Output factors and dose calculations for blocked x-ray fields.

Output factors for blocked fields have been measured in a polystyrene phantom for four collimator field sizes and two blocking schemes using 6-MV x rays. For all measurements the phantom surface was at the calibration source-surface distance (SSD) because, as is shown, the calculation of dose to any point in a phantom at an arbitrary SSD can be expressed in terms of the output factor for the field size at the calibration distance. It is found that output factors are a function of both the surface field size of the blocked field and the collimator field size. Specifically, the output factor for a blocked field is less than that for the collimator field size used but greater than that for an unblocked field of the same surface field size formed by collimator settings only. A method is proposed for utilizing these data to calculate the output factor for any collimator and blocked field size. The validity of the method is checked by using it to calculate dose to a point in a phantom and comparing this to the measured dose.