ESTRO 36 Abstract Book

S450 ESTRO 36 2017 _______________________________________________________________________________________________

hypofractionation is used and they are expected to translate into lower late toxicity and improved aesthetic outcome. PO-0841 Feasibility of dose decrease in a rectal sub- region predictive of bleeding in prostate radiotherapy C. Lafond 1,2,3 , J. N'Guessan 2 , G. Dréan 1,3 , N. Perichon 2 , N. Delaby 2 , O. Acosta 1,3 , A. Simon 1,3 , R. De Crevoisier 1,3,4 1 University Rennes 1, LTSI, Rennes, France 2 Centre Eugène Marquis, medical physics department, Rennes CEDEX, France 3 INSERM, U1099, Rennes, France 4 Centre Eugène Marquis, radiation oncology department, Rennes CEDEX, France Purpose or Objective The inferior–anterior hemi anorectum has been found as highly predictive of rectal bleeding in case of prostate cancer radiotherapy, shown in Figure 1 (Dréan et al., Radiother Oncol 2016) . The aim of this dosimetric study was to evaluate the feasibility of decreasing the dose in this rectal sub-region (SRR), while keeping a high PTV coverage. Two new and simple strategies were used: identifying the SRR during inverse planning and/or using a recent dosimetric model. This model was used allowing to better define the achievable mean dose to the rectal structures at the inverse planning step of IMRT (Moore et al., Int. J. Radiation Oncology Biol. 2011) . This model integrates the overlap volume between the OAR and the PTV.

were 0.22 for “Standard” plans, 0.19 for “SRR” plans, 0.18 for “Model” plans and 0.17 for “Model + SRR” plans. Plans “Model + SRR” showed slightly less dose homogeneity: mean homogeneity indexes varied from 0.077 for “standard” plans to 0.101 for “Model + SRR” plans. Dose conformity was very similar for all plans: the conformal index varied of 1% in average. “Model + SRR” plans required an increase of 22% in the number of MU compared to the “Standard” plan. The irregularity and modulation indexes increased of 58% and 10%, respectively.

Conclusion Compared to standard prostate VMAT plans, applying specific dose constrains to the SRR and rectal wall using the “Moore method” should decrease of around 8 Gy the mean dose to the SRR and decrease relatively of 23% the risk of rectal bleeding. PO-0842 Choosing the best heart sparing technique for breast and internal mammary chain radiotherapy A. Ranger 1 , A. Dunlop 1 , K. Hutchinson 2 , M. Maclennan 3 , H. Convery 4 , H. Chantler 2 , C. Rose 2 , N. Twyman 2 , E. Donovan 1 , E. Harris 5 , C. Coles 2 , A. Kirby 6 1 The Royal Marsden NHS Trust and The Institute of Cancer Research, Physics, London, United Kingdom 2 Cambridge University Hospitals NHS Trust, Physics, Cambridge, United Kingdom 3 Lothian and Dumfries NHS Trust, Oncology, Edinburgh, United Kingdom 4 The Royal Marsden NHS Trust, Physics, London, United Kingdom 5 The Institiute of Cancer Research, Physics, London, United Kingdom 6 The Royal Marsden NHS Trust and The Institute of Cancer Research, Clinical Oncology, London, United Kingdom Purpose or Objective Published data demonstrate a 4.4% overall survival benefit t associated with inclusion of the internal mamm ary chain (IMC) in the radiotherapy (RT) target volume in patients with breast cancer. Survival gains will be maximised by minimising radiation doses to heart and lungs. This dosimetry study compares the ability of breath-hold techniques in 3D conformal radiotherapy, arc therapy and protons to achieve target volume constraints whilst minimising dose to heart and lungs with a view to defining implementable class solutions for irradiating the IMC. Material and Methods Breast tissue, level I-IV axillary and IMC lymph nodes were outlined using ESTRO consensus guidelines in 14 patients scanned in both free breathing (FB) and breath hold (BH). Seventy two locoregional RT plans, prescribed to 40Gy/15

Material and Methods 60 patients data already treated for prostate cancer to a total dose of 78 Gy were used. For each patient, 4 VMAT plans were generated with Pinnacle v9.10 (Philips): one standard plan corresponding to the current practice (“Standard”), one plan adding specific objectives to the SRR (“SRR”), one plan using the Moore model applied to the rectal wall only (“model”) and one plan using the Moore model applied to both the rectal wall and the SRR (“model+SRR”). The plans were compared regarding dose distribution, indexes of conformity and homogeneity, risk of 3-year Grade > 1 RB using the Lyman–Kutcher–Burman NTCP model, and efficiency (Monitor Units and complexity indexes). Results Figure 2 shows the mean DVH of the 60 patients for each of the 4 plans. “Model + SRR” plans showed the most important SRR dose sparing, with mean dose decreases of 4.7 Gy, 5.3 Gy and 7.7 Gy relatively to the “Model”, “SRR” and “Standard” plans respectively. Mean NTCP values