Abstract Book

ESTRO 37

S474

literature. By itself, the gantry sag effect does not produce clinically perceptible dose changes for shift ranges up to ±1 mm, both for coplanar and non-coplanar techniques. At the same time, it also indicates that 2mm diameter tolerance of gantry isocenter is reasonable as variations in excess of this value start to affect the overall dosimetric and spatial uncertainty. G. Smyth 1 , P.M. Evans 2 , J.C. Bamber 1 , H.C. Mandeville 3 , A.R. Moore 1 , L.C. Welsh 4 , F.H. Saran 4 , J.L. Bedford 1 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom 2 University of Surrey, Centre for Vision Speech and Signal Processing, Guildford, United Kingdom 3 The Royal Marsden NHS Foundation Trust, Children's & Young Person's Unit, Sutton, United Kingdom 4 The Royal Marsden NHS Foundation Trust, Neuro- Oncology Unit, Sutton, United Kingdom Purpose or Objective Optimized trajectories for dynamic couch rotation VMAT (DCR-VMAT) reduce dose to organs at risk compared to coplanar VMAT. However, to date there has been limited work reported on the delivery of dynamic couch techniques. This work investigated the dosimetric accuracy and delivery efficiency of DCR-VMAT compared with VMAT for five primary brain tumour treatment plans. Material and Methods DCR-VMAT plans were produced using an in-house treatment planning system (TPS) for a digital linear accelerator (TrueBeam, Varian Medical Systems, Palo Alto, CA). Final dose calculation was performed in a commercial TPS (Pinnacle 3 v9.10, Philips Medical, Madison, WI). DICOM RTPLAN files were converted to XML files to enable dynamic delivery of each plan through the linac’s R&D mode. Dosimetric accuracy was investigated by comparing TPS predictions against measurements in a solid water phantom using: (1) radiochromic film in the sagittal and coronal planes, analysed using a gamma analysis with 3% dose, 3 mm distance and 20% low dose threshold criteria, (2) a centrally located 0.125 cc volume ionization chamber. Mechanical accuracy was analysed using linac log files by comparing predicted and delivered values for MLC leaf, couch, and gantry positions. Log file dose reconstruction was performed by calculating dose in the TPS from the delivered trajectory, and compared to the original treatment plan using 3D gamma analysis (2%/2 mm, 5% threshold). This work determined the dosimetric effect of static modelling of using control points spaced every 2°, and the impact of machine- recorded mechanical errors. Results Median (range) treatment delivery times were 125 s (123- 133 s) for DCR-VMAT, compared to 78 s (64-130 s) for coplanar VMAT. Absolute point dose differences were 0.8% (0.6-1.7%) for DCR-VMAT, compared to 1.3% (0.6- 1.6%) for VMAT. Percentage of pixels passing gamma analysis was 99.2% (96.7-100%) and 98.1% (92.9-99.0%) for coronal and sagittal films with DCR-VMAT, compared with 100% (97.6-100%) and 96.6% (81.0-99.1%) for VMAT. DCR- VMAT point dose and gamma analysis results are presented in Table 1. Coronal and sagittal film gamma analyses for DCR-VMAT Case 3 are presented in Figure 1. Root-mean-square error (RMSe) for MLC leaves was less than 0.03 mm for both techniques. Gantry rotation RMSe was 0.057° (0.051-0.059°) for DCR-VMAT and 0.055° PO-0893 Dosimetric accuracy and delivery efficiency of dynamic couch rotation VMAT (DCR-VMAT)

(0.051-0.067°) for VMAT. Couch rotation RMSe for DCR- VMAT was 0.091° (0.086-0.102°). Log file dose reconstructions showed that 100% of voxels within the patient external contour passed gamma analysis for both techniques.

Conclusion The dosimetric and mechanical accuracy of DCR-VMAT was comparable to coplanar VMAT. Delivery times were up to 69 s slower for DCR-VMAT than VMAT; differences, however, were case dependent. Static modelling of dynamic delivery and recorded mechanical errors did not significantly affect plan dosimetry. DCR-VMAT delivery is technically feasible and could be implemented for a given patient if improved OAR sparing was deemed clinically relevant. PO-0894 Reduced spot number for PBS proton therapy shortens delivery time without dosimetric plan compromise M.F. Belosi 1 , S. Van de Water 2 , F. Albertini 1 , D.C. Weber 1 , A.J. Lomax 1 1 Paul Scherrer Institute, Centre for Proton Therapy, Villigen PSI, Switzerland Purpose or Objective The degeneracy of spot-scanned proton therapy treatment plans can be exploited to shorten delivery times, by reducing the number of proton spots (i.e. pencil beams) while maintaining dosimetric plan quality. Because a strongly reduced number of spots can