ESTRO 2020 Abstract Book

S871 ESTRO 2020

Conclusion 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

directly while the shape Y movement took the leaf thickness into account by either using a threshold or interpolation algorithm. The MLC control system was running in 1ms cycle mode handling displacement information and commanding new leaf positions. Detectors were fixed to the moving phantom (cos 2 amplitude of 2cm) with breathing cycles of 12/min up to 60/min. Films were evaluated for penumbra, gamma index (3%, 2mm) and dose difference for static, non-tracked and tracked scenarios. Ionization chamber measurements were used to show dynamic processes.

Results In general, experiments performed with the HDMLC presented significant improvements in all aspects compared to the results without tracking. Higher cycle rates lead to a higher penumbra (1 mm for 12/min and 6mm for 60/min), which is up to 15 times better than untracked. The gamma distribution shows corresponding results (Fig 2). Interpolation for the y axis was most effective on more rounded target shapes. The main drawback observed in all experiments was the mechanical backslash of the leaves on the HDMLC, which could be resolved by a direct coupling of the position sensor into the leaf.