ESTRO 2020 Abstract book

S922 ESTRO 2020

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

Due to its high precision/accuracy, high speed and deterministic positioning properties, the new HDMLC system makes real-time tumor tracking possible. The MLC system design, using automation standards, leads to easy integration into existing devices and enables new real-time tracking possibilities. For the most important challenging open issues on real- time tumor tracking, such as MLC speed, imaging and re- planning, a feasible solution was shown being able to cope with most realistic tumor movements. Also modalities using high speed modulation can benefit from this new approach. PO-1601 electromagnetic localization and tracking for prostate cancer radiotherapy: impact of hip prostheses M. Bottero 1 , G. Dipasquale 1 , A. Lancia 2 , R. Miralbell 1 , M. Jaccard 1 , T. Zilli 1 1 Geneva University Hospital, Radiation Oncology, Geneva, Switzerland ; 2 Fondazione IRCCS Policlinico San Matteo, Radiation Oncology, Pavia, Italy Purpose or Objective To assess electromagnetic transponders (EMTs) ability to localize and track movements in prostate cancer (PCa) patients with metallic hip prostheses (MHP) treated with curative radiotherapy (RT). Material and Methods Datasets of eight PCa patients with MHP (3 bilateral and 5 unilateral) treated between 2016 and 2018 with RT and EMT tracking were retrospectively assessed. The distances between the 3 EMTs (apex to left, left to right, right to apex) and the isocenter were calculated both on planning computed tomography (CT) and cone beam CT (CBCT) at the first treatment fraction and compared with data reported by Calypso® (Varian Medical Systems, Palo Alto, CA). EMT-based barycenter position and treatment interruptions triggered by the EMTs corrections were analyzed for all evaluable treatment fractions (n=120). Localization accuracy was quantified by recording the geometric residual value (expected limit ≤ 0.2 cm) at the RT setup. Results The Calypso® system was able to localize and track prostate position without any detectable interference from MHP. For every treatment fraction, the agreement between the CBCT images and Calypso ® guidance was optimal, with EMTs always within the defined tolerance ( i.e., CT-Calypso or CBCT-Calypso measured differences in inter-EMT distances within 0.3 cm). EMTs to isocenter distances measured by Calypso® reproduced CT data and were confirmed on CBCTs. During RT, the EMTs centroid exceeded the threshold 24 times (20% of all fractions): 5 times in the left-right, 15 times in the anterior-posterior (AP), and 4 times in the superior-inferior directions. The largest motions recorded were in the AP axis: 0.6 cm anteriorly and 0.5 cm posteriorly in patients with unilateral and bilateral MHP, respectively. Conclusion Our study represents the first clinical experience assessing the localization and tracking accuracy of Calypso® EMTs during curative RT of PCa patients with of unilateral or bilateral MHP. PO-1602 Great vessel motion mapping from 4D MRI to 3D CT - Implications for Oligometastatic spinal SBRT B. Patneedi 1 , H. Vyas 1 , S. Vangipuram 1 , P. Bhaskar 2 , A. Bhange 1 1 HCG Cancer Center, Radiation Oncology, Mumbai, India ; 2 Hcg Cancer Center, Radiation Oncology, Vishakapatnam, India Purpose or Objective 4D-MRI imaging offers good soft tissue details & a real time motion assessment. We hypothesize that the actual volume of the aorta and the dose received by it would be