ESTRO 36 Abstract Book

S435 ESTRO 36 _______________________________________________________________________________________________

4.97% for AXBDm and AXBDw, respectively. For these organs at risk, differences up to -4.56% and to +3.37% were found for the D2% for AXBDm and AXBDw, respectively. Conclusion The studied lung cases present small mean differences among all calculation modalities; AXBDm and AXBDw calculations are similar. The spine cases show strong difference between AXBDm and AXBDw inside the PTV and the spinal cord or the cauda equina. Acuros is known to provide an accurate alternative to Monte Carlo calculations for heterogeneity management [1] and reporting dose to medium is the preferred choice [2]. Nevertheless, moving from AAA to AXBDm for SBRT treatments, in particular for spine or for lung of low density, has to be carefully evaluated. [1] A. Fogliata et al. “Dosimetric evaluation of Acuros XB Advanced Dose Calculation algorithm in heterogeneous media.,” Radiat. Oncol. , vol. 6, no. 1, p. 82, Jan. 2011. [2] P. Andreo “Dose to ‘water-like’ media or dose to tissue in MV photons radiotherapy treatment planning: still a matter of debate.,” Phys. Med. Biol. , vol. 60, no. 1, pp. 309–37, Jan. 2015. PO-0813 Cardiac Toxicity after Radiotherapy for Hodgkin Lymphoma: Impact of Breath Hold and Proton Therapy L.A. Rechner 1 , M.V. Maraldo 1 , I.R. Vogelius 1 , P.M. Petersen 1 , R.X. Zhu 2 , B.S. Dabaja 3 , N.P. Brod in 4 , L. Specht 1 , M.C. Aznar 5 1 The Finsen Center - Rigshospitalet, Department of Oncology, Copenhagen, Denmark 2 MD Anderson Cancer Center, Radiation Physics, Houston, USA 3 MD Anderson Cancer Center, Radiation Oncology, Houston, USA 4 Albert Einstein College of Medicine, Institute for Onco- Physics, Bronx, USA 5 University of Oxford, Nuffield Department of Population Health, Oxford, United Kingdom Purpose or Objective We undertook this work to quantify the impact of deep inspiration breath hold (DIBH) and proton therapy, alone and in combination, relative to treatment in free breathing (FB) with IMRT with respect to the estimated risk of cardiac toxicity after radiotherapy for patients with early-stage mediastinal Hodgkin lymphoma (HL). Material and Methods Treatment plans were generated for 22 patients in both FB and DIBH to 30.6 Gy (Gy(RBE) for proton therapy) in 17 fractions. IMRT plans were created according to the clinical procedure at the presenting author’s institution. Proton plans were created with guidance from the authors with clinical proton therapy expertise. Mean doses to the heart, heart valves, and left anterior descending coronary artery (LADCA) were exported and excess relative risks (ERRs) of radiation-induced myocardial infarction, heart failure, and valvular disease were estimated. Dose volume histograms (DVHs) for the heart were extracted and mean DVHs were created for each treatment technique. The Friedman test was used to assess statistical significance, and analysis was performed in Matlab (The MathWorks, Inc). Results The use of both DIBH and proton therapy were found to reduce the dose as well as the estimated risk to cardiac structures (Table 1). Mean doses to the heart, valves, and LADCA, and the ERRs of radiation induced myocardial infarction, heart failure, and valvular disease were statistically significantly reduced for all other treatment Poster: Physics track: Radiation protection, secondary tumour induction and low dose (incl. imaging)

technique combinations when compared with IMRT in FB, and when proton therapy in DIBH was compared to proton therapy in FB. Heart dose and the ERRs of myocardial infarction and heart failure were significantly reduced when proton therapy in DIBH was compared to IMRT in DIBH. However, when proton therapy in FB was compared to IMRT in DIBH no statistically significant differences were seen for any doses or ERRs. To further analyze the differences between proton therapy in FB and IMRT in DIBH, the paired differences in heart dose from the techniques were calculated (proton therapy in FB minus IMRT in DIBH). The resulting median difference was 0.0 Gy (range -4.3 to 3.5), revealing that the relative benefit of these two techniques with respect to heart dose is patient-specific. Furthermore, mean DVHs for the heart show that, on average, the volume of the heart receiving a dose above about 7 Gy is greater for proton therapy in FB than it is for IMRT in DIBH (Figure 1).

Conclusion DIBH and proton therapy both reduced the dose to cardiac structures and the risk of cardiac toxicity, compared to IMRT in FB, but no significant difference was found between IMRT in DIBH and proton therapy in FB. Therefore, with respect to cardiac toxicity, these data suggest that given a choice in techniques, IMRT in DIBH and/or proton therapy should be selected. However, the difference between IMRT in DIBH and proton therapy in FB is variable and should be evaluated on a patient-specific basis. PO-0814 The Influence of scans parameters on effective dose of CBCT scans used for IGRT proce dures Abuhaimed 1 , C. J. Martin 2 , M. Sankaralingam 3 1 King Abdulaziz City for Science and Technology, Department of Applied Physics, Riyadh, Saudi Arabia 2 University of Glasgow, Department of Clinical Physics, Glasgow, United Kingdom 3 Beatson West of Scotland Cancer Centre, Department of Radiotherapy Physics, Glasgow, United Kingdom Purpose or Objective A new software with a version of (V2.5) of On-Board imager (OBI) system, which is utilized in the clinic for image guided radiation therapy (IGRT) procedures, was