Abstract Book

S1008

ESTRO 37

EP-1867 Comparison of robust optimized proton planning strategies for dose escalation in pancreatic cancer S. Stefanowicz 1,2 , S. Zschaeck 3 , M. Rehm 4 , A. Jakobi 1,2,4 , K. Stützer 1,2 , E.G.C. Troost 1,2,4,5,6 1 OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden- Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany 2 Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology – OncoRay, Dresden, Germany 3 Charité Universitätsmedizin Berlin, Department of Radiation Oncology- Klinik für Radioonkologie und Strahlentherapie, Berlin, Germany 4 Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden, Department of Radiotherapy and Radiation Oncology, Dresden, Germany 5 German Cancer Consortium DKTK, partner side Dresden- and German Cancer Research Center DKFZ, Heidelberg, Germany 6 National Center for Tumor Diseases NCT, partner side Dresden, Dresden, Germany Purpose or Objective In patients with locally advanced unresectable pancreatic cancer, neoadjuvant or primary radiochemotherapy (RCT) are alternative treatment options. Today, treatment outcome after RCT is poor, in part due to radiosensitive organs at risk (OARs) limiting the prescribed dose to the target volume. Proton beam techniques enable delivering high radiation doses to the target volume while sparing OARs. In this in-silico feasibility study, we assessed different strategies for dose escalation to 66Gy(RBE) using a simultaneous integrated boost technique and robust multi-field optimized intensity modulated (rMFO- IMPT) pencil beam scanned protons and evaluated their robustness. Material and Methods For each of six pancreatic cancer patients, four different rMFO-IMPT plans were optimized on free-breathing treatment planning CTs using the RayStation treatment planning system (V5.99, RaySearch Laboratories AB, Sweden). These planning strategies consisted of equally- weighted beams: (S1) two posterior oblique (PO) beams, (S2) lateral right beam and left PO beam, (S3) two PO beams plus right non-coplanar beam, and (S4) three non- coplanar beams. At least 95% of 66Gy(RBE) in 30 fractions was prescribed to 95% of the boost volume (GTV), and 51Gy(RBE) was prescribed to 95% of the CTV (GTV with margin and elective volume). A dose fall-off range of 10 mm around the GTV was preset, and setup and range uncertainty parameters of 3 mm and of 3.5% for GTV and CTV coverage were chosen, respectively. The OAR dose constraints adhered to local guidelines and QUANTEC. For each patient and planning strategy, conformity and homogeneity index (CI, HI) of target doses as well as doses to GTV, CTV, and OARs were calculated. Together with additional robustness evaluations of the worst-case scenarios (±3 mm, ±3.5%) the best planning strategy for dose escalation was sought for. Results All nominal plans reached the prescribed dose to the GTV and CTV (Fig. 1a). The CI of all planning strategies was similar (mean CI: 0.6-0.7) even though S3 and S4 were more homogeneous. In some patients, S1 was associated with excess dose to the kidneys (Fig. 1b). Radiation doses (D 2% , V 45Gy ) to the duodenum exceeded the constraints since that OAR was next to or within the target volume, while for the remaining gastrointestinal organs radiation doses were similar for the different strategies and within preset limits (Fig. 1c, d). Overall, S3 and S4 showed the best dose distribution for all OARs. Robustness evaluation of all plans revealed that in total only 38% of the D 95% values (S1: 31%, S2: 31%, S3: 39%, S4: 51%) in the worst-

Electronic Poster: Physics track: Treatment plan optimisation: algorithms

EP-1866 Influence of gantry angle increment and number of control points on prostate cancer using VMAT S. Poeta 1 , Y. Jourani 1 , C. Vandekerkhove 1 , O. Koshariuk 1 , D. Rodriguez 1 , P. Fernandes 1 , F. Kert 1 , S. Simon 1 , N. Reynaert 1 1 Institut Jules Bordet ULB, Medical Physics, Brussels, Belgium Purpose or Objective In the Monaco treatment planning system several parameters can be selected that have an important impact on the quality of the VMAT prostate plans. In this study, the variation of the gantry angle increment (GAI) and the maximum number of control points (CPs) allowed per arc were studied for 10 prostate cases. Material and Methods All plans were calculated in Monaco version 5.00.04 for an Elekta Infinity TM linac with Agility TM treatment head. The GAI analysed were 10°, 20°, 30° and 40°, while fixing all other TPS parameters. To evaluate the impact of the maximum number of control points allowed, plans for 40, 90 and 150 CPs were obtained, again fixing all remaining parameters. For plan comparison MUs, D 95% of the PTV, D max , mean rectum dose, PTV Homogeneity Index, PTV Conformity Index, total number of CPs, delivery time (seconds) and planning time (minutes) were analysed using rANOVA and paired t- test. Results The parameters most influenced by variation of the GAI were the number of MUs, the D 95% on the PTV, the planning time and the treatment delivery time. When decreasing the GAI, D 95% coverage was degraded up to 1,5Gy and lead to a 13 % increasing number of MUs. Low GAI was time consuming, increasing the planning time by about 20 min per plan. For the maximum numbers of CPs allowed, the most influenced parameters were the number of MUs, the D 95% on the PTV, the number of CPs used by the TPS, the treatment delivery time and the mean rectum dose. All other parameters did not vary significantly. High values of CPs doubled the number of MUs and the delivery time was increased by half a minute. Increasing the maximum number of CPs allowed from 40 to 90 CPs increased the number of CPs used by the TPS by about 80% and when comparing the plans of 40 with 150 CPs, the difference was about 200%. The mean rectum dose was reduced by about 1,5 Gy (3%) for plans with higher CPs. Conclusion Regarding the GAI, extremely low values do not provide an optimal plan for prostate cases. The planning process becomes time consuming and the MUs are increased. A GAI of 40 does not show any clear advantage. Regarding the number of CPs, the system will always use more CPs when allowed, even if not needed. This will have a direct impact on the number of MUs and the treatment delivery time. The increase of CPs also leads to a decrease of robustness, due to the increased level of modulation. In cases of very complex target shapes, the increase of number of CPs can be justified, otherwise this increase is not necessary. In this study, those patients would not benefit from a high number of CPs. In conclusion, a GAI between 20 and 30 seems to be the optimal choice for prostate plans and it is recommended to use a lower value of CPs.