Evaluation of treatment planning system monitor unit calculations for three intensity modulated radiotherapy delivery techniques

ABSTRACT

We have validated the monitor unit calculations from a commercially available treatment planning system [TPS] for three intensity modulated radiotherapy [IMRT] planning techniques for tangential breast irradiation by using ionization chamber measurements. Treatment plans were generated for forty-two breast patients by a forward planned field in field technique, electronic tissue compensation [ETC], and an inverse planned sliding window technique. We also performed a reproducibility of delivery and dose linearity analysis for each technique. The treatments were delivered to a phantom using a Varian CL21EX linear accelerator. A 2571 0.6 cm3 Farmer type ionization chamber and Farmer 2570/1 electrometer from NE Technology was used to measure output of the linear accelerator and the dose at predefined point in the verification plan. The agreement between the measured and calculated dose was-0.87% +/- 0.54% for field in field technique,-0.74% +/- 0.23% for electronic tissue compensators, and-1.26% +/- 0.48% for the inverse planning technique and. In terms of reproducibility the mean deviation was-1.10% +/- 0.44% for the field in field technique,-0.38% +/- 0.42% for electronic tissue compensators,-1.04% +/- 0.42% for inverse planning technique. Dose linearity experiments showed no significant variations for clinical situations but a breakdown was observed in relative dose for very low monitor units. We have found that the monitor unit calculations for all three planning techniques are correct to the order of 1%, and that the plans can be delivered in a reproducible and accurate manner