ESTRO 36 Abstract Book

S430 ESTRO 36 2017 _______________________________________________________________________________________________

technical drawings. Difference of just 3.5 % was obtained for the experimental and calculated FWHM. Mean energies calculated from peripheral photon spectra energies ranged from 0.242 MeV in the mediastinum to 0.171 MeV in the pelvis. Based on these results, an average energy correction factor of 10% was applied to TLD readings. A maximum of 15% difference between calculated and measured peripheral doses were obtained. Conclusion The use of Egspp allowed us to model accurately the Gamma Knife. The level of detail achieved in the modeled geometry is essential for the peripheral dose calculation, since it is dependent on the radiation leakage. The agreement between simulations and measurements is good, with higher discrepancies observed in the points located on the limbs of the phantom. A MC methodology for peripheral dose characterization has been validated and therefore, different scenarios regarding patient size and/or beam geometry can be estimated for future references. References [1] Radiother Oncol 2012;10(5):122-126 [2] JACFMP, vol 14, N2, 2013 [3] Phys. Med. Biol. 57 (2012) 6167–6191 [4] Biomed. Phys. Eng. Express 1 (2015) 045205 PO-0812 Dosimetric impact of using Acuros algorithm for stereotactic lung and spine treatments L. Vieillevigne 1,2 , T. Younes 1,2 , A. Tournier 1 , P. Graff Cailleaud 1 , C. Massabeau 1 , J.M. Bachaud 1 , R. Ferrand 1,2 1 Institut Claudius Regaud- Institut Universitaire du Cancer de Toulouse Oncopole, Radiophysique, Toulouse, France 2 Centre de Recherche et de Cancérologie de Toulouse CRCT- UMR1037 INSERM - Université Toulouse 3, Radiophysique- équipe 15, Toulouse, France Purpose or Objective The main aim was to assess the dosimetric impact of calculating with the Acuros (AXB) algorithm instead of Anisotropic Analytical Algorithm (AAA) for stereotactic (SBRT) lung and spine cancer treatments. Material and Methods Ten stereotactic lung patients and ten stereotactic spine patients were selected to investigate the dosimetric impact of using AXB instead of AAA. Dynamic conformal arc was used for SBRT lung patients with a prescription of 50 to 55 Gy in 3 or 5 fractions to the 80% isodose. For the SBRT spine patients, Rapid Arc plans were prepared and 27 Gy was prescribed in 3 fractions to the PTV median dose. The plans were recalculated with the AXB algorithm by using the same beam settings and monitor units as the AAA. Two dose reporting modes of AXB, dose to medium (Dm) and dose to water (Dw) were studied. Relative dose differences between algorithms were calculated for PTV (D98%, D95%, D50% and D2%) and for organs at risk (D2% and mean dose for the ipsilateral lung, the spinal cord or the cauda equina). Results For the 10 SBRT lung patients, the mean lung density was around 0.18 g/cm 3 which corresponded to a normal lung tissue. The dosimetric impact on PTV dose (D98%, D95%, D50%) using AXB instead of AAA was quite small with a maximum underdosage up to 3.1% for D98%. Larger differences were obtained on D2% with a maximum deviation of 7.79%. The average difference on the mean ipsilateral lung dose was 0.22% and 0.44% for AXBDm and AXBDw, respectively. AXBDm and AXBDw presented similar results. For the 10 SBRT spine patients, large relative dose disagreement of up to -5.02% for the D98% of the PTV was observed with AXBDm. On average, for the D50% of the PTV, AXBDm revealed a relative underdosage of -2.36%, whereas AXBDw lead to a relative overdosage of +1.64%. Concerning the spinal cord or the cauda equina, the

average mean dose was reduced up to -6.93% and up to - 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 Poster: Physics track: Radiation protection, secondary tumour induction and low dose (incl. imaging)