Effect of amplitude of respiratory movements on dose distribution of static intensity-modulated radiotherapy

Abstract

Objective To study the effect of amplitude of respiratory movements on dose distribution of static intensity-modulated radiotherapy (IMRT), and to provide a basis for dose correction against respiratory movements. Methods A two-dimensional matrix driven by the QUASAR program-controlled respiratory movement instrument was used to simulate human respiratory movements in the head/foot direction. The dose distribution was evaluated on the isocenter plane with different amplitudes of respiratory movements. The Verisoft software and absolute dose analysis were used to analyze dose distribution, percentage errors of absolute dose, and passing rates of radiation field for both collected data and planned dose distribution. Results In spite of little effect on dose distribution in target volume, respiratory movements increased the dose outside the marginal target volume along the movement direction. When the respiratory amplitude was lower than 6 mm, the passing rate of γ distribution met the requirement of clinical dose verification; when the respiratory amplitude was larger than 8 mm, the passing rate of γ distribution was reduced with the increasing respiratory amplitude and fell below the clinical standard. Conclusions Respiratory movements have a blurring effect on static IMRT. It is possible to introduce some compensation methods to static IMRT in the treatment of tumor with periodic respiratory movements. The normal tissue at the edge of target volume along the respiratory movement direction is exposed to a higher radiation dose than expected. Therefore, the radiation dose for the normal tissue around target volume should be made as low as possible in the radiotherapy plan. For the patients with relatively large respiratory amplitude, action should be taken to lower respiratory amplitude and elevate the radiation dose in target volume. Key words: respiratory movement amplitude; static intensity modulated radiotherapy; dose distribution; QUASAR program-controlled respiratory movement instrument