In radiotherapy planning, it is vital that the breathing patterns at simulation scan mimic treatment scenario as accurately as possible. A number of previous studies assumed that a patient has the same breathing trace shape at both simulation and treatment delivery. This is not always the case and this work was to evaluate the dosimetric effect of a change in breathing patterns between planning and delivery in lung volumetric modulated arc therapy (VMAT) stereotactic body radiation therapy (SBRT). A TrueBeam LINAC with 6MV flattened filter free (FFF) beams was used. A dynamic motion phantom with sinusoidal (reference) motion (20 mm diameter ‘tumour’; period = 3 s; amplitude = 30 mm) underwent a 4D-CT scan. A treatment plan was created using Eclipse version 15.1 and optimised according to St Luke’s protocol. The delivered plan was evaluated in five situations: using the same trace as simulation, using a modified trace which contained variabilities in amplitude (VA), using a modified trace which contained variabilities in period (VP), using a highly regular trace (RT), and finally using a highly irregular trace (IRT). Measurements were made with EBT-XD films and evaluated against convolved dose distribution using a gamma analysis of 3% dose difference and 2 mm distance to agreement. Results indicate that both sinusoidal and RT traces showed a high passing rate of more than 98%. However, there was more variability in passing rates for the irregular motion. Results and dosimetric analysis will be presented.
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