A new margin concept to compensate for breathing motion during stereotactic treatment of lung tumours

In radiotherapy, dose distributions are optimized patient individually to ensure high dose to the tumour while preserving the surrounding healthy tissue as much as possible. Treatment planning and dose calculation are performed using static image data sets of the patient. Due to breathing, lung tumours are subject to regular movements. The magnitude of motion affects the applied dose to the tumour leading to dose blurring and underdosage in the tumour region. To compensate for motion during treatment, margins around the tumour are applied to account for motion induced effects on dose distributions. By applying the margin, the irradiated volume is increased to ensure that the tumour is irradiated even under motion.In this work, the compensation of motion by applied margins is methodically analysed based on dose calculations using the treatment planning system. This is done for stereotactic treatments of lung tumours using the 3D conformal treatment planning technique. A new optimized margin concept is developed leading to a margin formula to calculate an optimized margin for motion compensation. The optimized margin concept is different from existing margin concepts by taking into account tumour size in motion direction beside motion amplitude as parameters having an impact on the margin. The optimized margin concept is evaluated by measurements using an in-house developed robotic 4D motion phantom and film measurements. Due to this measurements it is demonstrated that the optimized margin compensates for motion induced effects on applied dose distributions. The dose to the tumour is ensured by preserving the surrounding tissue at the same time due to highly conformal dose distributions