Purpose/Objective: Volumetric Modulated Arc Therapy (VMAT) is increasingly popular as a treatment method in radiotherapy due to the speed with which treatments can be delivered. However, there has been little investigation into the effect of increased modulation in lung plans with regard to inter-fraction organ motion. This is most likely to occur where the planning target volume (PTV) lies within areas of low density. This study aims to investigate the effect of modulation on the dose

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... ion using simulated patient movement, and to propose a method which is less susceptible to such movement. Materials and Methods: Simulated inter-fraction motion is achieved by moving the plan isocentre in steps of 0.5cm and 1.0cm in 6 directions, for 5 clinical VMAT patients. The proposed planning method involves optimising using a density override to 1g/cm 3 , within the PTV in lung, to reduce segments boosting the periphery of the PTV. Results: This investigation shows that modulation can result in an increase in maximum dose of more than 25%, an increase in PTV D2 of 17% and a reduction in D98 by 46%. Unacceptable organ at risk (OAR) doses are also seen. The proposed method reduces modulation, resulting in a maximum dose increase of 10%. Conclusions: Whilst safeguards are in place to prevent increased dose to OARs from patient movement, there is nothing to prevent increased dose as a result of modulation in lung. A simple planning method is proposed to safeguard against this effect. Investigation suggests that where modulation exists in a plan this method reduces it and is clinically viable.