ESTRO 36 Abstract Book

S810 ESTRO 36 2017 _______________________________________________________________________________________________

reduce the incidence and severity of symptoms. Although Intensity Modulated Radiotherapy (IMRT), over 3D-CRT, allows a better conformal treatment of the targets and to spare the organs at risk (OARs), only a few studies have assessed the role of patient positioning in IMRT planning for OARs sparing. We evaluated the effect of a PP or supine position (SP) with full bladder to spare OARs in pelvic IMRT in gynaecologic malignancies. Material and Methods A PP and a SP Computed Tomography scan, slice thickness of 3 mm, full bladder and empty rectum, were performed in 13 patients with endometrial or cervical cancer, 8 of whom submitted surgery. Target volumes, nodes and uterus or vaginal cuff, and OARs were delineated by one young in training radiation oncologist and review by a senior radiation oncologist. Step and shoot technique IMRT plans were elaborated for each position. A dose of 50.4 Gy in 28 fractions was prescribed. Dosimetric parameters were compared by non-parametric Wilcoxon exact signed rank test for paired data and for unpaired data with Mann Whitney test and Kruskal-Wallis test (SPSS 22.0, Inc., Chicago, IL). Statistical significance was assumed for p ≤ 0.05. Results In prone and supine plans the mean PTV volumes were 1374.93 cc for PP and 1413.47 cc for SP, median Dmean were 50.27 Gy in PP and 50.18 Gy in SP, and PTV D50% were 50.4 Gy for PP and 50.3 Gy for SP. Data regarding conformity and homogeneity of IMRT plans for PP and SP gave similar results. All parameters were calculated according ICRU 83. We found that PP permits to spare irradiated rectal volume from 10 to 45 Gy compared with SP, but the difference was not significant. The dose- volume histogram for the bladder was significant better in SP at V45 (p = 0.03), V40 (p = 0,011), V30 (p = 0.033), V20 (p = 0.039), V10 (p = 0.039). The analysis of tabular dose- volume histograms showed a significant decrease of the small bowel volume at V20 (p = 0.005), V30 (p = 0.019), V40 (p = 0.046), V45 (p = 0.028) and V50.4 (p = 0.019) in favour of the PP. For V10 the reduction of irradiated bowel was not significant (p = 0.055). Dmax and NTCP were significantly lower in PP. In the operated group, a significant difference was observed in small bowel NTCP reduction for both PP and SP (p= 0.003 and 0.006, respectively) compared with non operated group, but not for rectum and bladder. Conclusion PP with a full bladder in pelvic IMRT for gynaecologic malignancies permits a significant bowel sparing for doses > 20 Gy providing similar target coverage and target conformity. This is very useful when higher dose lymph- node boost is planned. SP allows a larger bladder sparing. Small bowel NTCP reduction in both position in operated patients could be linked to the smaller target volume. EP-1528 RapidPlan Head and Neck model: the objectives and possible clinical benefits L. Cozzi 1 , G. Reggiori 2 , C. Franzese 2 , F. Lobefalo 2 , M. Scorsetti 1 , A. Fogliata 2 1 Humanitas Cancer Center and Humanitas University, Radiation Oncology, Milan-Rozzano, Italy 2 Humanitas Cancer Center, Radiation Oncology, Milan- Rozzano, Ital y Purpose or Objective RapidPlan TM is the knowledge based planning process recently implemented in the Varian Eclipse treatment planning system. It estimates, according to the model data, the organ at risk (OAR) DVHs to generate the optimization objectives, tailored on any new patient, for the plan optimization process. Advanced head and neck cancer (AHNC) planning presents complexities due to the anatomy and the low tolerance dose levels for the surroundings OARs. In the present work a RapidPlan (RP) model is configured and subsequently validated to

Conclusion If the GTV is static, it should receive a constant dose, but step 1 shows that the dose delivered to GTV, when it reaches a position inside the PTV (where the photon fluence is optimized for low electron densities), is higher than what estimated on the original EDo map. The GTV is thus irradiated in a more homogeneous way in step 2 in which the fluence is optimized for its mean ED everywhere in the PTV. We propose that, in lung small lesions, the PTV is modified in terms of electron density considering the GTV mobility. Optimizing the photon fluence for the “forced” electron density map appears an effective way to evaluate the real dose delivered to the GTV. EP-1527 Pelvic Intensity-Modulated Radiotherapy in prone and supine position in gynaecological cancer E. Perrucci 1 , G. Montesi 2 , M. Marcantonini 3 , C. Mariucci 2 , M. Mendichi 2 , S. Saccia 1 , A. Cavalli 1 , A.M. Didona 3 , V. Lancellotta 4 , V. Bini 5 , C. Aristei 4 1 Perugia General Hospital, Radiation Oncology, Perugia, Italy 2 University of Perugia, Radiation Oncology, Perugia, Italy 3 Perugia General Hospital, Medical Physics Unit, Perugia, Italy 4 University of Perugia and Perugia General Hospital, Radiation Oncology, Perugia, Italy 5 University of Perugia, Internal Medicine- Endocrin and Metabolic Sciences, Perugia, Italy Purpose or Objective Pelvic radiation is linked to high rate of toxicity, mainly gastrointestinal. In 3D-conformal radiotherapy (3D-CRT), prone position (PP) and a belly-board device are used to