The aim of this work is to analyze the dependence of the dose profile uncertainties for the sliding window IMRT (SW-IMRT) beams under the condition of an extreme dose rate (DR) and leaf velocity (LV). The deviations of the edges and plateau for the beam profiles of small number of MUs delivered using the dynamic MLC were studied. Field sizes with lengths of 5 and 10 cm were irradiated by photon beams of 2-8 MU/beam, DR = 100-600 MU/min and LS = 1-5 cm/s. Kodak TL radiographic films were used in the measurement. The photon beams (6 and 15 MV) were produced by a Varian 21EX Linac with a 120-leaf MLC. It is found that the MLC cannot keep the leaves moving with a proper speed continuously under a stable DR when beam of small MUs are irradiated. For example, the dynamic MLC needs 1.2 s and 12 MU to irradiate a field of 5 cm length with 2 MU using DR = 600 MU/min and LC = 5 cm/s. The plateau of the beam profile has several sinusoidal periods of about 150 ms. The magnitude of the plateau uncertainties was about 7% and 15% for the dose of the beam with DR = 400 and 600 MU/min (2 MU/beam), respectively. It is concluded that SW-IMRT beams of more than 10 MUs, delivered with 1 cm/s ⩽ LV ⩽ 5 cm/s and 100 MU/min ⩽ DR ⩽ 600 MU/min, have a good agreement between the delivered and planned dose profiles.
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