Comparison of dosimetric characteristics of Siemens virtual and physical wedges

Dosimetric properties of Virtual Wedge (VW) and physical wedge (PW) in 6 an d 23 MV photon beams from a Siemens Primus linear accelerator, including we dge factors, depth doses, dose profiles, peripheral doses and surface doses , are compared. While there is a great difference in absolute values of wed ge factors, VW factors (VWFs) and PW factors (PWFs) have a similar trend as a function of field size. PWFs have a stronger depth dependence than VWF d ue to beam hardening in PW fields. VW dose profiles in the wedge direction, in general, match very well with PW, except in the toe area of large wedge angles with large held sizes. Dose profiles in the nonwedge direction show a significant reduction in PW fields due to off-axis beam softening and ob lique filtration. PW fields have significantly higher peripheral doses than open and VW fields. VW fields have similar surface doses as the open field s while PW fields have lower surface doses. Surface doses for both VW and P W increase with field size and slightly with wedge angle. For VW fields wit h wedge angles 45 degrees and less, the initial gap up to 3 cm is dosimetri cally acceptable when compared to dose profiles of PW. VW fields in general use less monitor units than PW fields. (C) 2000 American Association of Ph ysicists in Medicine. [S0094-2405(00)02710-3].