ESTRO 2020 Abstract Book

S872 ESTRO 2020

volume of the aorta and the dose received by it would be more than what it seems on a static planning CT-image due to the dynamic pulsatile motion of the great vessel. In this study, we made an effort to quantify the volume and dose variations of the aorta with the volumes marked on planning CT scan Vs 4D-MRI scan based PRV generation in cases of oligo metastatic spine SBRT. Material and Methods Five patients of oligo metastatic spine SBRT (Lumbar-2; Dorsal-3) were chosen for this study. All patients underwent planning CT scan using deep inspiratory breath hold (DIBH) technique with RPM device. All patients underwent 4D-MRI scan sequences (FIESTA 4-chamber view, contrast LAVA, DEFFICO sequences) using pulse gated technique with breath holding in the treatment planning position on 1.5T MRI machine. Breathhold 4D MR Scan mitigates respiratory motion and allows us to capture true pulsatile motion. Aorta was delineated in 2 clinical contexts (1) Aorta MRI: Aorta delineated in one of the 4D-MRI bins (4 chamber view) was deformably propagated onto the rest of the bins using intensity based deformable registration algorithm software and the PRV for the aorta was generated. The PRV Aorta MR volume was registered with BH CT Scan and contours were mapped. (2) Aorta CT: On static BH planning CT scan. The target volume and all other OARs were contoured on the Planning CT scan and transferred to TPS for planning. The target coverage parameters and OAR constraints were achieved as per RTOG protocol. The PRV Aorta volumes on MRI and CT along with the D max and Threshold dose received by the respective PRV Aorta (Aorta MRI & Aorta CT) where assessed. These parameters were analyzed using Paired sample t test in SPSS software Results The median motion of aorta due to its pulsations was 2mm (Range 1-2.5mm). The PRV Aorta Volumes in MR were significantly more compared to CT (p value-0.008). Similarly, the D max and threshold dose received by the PRV Aorta MR as compared to CT were more and were statistically significant (p Value- 0.05 and p Value-0.008 respectively). Conclusion We noticed statistically significant change in the volumes and doses (max and threshold) of aorta contoured on 4D-MRI vs Planning CT scan. As the tolerance limit of great vessel (aorta) for single fraction spine SBRT is higher than the maximum doses delivered to the target, these results may not hold any significance but this study may form a basis for future studies of SBRT in abdominal malignancies close to great vessels in terms of dose fluctuations due to their pulsations. This is the first study reported in literature and proof-of-concept to map the 4D MR motion on the 3D CT datasets & analyse the dose deformations. PO-1603 Residual intra-fraction error in tracked spinal SBRT: effect of site, fractionation and patient pain E. Rossi 1 , C. Fiorino 1 , A. Fodor 2 , G.M. Cattaneo 1 , C. Deantoni 2 , F. Zerbetto 2 , R. Calandrino 1 , S. Broggi 1 , N.G. Di Muzio 2 1 San Raffaele Scientific Institute, Medical physics, Milan, Italy ; 2 San Raffaele Scientific Institute, Radiotherapy, Milan, Italy Purpose or Objective Residual intra-fraction error in the delivery of spinal radiosurgery with the Cyberknife (Accuray Inc) robotic system was previously assessed for no-immobilized patients (pts). We aimed to explore possible correlations between residual error and clinical/geometrical parameters: site, single vs multi-fraction sessions and VAS (Visual Analog Scale) score for pain Material and Methods Delivery data of 42 pts (128 fractions, 4220 images) were analyzed, including 27 thoracic (T), 21 lumbar (L), 2 cervical and 4 sacral lesions adding up to 54 treatments

delivered in 1, 3 and 5 fractions (27, 17, 10). X-ray images were regularly taken for tracking every 30-90sec and offsets were compensated by the robotic arm. Residual error was quantified as the difference between corresponding measured translational/rotational shifts of consecutive images. The error distribution for each fraction, patient and the whole population was assessed for each axis/rotation angle. The overall error (OE) [x²+y²+z²]½ (translations along each axis) was also calculated. The OEs for pts treated to T and L spine metastases were estimated and compared, as well as OEs from different sessions for multi-fraction treatments. We verified a possible correlation with pain estimating and comparing the OE of pts with VAS=0 and VAS>0. Significance was evaluated by Mann-Whitney tests. Finally, we simulated the delivered treatments of 4 pts identified as the worst case scenarios (2 pts with the highest single offset and 2 pts with the highest mean error) editing the original beams coordinates, based on the real shifts between consecutive corrections. Resulting “true” dose distributions and planned ones were compared Results The whole population mean error was 0 (SD 0.1mm). Translational (cranio-caudal, lateral, anterior-posterior) shifts >1mm were <2%; rotational (roll, pitch, yaw) shifts >1° were < 1%. No significant differences were found between OE for pts treated to T or L spine (0.08mm, 0.06mm) or between different fractions (0.09mm at fraction 1, 0.07mm at fraction 5). The difference between the OE of a patient group with 0 VAS score (0.05mm) and the others (0.06mm) was not significant; still, the corresponding values of OE in case of no-tracking correction showed a significantly higher fraction of OE≥2mm for pts with VAS>0 compared to VAS=0 (4/14 vs 0/23, p=0.03): Fig.1 summarizes both situations. No relevant differences between the planned and delivered dose distributions (with tracking) were found in the selected worst scenarios: for all pts D0.03cc for OARs differ less than 0.5Gy and V100 for targets differ less than 1%. Fig.2 pictures an example of the worst expected effect on a representative slice

Conclusion The high efficiency of spine tracking for non-immobilized pts has been confirmed also in terms of dosimetric impact to the patient. OE seems to be independent from different factors such as target localization, reported pain and