The effect of the focal distance parameter on the dose rate of radiotherapy equipment

: In radiotherapy it is sometimes necessary to use nonstandard treatment distances, i.e. in mantle-field irradiation or in total-body irradiation. The design of the collimating system as well as other components of the treatment head are influencing the dose rate depending on focus-patient respectively focus-phantom distance (FPD). The quality of the collimator and the dependence on FPD, which deviates from the inverse-square law, were described by a new defined parameter INV. This parameter INV was checked experimentally on various treatment units. For approximation and explanation of this INV factor a combination of a point, line and area source is proposed.