Dosimetric impact of intra- and interfraction motion on organ at risk dose during Volumetric arc therapy Total body irradiation

Abstract

Purpose: Multi-isocenter Volumetric Modulated Arc Therapy (VMAT) allows for better dose homogeneity and individualized organ-at-risk (OAR) sparing compared to 2D conventional total body irradiation (TBI). However, VMAT steep dose gradients may lead to OAR dose uncertainties due to patient positioning variations and organ motion. The impact of respiration (intrafraction motion) and interfraction patient motion on the lung and kidney dose during VMAT-TBI was evaluated in a cohort of paediatric patients. Methods: Intrafraction motion Lung and kidney motion amplitudes were determined by a mid-position image created from radiotherapy planning 4D CT scans in a surrogate paediatric patient population. The average motion amplitude was used to expand and reduce the lung and kidney volumes on the planning CT. The mean dose (Dmean) of the expanded and reduced volumes was compared to the planned Dmean. Interfraction motion The planning CT and delineations were deformed according to the anatomy imaged on daily acquired cone beam CT images. The dose distribution was recalculated on the deformed CT per fraction and compared to the planned one. Results: Intrafraction motion The maximum Dmean absolute difference between expanded/reduced contours compared to the original contour for both kidneys, left and right lung was 0.5 Gy, 0.7 Gy and 1.0 Gy, respectively. Interfraction motion The average difference, over all patients, in Dmean between the maximum and minimum value among the fractions was 0.04 Gy for both OAR. Conclusion: The impact of respiration and patient motion on the lungs and kidneys Dmean during VMAT-TBI is ≤ 1.0 Gy and ≤ 0.5 Gy, respectively, in this cohort.