SU‐E‐T‐365: Respiratory Motion‐Induced Dose Uncertainty in Step‐And‐Shoot IMRT

Purpose: The objective of this study was to quantify respiratory motion induced dose uncertainty at the planning stage for step and shoot intensity modulated radiation therapy (IMRT) using a novel analytical technique. Methods: Ten patients with stage II/III lung cancer who had undergone a planning 4DCT scan and step and shoot IMRT planning were selected with a mix of motion and tumor size for this study. The maximum and minimum doses with respiratory motion were calculated for each plan, and the mean deviation from the 4D dose was calculated, taking delivery time, fractionation, and patient breathing cycle into consideration. Results: For all patients evaluated in this study, the mean difference between the total delivered dose and the 4D dose in the planning target volume (PTV) was <2.5%, with a standard deviation <1.2%, and maximum point dose variation from the 4D dose was <6.2% in the PTV with delivery dose rate (DR) of 200 MU/min and patient breathing cycle of 8 seconds. The motion induced dose uncertainty is a function of fractionation, MU (plan modulation), dose rate, and patient breathing cycle. Conclusion: Respiratory motion induced dose uncertainty varies from patient to patient. Patient specific respiratory motion induced dose uncertainty quantification could be useful for plan evaluation and treatment strategy determination.