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.