Dosimetric accuracy of the ITP inverse treatment planning system

The dosimetric accuracy of the ITP system for intensity modulated radiation therapy was determined for 19 patient plans. Treatments were given with a Varian 2100C and a 120-leaf multileaf collimator using step and shoot delivery. Both absolute dose, determined by an ionization chamber, and relative dose, determined by film, were assessed. It was found that absolute dose agreement was within 0.1% +/- 1.5%, isodoses in the high dose-low gradient region were within 1.7% +/- 2%, and the distance to agreement for isodoses between 20% and 90% was 1.4 mm +/- 1 mm. This agreement is at least as good as that found for standard wedged fields in most treatment planning systems.