ESTRO 36 Abstract Book

S454 ESTRO 36 _______________________________________________________________________________________________

Department of Oncology and Hemato-oncology, MIlan, Italy Purpose or Objective The aim of the study (partially supported by a research grant from Accuray Inc. entitled “Data collection and analysis of Tomotherapy and CyberKnife breast clinical studies, breast physics studies and prostate study”) is to assess the dosimetric benefit of intensity-modulated radiotherapy (IMRT) in postmastectomy patients with implant–based immediate breast reconstruction (IBR), candidates to locoregional radiotherapy with hypofractionation. Material and Methods Data of the first 121 consecutive post-mastectomy locoregional patients treated with Helical Tomotherapy between May 2012 and May 2015 with a hypofractionated scheme (2.67Gy/fr, 15 fractions) have been prospectively collected. At the time of surgery, all patients underwent IBR using either temporary tissue expander or permanent prosthesis. The impact of immediate breast reconstruction on the planning was analyzed. Treatment plans were scored in terms of coverage of the PTVs (chest wall and supraclavicular region) and sparing of organs at risk (heart, lungs and contralateral breast). The coverage of chest wall and supraclavicular region was evaluated according to the amount of volume receiving the 90% of the prescribed dose (V 90% ) while the sparing of each OAR was evaluated according to the number of satisfied constrains (Tab.1). A plan with optimal coverage of both PTVs had 2 PTV points, while a plan with optimal sparing of all OARs had 4 OARs points. An overall score was assigned to each plan.

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.

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

Results 71.1% (86/121) of the 121 post-mastectomy radiotherapy plans had high total scores (total score=6 points) as a result of an optimal coverage of both chest wall and supraclavicular region and optimal sparing of all OARs. The remaining 28.9% (35/121) of plans had a compromised distribution of dose (total score<6 points). In particular, 13.2% (16/121) of plans fully satisfied all the OAR constraint but at a cost of moderate coverage of chest wall (7/121 plans) or supraclavicular region (9/121 plans) target volumes. On the other hand, 13.2% (16/121) of plans fully satisfied coverage of both PTVs compromising the sparing of OARs (heart, ipsilateral lung, or contralateral breast). The residual 2.5% of plans (3/121) had both coverage of PTVs and sparing of OARs compromised. Conclusion In patients having implant-based IBR, IMRT allows optimal treatment plans in more than 2/3 of cases. Superior dosimetric results are even more important when