Dosimetry limitations and pre-treatment dose profile correction for sliding window IMRT

Background: this work investigated the dosimetry limitations of the random and systematic uncertainties of sliding window (sw) intensity modulated radiation therapy (imrt). materials and methods: a varian 21ex linear accelerator, pinnacle3 treatment planning system and radiographic film dosimetry was used. the limitations of the sw were studied using beam modulation ranging from 2 to 100 mu/beam, dr from 100 to 600 mu min-1, lv from 1 to 5 cm s-1 and field size up to 12 × 12 cm2. the random and systematic errors were investigated using clinical and flat beams, as well as beams of high profile modulation including linear, exponential, and sinusoidal profiles. results: the leading edge and plateau of the sw profiles have a significant deformation for higher dr and for beams of < 10 mus/beam. it was found that the error is directly proportional to the dr and lv, and inversely propor- tional to the number of mu/beam. conclusion: the high dr and lv are limiting factors, producing random profile deformation when sw beams of small number of mu/beam are delivered. a very good agreement was found between the planned and delivered geometrical and clinical dose profiles when beams > 10 mus irradiated by a dr from 100 to 600 mu min-1 and lv from 1 to 5 cm s-1. after the proposed correction, an average difference < 0.5% for clinical profiles was measured for beams irradiated with dr = 600 mu min-1 and lv= 5 cm s-1. it was concluded that this correction methodology may serve as a pre-treatment quality assurance tool for sw imrt beams.