ESTRO 37 Abstract book

S1108

ESTRO 37

the same criteria were created for DS. The prescribed dose to the CTV was 63 Gy in 15 fractions. A customized and experimentally validated interplay effect routine [2] served to compute 4DDs for PBS based on an empirical beam time model, a 4D CT and deformable image registrations. For DS, the dose delivery was distributed equally over all motion phases to compute 4DDs. Resulting treatment plans were analyzed in terms of DVH metrics, i.e. the percentage of over- (V107%) and underdosage (V95%) and the homogeneity index: Results The percentage of over- and underdosage was zero for DS, whereas for PBS it ranged typically from 5 to 10% for a single fraction and approached 0% when considering all fractions. There was almost no difference in the HI for the static dose and the 4DD in DS. In PBS, the HI was around 10 for a single fraction and went down to below 5 (close to the static case) when averaging over all fractions. OARs proximal to the target, such as ribs, could be spared very well with PBS in cases where significant OAR volumes received the prescribed target dose in DS. Furthermore, the dose to the normal liver tissue was reduced in PBS. Conclusion The developed interplay effect model was proven to be useful for 4DD comparisons in proton therapy. It can help to evaluate the available treatment options for a patient and to access residual uncertainties related to organ motion and proton range. For the investigated HCC patients PBS without further motion mitigation techniques was shown to be inferior to DS in terms of homogeneous target coverage for a single fraction. But the study demonstrated the expected high potential of PBS to spare OARs proximal to the target and liver tissue in comparison to DS. [1] Chang et al., Int J Radiation Oncol Biol Phys 2017, 99:41e50 [2] Pfeiler et al., Z Med Phys. 2017, https://doi.org/10.1016/j.zemedi.2017.07.005 EP-2025 Patient setup accuracy in spinal SBRT J.M. Perez moreno 1 , L. Alonso Iracheta 1 , O. Hernando Requejo 2 , R. Alonso Gutierrez 2 1 Hospital Universitario HM Puerta del Sur, Radiofísica y Protección Radiológica, Móstoles, Spain 2 Hospital Universitario HM Puerta del Sur, Radiation Oncology, Móstoles, Spain Purpose or Objective In the treatment with SBRT of vertebral and paravertebral lesions it is usual not to make any expansion to the defined target volume. In addition, it is very close, if not in contact, with the spinal cord. Since high dose gradients are present in this type of treatment and the regions of interest are small in size, a slight deviation in patient placement during treatment may lead to significant deviations from the treated versus planned schedule. It is intended to evaluate the setup error of patients undergoing SBRT treatments of vertebral and paravertebral lesions. Material and Methods Spinal SBRT treatments have been delivered in a VERSA HD linac using Agility MLC and 6FFF energy. Patient setup have been performed with cone beam (CB) based IGRT XVI R5.0, wich allows CB acquisitions simultaneously to treatment delivery. 80 CB images have been acquired during 23 spinal SBRT treatment fractions of 13 patients (10 single dose tretments). Every treatment consist of 4 full VMAT arcs. Workflow of treatment delivery is as follows: 1-initial patient setup 2-initial CB to setup error correction

Conclusion AlignRT with an Elekta Linac showed to be suitable and might result in an improvement of the delivered dose distribution. However, a commissioning program of the complete system (TPS, Linac, and AlignRT) should be conducted to identify the limitations and to define clinical protocols. EP-2024 4D evaluation of proton pencil beam scanning and double scattering for hepatocellular carcinoma T. Pfeiler 1,2 , C. Bäumer 1 , O. Blanck 3,4 , M. Chan 3,5 , E. Engwall 6 , D. Geismar 1,7,8 , S. Peters 1,7,8 , B. Spaan 2 , J. Wulff 1 , B. Timmermann 1,7,8 1 West German Proton Therapy Center Essen WPE, Essen, Germany 2 TU Dortmund University, Experimental Physics 5, Dortmund, Germany 3 University Clinic Schleswig-Holstein, Department of Radiation Oncology, Kiel, Germany 4 Saphir Radiosurgery Center, Güstrow and Frankfurt, Frankfurt, Germany 5 Imperial College London Healthcare Trust, Department of Radiation Physics, London, United Kingdom 6 RaySearch Laboratories AB, Stockholm, Sweden 7 University Hospital Essen, West German Cancer Center WTZ, Essen, Germany 8 University Hospital Essen, Clinic for Particle Therapy, Essen, Germany Purpose or Objective In proton therapy there is a rising trend towards pencil beam scanning (PBS) for the treatment of moving tumors. Contrary to passive delivery techniques such as double scattering (DS), active PBS offers a high conformal dose distribution both at the distal and proximal edge of the target, enables intensity modulation and does not require patient specific hardware. However, it suffers from the interplay of organ motion and pencil beam motion which might lead to serious distortions of the intended dose distribution. Modeling of interplay effects becomes thus a crucial ingredient of motion management in PBS [1]. In this study we investigate 4D dynamic accumulated dose distributions (4DDs) of PBS and DS plans regarding target coverage and sparing of organs at risk (OARs) for hepatocellular carcinoma (HCC), representative for targets with moderate motion amplitudes. Material and Methods 4D robust optimized PBS plans and DS plans with beam specific PTVs were created in RayStation 5.99 for 7 HCC patients. 2 mm setup error, 5% range uncertainty and 10 motion phases were used in the optimization of PBS plans. Corresponding internal target volumes based on