ESTRO 2020 Abstract Book

S893 ESTRO 2020

Conclusion Robustly optimized PBS-PT for cervical cancer patients compared to VMAT shows equivalent robustness against inter-fraction variability, but offers significantly better OAR sparing. PO-1632 Planning target volume (PTV) reduction for gated stereotactic body radiotherapy(SBRT) of lung tumors K. Kraus 1 , J.J. Wilkens 1 , M. Oechsner 1 , S.E. Combs 1,2,3 1 Technical University Munich, Radiation oncology, Munich, Germany ; 2 Deutsches Konsortium für Translationale Krebsforschung DKTK, Partner Site Munich, Munich, Germany ; 3 Helmholtz Zentrum München HMGU- - Institute of Radiation Medicine IRM, Department of Radiation Sciences DRS, Neuherberg, Germany Purpose or Objective A reduced PTV can result in better sparing of organs at risk (OARs). The presence of breathing motion, however, necessitates the enlargement of the PTV to cover the tumor in all respiratory phases. This is commonly known as internal target volume (ITV) concept. Delivery of the dose in defined respiratory phases known as gating windows (GWs) only allows for reduction of the ITV. Material and Methods We investigated 4 peripherally and 3 centrally located NSCLC cases. The gross tumor volume (GTV) was defined on the planning CT (pCT). An ITV was defined comprising the GTV in all respiratory phases of a 4D Computed Tomography (CT) acquired on 10 different motion phases indicated as 0 to 100. To define the PTV a spherical margin of 5 mm was added to the ITV. Different GWs were assigned. For one of the centrally and one of the peripherally located tumors GWs were defined covering 1 to 7 phases around the end-expiration phase. PTV reductions are reported with respect to the pCT contours. Treatment plans were calculated on the pCT and on one base phase CT (bCT) for the gating plans. A dose of 45 Gy delivered in 3 fractions was prescribed to the 65% isodose covering 98% or 99% of the PTV. The dose was recalculated on the corresponding GW phases. Deformable image registration was applied between the pCT or bCT and the included phase CTs. By applying the resulting transformation, the phase dose was mapped to the pCT or bCT and accumulated for all included phases. Dose constraints are analyzed and presented in table 1. 3D tumor motion was investigated by GTV center motion analysis. Results Results are reported in table 1. Figure 1 shows the PTV reduction for all GWs for patient 7. Maximum PTV reduction of 38.4% and 33.1% was observed for patient case 4 and 7, respectively. These were the tumors with maximum motion of 17.8 mm and 5.3 mm, respectively. PTVs ranged from 17.3 ccm for patient 5 to 79.8 ccm for patient 4. V20[lung] did not exceed 10% for all GWs and almost all peripherally located tumors. For central tumors V20[lung] was generally higher except for case 5 where the tumor volume was small. However, analyzing the doses for patient 7 a general trend of reduced irradiated lung volumes with decreasing GW sizes could be noted. For central tumors Dmean[heart] was below 2.7 Gy for all cases. However, Dmax[heart] for patient 7 exceeded 30 Gy. For almost all cases the V17.7[esophagus] exceeded 5 ccm. However, the reduction of esophageal dose with decreasing GW size is evident. Whereas Dmax[esophagus] could not be kept below 30 Gy for the plan including all phases for patient 5, with gating Dmax[esophagus] was reduced to 29.3 Gy.

Conclusion Gating can significantly reduce the PTV size and the treated healthy lung volume. Also for central tumors doses to healthy tissue could be significantly reduced making dose escalated gated SBRT a considerable solution for central lung Tumors. PO-1633 Impact of IGRT frequency on the dose reconstruction based on deformable registration of Halcyon CBCT H. Wang 1 , H. Wu 1 , W. Wang 1 , Y. Zhang 1 1 Beijing Cancer Hospital & Institute- Peking University Cancer Hospital & Institute, Department of Radiotherapy, Beijing, China Purpose or Objective Fractional image guided radiation therapy (IGRT) is beneficial for reducing setup errors and can be used for monitoring dose delivery based on deformable image registration (DIR). However, frequency of imaging guidance is sometimes sacrificed concerning the concomitant dose and secondary cancer risk, without tracking of its dosimetric influence. Using identical beams for treatment and MV cone beam CT (MVCBCT), the integrated dose can be optimized collectively by Eclipse hence fractional guidance has been made mandatory on Halcyon system. To investigate the impact of imaging frequency on the accuracy of target dose reconstruction using DIR, this work resampled the fractional Halcyon MVCBCT images to simulate various frequencies.