation of head scatter factors at isocenter or at center of field forany arbitrary jaw setting

The purpose of this work is to calculate the head scatter factors for any a rbitrary jaw setting by using two different semi-empirical methods. The hea d scatter factor at the center of field (COF) for any arbitrary jaw setting can be defined as H-COF(X-1, X-2, Y-1, Y-2, r) = D-COF(air)(X-1, X-2, Y-1, Y-2, r)/[D-air(5,5,5,5,0)* OAR(r)], where X-1, X-2, Y-1, and Y-2 are the j aw positions; r is the distance between COF and isocenter (IC); OAR(r) is t he Off-Axis-Ratio; D-COF(air)(X-1, X-2, Y-1, Y-2 r) is the dose in air meas ured at COF; D-air(5,5,5,5,0) is the dose in air measured at IC for the 10 X 10 cm(2) field. In certain clinical situations, doses are prescribed at I C instead of COF for asymmetric fields. In these cases, head scatter factor s should be determined at IC. It is found that the head scatter factors at IC for asymmetric fields [H-IC(X-1, X-2, Y-1, Y-2)] are lower than H-COF(X- 1, X-2, Y-1, Y-2, r) for the same jaw setting by up to 4%. The values of H- IC(X-1, X-2, Y-1, Y-2) and H-COF(X-1, X-2, Y-1, Y-2, r) for a variety of ja w 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 th e source plane for any jaw setting, was used to calculate H-COF(X-1, X-2, Y -1, Y-2, r). The calculation and the measurement agree within +/-1% (+/-0.5 % for most clinical situations). To calculate H-IC(X-1, X-2, Y-1, Y-2), we have generalized the Day's ''quarter-field" method, i.e., H-IC(X-1, X-2, Y- 1, Y-2) = [H(X-1, X-1, Y-1, Y-1) + H(X-1, X-1, Y-2, Y-2) + H(X-2, X-2, Y-1, Y-1) + H(X-2, X-2, Y-2, Y-2)]/4. We found that the calculation and the mea surement agree within +/-0.8% for the beams studied. (C) 1999 American Asso ciation of Physicists in Medicine. [S0094-2405(99)00804-4].