ESTRO 37 Abstract book

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ESTRO 37

Conclusion STAR for VT can be delivered totally non-invasively by tracking ICD electrodes using Cyberknife ® in patients with an already implanted cardiac device. EP-2202 Accuracy of and quality assurance for VMAT- based, single isocenter SRS for multiple targets G. Ezzell 1 , M. Foster 1 1 Mayo Clinic AZ, Radiation Oncology, Phoenix, USA Purpose or Objective SRS for patients with multiple brain metastases is becoming a frequent alternative to whole brain irradiation. Single isocenter, VMAT-based planning makes such treatments practical, but the dosimetric and geometric accuracy must be established and assured for each patient. Previously published work established that at least a 1 mm PTV margin should be added to account for additional geometric uncertainty associated with off- center targets. This effort focused on testing minimum target size to assure dosimetric accuracy and establishing gamma criteria appropriate for the quality assurance of this treatment technique. Material and Methods In our facility, treatment planning for multi-target, VMAT-based SRS treatments is done on Varian Eclipse (V13, Varian Medical Systems) treatments are delivered on a Varian iX linac with Millenium MLC, and per-patient IMRT QA is done using an ArcCheck device (Sun Nuclear Corporation). The dosimetric accuracy was studied by creating test plans on an anthropomorphic water phantom (RTsafe) in which a micro ion chamber (Exradin model A16). Target sizes were varied from 0.6 – 2.0 cm diameter (0.11 – 4.2 cm 3 ) and distances from the isocenter up to 5.6 cm. The plans used four arcs, three non-axial, following our standard protocol. Readings were corrected for volume averaging. The plans were also calculated on a rectangular solid water phantom and measured using radiochromic film (EBT3). QA plans were calculated on the ArcCheck device and measured with deliberate couch rotations of 0-3 degrees to force geometric inaccuracy. At 2 degrees, a target 5 cm from the isocenter would see a 1.7 mm displacement and at 3 degrees a 2.6 mm displacement. Measurements at the nominal couch angle were repeated with the monitor units changed by +/- 5% and 10%. The ArcCheck measurements were analyzed (Sun Nuclear SNC PA) with different gamma criteria to determine what combination could reliably pass plans known to be accurate and fail plans with known geometric or dosimetric inaccuracies.

respectively. For γmean, CL and UCL were 0.42, 0.36, 0.29 and 0.54, 0.49, 0.40, for groups 1, 2 and 3 respectively. For γ1%, CL and UCL were 1.87, 0.98, 0.94 and 2.69, 1.34, 1.31, for groups 1, 2 and 3 respectively. For the groups 2 and 3, all processes are in control and within clinical specifications (State I). On the contrary, for group 1 only the γmean process is in a state I; the γ% process is in a state IV (out of control and out of clinical specifications) while the γ1% process is in a state II (within the control limits but out of clinical specifications). The capability index of the seven29 array was 6.5, implying that this device is statistically capable. The Cpl/Cpu capability indices for γ%, γmean and γ1% resulted equal to 0.33, 1.07 and 0.10 in group 1; 2.85, 1.64 and 2.11 in group 2; 4.20, 2.60 and 2.01 in group 3, respectively. With 5%-3mm specifications, also γ% and γ1% processes in group 1 moved in a state I (Cpl and Cpu were 1.87 and 1.57, respectively). Conclusion SPC is a powerful tool to quantifiably evaluate the QA process performance in advanced radiotherapy. EP-2201 Stereotactic arrhythmia radio-ablation using ICD electrode tracking by Cyberknife® R. Jumeau 1 , E. Pruvot 2 , J. Schwitter 2 , C. Teres Castillo 2 , F. Duclos 1 , V. Vallet 3 , R. Moeckli 3 , E.M. Ozsahin 1 , J. Bourhis 1 1 Centre Hospitalier Universitaire Vaudois, Department of Radiation Oncology, Lausanne Vaud, Switzerland 2 Centre Hospitalier Universitaire Vaudois, Department of Cardiology, Lausanne Vaud, Switzerland 3 Centre Hospitalier Universitaire Vaudois, Institute of Radiation Physics, Lausanne Vaud, Switzerland Purpose or Objective Stereotactic arrhythmia radio-ablation (STAR) is an emerging treatment in the management of complex cardiac arrhythmias, including ventricular tachycardia (VT) and atrial fibrillation. A few case reports have reported individual experiences. However, for most patients treated with STAR for VT, a temporary intracardiac wire has been placed as an imaging fiducial marker before the procedure. This invasive step may increase the risk of morbidity. To determine if STAR could be delivered non-invasively using electrodes of already implanted cardiac devices for tracking, a patient with refractory VT was assessed. Material and Methods A 73-year-old man suffering from cardiac sarcoidosis with recurrent VT and arrhythmic storms was screened and consented. The patient had an implantable cardioverter defibrillator (ICD) placed three years before. The patient had a pair of planning computed tomography (CT)-scans: a non-contrast 4D planning CT-scan and a breath hold planning CT-scan with contrast. Additionally, a cardiac magnetic resonance imaging was fused to the planning CT-scan to aid in volume definition. The VT substrate was delineated with the help of the cardiologist. An internal target volume was created to compensate for heart movement and an additional 1mm margin was added to yield the planning target volume (PTV). The right ventricular electrode of the ICD was used as a fiducial marker for tracking. The dose of 25 Gy was prescribed to the 82% isodose line. STAR planning was performed with attention to coronary arteries close to the PTV. Results The VT substrate volume drawn by the cardiologist was 10cc and the final PTV volume was 19cc. The mean dose to the heart was 7 Gy. The maximal point dose to the left and right coronary arteries were 9.6 Gy and 7.9 Gy, respectively. STAR was delivered using near-real-time tracking on the ICD electrode without any interruption in a 41-minutes procedure as originally planned. The patient tolerated the procedure well and was discharged a day later.

Results The ratio of measured to planned dose averaged 0.991 (range 0.958 – 1.022) for the ion chamber measurements