[The improved determination of the dose rate in irregular photon fields]

A revised formula for calculation of dose per time or monitor unit ("dose rate") of irregular megavoltage photon beams is proposed. The formula considers additionally the shading by secondary shieldings of the plane source describing the scattered radiation in the head. Therefore, in addition to the primary output factor C(ac) as function of the equivalent square ac x ac of the collimator field size a secondary output factor S(as) is introduced denoted as function of the equivalent square as x as of the irregular field. S is determined using a satellite diaphragm formed of shielding blocks in combination with a totally opened collimator. In this definition, S includes both the effects of radiation scattered in tissue (phantom scatter factor PSF) and of missing photons from the plane source by the blocks (satellite diaphragm factor SAT). In the usual formula to calculate the "dose rate" (output) at reference point in depth t, PSF is replaced by S: D = D0 x C(ac) x [PSF(as)/PSF(ac)] x T x R(as,t). There T is representing the block tray factor and R(as,t) the tissue phantom ratio or relative depth dose, depending on the irradiation technique used. Thus the modified formula is given by D = D0 x C(ac) x [S(as)/S(ac)] x T x R(as,t). Measurements show that the revised formula provides an additional precision of up to 2% in the calculation of dose, depending on the type of accelerator respectively on the size of the plane source of scattered radiation.