ESTRO 36 Abstract Book

S821 ESTRO 36 _______________________________________________________________________________________________

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 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 evaluate the RP quality relative to the clinical plans (CP). Secondary, through normal tissue complication probability (NTCP) estimations, the possible effective clinical benefit in planning with RP is evaluated. Material and Methods 83 patients presenting AHNC were selected from the department database. The patients were chosen as their plans were considered as dosimetrically optimal. All plans were optimized for VMAT technique (RapidArc), with 2-4 arcs, 6 MV beam quality, treated on a department linac equipped with Millennium 120-MLC or HD-MLC. Inverse planning used the PRO optimizer, and final calculations were with AAA. Dose prescription was to 69.96 Gy and 54.45 Gy to PTV2 and PTV1, respectively, in 33 fractions. A RP model was generated for the OARs: spinal cord, brain stem, oral cavity, parotids, submanidbular glands, larynx, constrictor muscles, thyroid, eyes. To constrain the uninvolved healthy tissue, the ‘body’ with all the targets subtracted was included in the model. The optimization objectives in the model included the line objective for all OARs with generated priority. For serial organs, an upper objective was added with generated dose at 0% volume with a fixed priority of 90. For parotids and oral cavity, a mean objective was added with generated dose and fixed priority of 60. Targets upper and lower objectives were placed in a very narrow interval, with priority 110 and 120. The automatic Normal Tissue Objective NTO was added with priority 280. The model was validated on a set of 20 similar patients selected from the clinical database. The possible clinical benefit was evaluated through NTCP estimation for some of the OARs, using the biological evaluation availabile in Eclipse, based on LQ-Poisson Regarding target dose homogeneity, the standard deviation was reduced by 0.3 Gy with RP (p<0.05). The mean doses to parotids, oral cavity, and larynx were reduced with RP of 2.1, 5.2, and 7.0 Gy, respectively. Maximum doses to spinal cord and brain stem were reduced of 7.0, and 6.9 Gy, respectively (p<0.02). NTCP reductions of 11%, 16%, and 13% were estimated for parotids, oral cavity, and larynx, respectively, with RP planning. Conclusion Model validation confirmed the better plan quality with RP plans. NTCP estimation suggests that this dosimetric effect could positively affect also the toxicity profiles for patients receiving RP planning with an adequate model. EP-1529 Reducing total Monitor Units in RapidArc™ plans for prostate cancer K. Armoogum 1 , M. Hadjicosti 1 1 Derby Hospitals NHS Trust, Department of Radiotherapy, Derby, United Kingdom Purpose or Objective A retrospective planning study was performed on prostate cancer RapidArc (RA) plans to evaluate the use of the ‘MU model. Results