DOSIMETRY OF WEDGED FIELDS WITH ASYMMETRIC COLLIMATION

Shaping of wedged fields with asymmetric collimators (independent jaws) produces dosimetric effects which need to be taken into account when calculating isodose distributions and accelerator monitor units. These effects were studied using ion chamber dosimetry in a water phantom. Wedged fields of 30-degrees to 60-degrees wedge angles were shaped by an asymmetric collimator. A system of dose calculation was developed to take into account the changes in dose in the open portion of the field. The formalism utilizes regular symmetric field parameters (e.g., S(c), S(p), and TPR). Calculated point doses and profiles agreed well with the measured data.