Surface-Guided Radiation Therapy (SGRT) during Stereotactic Body Radiation Therapy Treatments (SBRT) of the Lung: Dosimetric Implications of Intrafraction Motion

Abstract

SGRT for set up followed by IGRT with cone-beam computed tomography (CBCT) is used in our clinic as a standard technique for patients (pts) receiving SBRT to the lung. SGRT is also utilized for continuous monitoring during treatment to detect intrafraction motion during SBRT. We previously reported the reliability of SGRT in detection and quantification of intrafraction motion for SBRT treatments in the lung. The purpose of this study is to analyze the potential dosimetric effects on target coverage and spinal cord dose of observed intrafraction motion. An IRB approved retrospective chart review was conducted to evaluate IGRT shift information for 58 pts with malignant thoracic tumors treated with SBRT. SGRT was utilized for patient set up and CBCT was taken for volumetric ITV matching. After a new SGRT reference image was captured, pts were then monitored continuously with SGRT during treatment. If an intrafractional shift in any direction >2mm for longer than 2 seconds was detected by SGRT, then the treatment was stopped, CBCT was repeated, and any IGRT shifts were recorded. Of the 25 pts and 34 fractions that had detected intrafractional shifts, the 11 highest shift quantities were analyzed for potential dosimetric effects on target (ITV and PTV) coverage and minimum dose, as well as max spinal cord dose. Paired t-tests were used to compare the differences in these parameters and were considered significant if the p-value was less than α=0.05. In addition, plans were recreated for PTV margins of 2mm and 4mm to analyze the potential effects of intrafraction motion with reduced PTV margins. 11 shifts during SBRT treatments of the lung were included in the analysis. The mean vector intrafraction shift was 0.51 cm. Mean PTV coverage was significantly reduced due to intrafraction motion (95% vs. 88.2%, p=<0.001). Mean PTV minimum dose was lower (4509 cGy planned vs. 4192 cGy, p=0.017). Mean ITV coverage was not changed due to intrafraction motion and remained at 100%. Mean ITV minimum dose was also not significantly reduced (5944 cGy vs. 5785, p=0.078). Mean spinal cord maximum dose was not significantly changed (990 cGy vs. 987.8 cGY, p=0.864). As PTV margins decreased from 5mm to 2mm, ITV coverage started to decline but remained >95%. SGRT is valuable in detecting potentially clinically meaningful intrafraction motion that may result in decreased PTV coverage if not accounted for during treatment. Accurate treatments with reduced PTV margins may be achievable when continuous monitoring or imaging of pts is performed.