ESTRO 2020 Abstract book

S840 ESTRO 2020

distributions it was observed that plans with PCI ≥ 0.89 were 76% prior to the clinical implementation and 86% after the clinical implementation of the guiding value showing an increase of 10 percentage points (See Figure, red bars indicate PCI < 0.89 and green bars indicate PCI ≥ 0.89. Dashed line indicates mean PCI).

consensus guidelines. The priority of the planning was to achieve optimal target coverage (> 98% of the CTV covered with 95% isodose) with acceptable sparing of the heart, LAD and ipsilateral lung. Prescribed dose was 50Gy for photons and 50Gy(RBE) for protons in 25 fractions. Statistical analysis was performed with Wilcoxon signed rank test. Results Proton DIBH and FB plans did not differ significantly in terms of heart, LAD and ipsilateral lung dose reduction. IMPT FB technique allowed for substantial sparing of the heart comparing to photon DIBH IMRT. Mean heart dose (MHD) was 0.14Gy(RBE) vs. 2.12Gy; p=0.04; respectively. Mean ipsilateral lung dose (7.2 Gy(RBE) vs. 12.6Gy; p=0.08) and lung V20 (15,1 Gy(RBE) vs. 22.4Gy; p=0.08) were not significantly different between proton and photon plans. Regarding analyzed LAD parameters mean LAD dose was significantly lower in proton plans comparing to photons (1.16Gy(RBE) vs. 5.57Gy; p=0.043; respectively), whereas LAD maxium dose and D max 0.2cm 3 were not different. Conclusion DIBH does not lead to further improvement of organs at risk dose reduction in proton radiotherapy. Proton FB technique comparing to photon DIBH IMRT ensures significant sparing of the heart in patients with left-sided breast cancer treated with regional lymph nodes. It also allows reducing dose to the left coronary artery. Proton beam radiotherapy could be a promising solution for breast cancer patients demanding locoregional adjuvant irradiation who have particularly high heart and LAD doses and do not benefit from photon DIBH radiotherapy technique. PO-1473 On the clinical implementation of conformity index for radiotherapy of prostate J. Scherman 1 , E. Wieslander 1 1 Skåne University Hospital, Radiation Physics- Department of Hematology- Oncology and Radiation Physics, Lund, Sweden Purpose or Objective The purpose of this study was to evaluate if any difference was observed in the conformity index for prostate volumetric arc therapy (VMAT) treatment plans after an implementation of a clinical guiding value. Material and Methods The conformity index used was Paddick’s Conformity Index (PCI) [Paddick 2000]. PCI is a measure of how well a prescribed isodose volume conforms to the size and shape of a target volume and is calculated according to: PCI = (TV PIV ) 2 /(TV·PIV), where TV PIV is the target volume covered by the prescriptions isodose volume, TV is the Target Volume and PIV is the prescription isodose volume. A retrospective analysis of conformity index for four pelvic VMAT- treatments was performed by the authors [Scherman and Wieslander 2018], where a specific PCI clinical guiding value of 0.89 was implemented for prostate VMAT-plans. A small script was created using Eclipse Scripting API (Varian Medical Systems) where treatment planners could calculate the PCI during the treatment planning process. Treatment plans were retrieved by extracting data for prostate patients 7 months prior to and 7 months after the implementation of the guiding value. A total of 575 prostate VMAT-plans were included in this current study, 284 prior and 291 after the clinical implementation. PCI distributions were compared prior and after implementation using a two-sided unpaired Wilcoxon signed-rank test, where p < 0.05 was considered statistically significant. Results Mean PCI values for the two distributions were both 0.91. No statistically significant difference was observed between the two distributions (p = 0.81, using a two-sided unpaired Wilcoxon signed-rank test). Comparing the two

Conclusion A successful clinical implementation was performed and a decreased number of treatment plans with low PCI was observed, however, no statistically significant difference was found between the two distributions. The script is continued to be used in the clinic, also for other diagnoses, as guidance during the treatment planning process to achieve more conformal treatment plans. PO-1474 Automated VMAT-SBRT treatment planning for complex spinal metastases: a dosimetric analysis A. Ianiro 1 , F. Cellini 2 , C. Romano 1 , F. Deodato 3 , G. Macchia 3 , A. Zamagni 4 , M. Buwenge 4 , S. Cammelli 4 , L. Strigari 5 , L. Azario 6 , M. De Spirito 6 , V. Valentini 2 , A.G. Morganti 4 , S. Cilla 1 1 Fondazione di Ricerca e Cura Giovanni Paolo II, Medical Physics Unit, Campobasso, Italy ; 2 Fondazione Policlinico Universitario A. Gemelli, Radiation Oncology Department, Roma, Italy ; 3 Fondazione di Ricerca e Cura Giovanni Paolo II, Radiation Oncology Unit, Campobasso, Italy ; 4 University of Bologna- S.Orsola-Malpighi Hospital, Radiation Oncology Department, Bologna, Italy ; 5 S.Orsola-Malpighi Hospital, Medical Physics Unit, Bologna, Italy ; 6 Fondazione Policlinico Universitario A. Gemelli, Medical Physics Unit, Roma, Italy Purpose or Objective Stereotactic body radiation therapy (SBRT) has become a common option for the treatment of spinal malignancies. Dose escalation by simultaneous integrated boost (SIB) may improve local tumor control. This anatomical site presents a major challenge for planning optimization due to the complex relationship between targets and adjacent critical structures. We evaluated the feasibility of Pinnacle 3 Autoplanning engine for SBRT planning with VMAT technique and SIB strategy for spine treatments. Automatically generated plans (AP) were compared with manually-generated ones (MP) by expert medical physicists. Material and Methods Six patients with metastases to the body of lumbar and thoracic spine were included. GTV was defined as the macroscopic visible lesion at vertebral body level (based