ESTRO 38 Abstract book

S948 ESTRO 38

Figure1 Histogram of Δ CTV D mean

for 113 fractions is shown

Purpose or Objective Purpose: at our institution in the last decades stereotactic radio surgery (SRS) treatments have been performed with frame-based targeting methods on linear accelerators. Recently a CyberKnife M6 (CK) with InCise2 MLC collimator has been installed in our department. Simultaneously the transition from frame-based to frameless SRS treatments was started on a TrueBeam v2.5 (TB) linear accelerator. The aim was to implement a unified method for quantification of spatial accuracy for these delivery systems. Material and Methods Materials and methods: on the CK the end-to-end tests were implemented according to the AAPM TG135 report with an antropomorphic phantom and Ball-cube insert. The concept of the method is to plan and deliver 4,2 Gy (70% isodose line) as conformal and symmetrically centered on the ball inside the phantom as possible, then calculate the distance between the mass center of the irradiated circle on EBT3 films and the center of the ball. For measurements pre-cut Gafchromic EBT3 Ball Cube II films were orthogonally placed inside the phantom and Accuray developed Matlab software was used for analysis. This way vertical, longitudinal and lateral targeting errors can be measured. The total targeting error is the vectorial sum of these three values. 7 deliveries were performed with both CK and TB. MLC collimator and 6DSkull tracking were used on CK. Rapid Arc delivery with kV-CBCT verification and 6DoF couch were used on TB. 3 of 7 TB deliveries were coplanar arcs without couch movement, while the others had 4 different couch angles, to evaluate the possible errors originating from couch movement during treatment.

in this figure. Conclusion

Results of in vivo transit dosimetry using a commercial system are reported in this paper. A new tolerance level for abdominal and pelvic SBRT was established in this work. EP-1756 Ion recombination and polarity correction for a plane-parallel ionization chamber in hadrontherapy D. Maestri 1 , A. Mirandola 1 , G. Magro 1 , A. Mairani 1 , E. Mastella 1 , S. Molinelli 1 , S. Russo 1 , A. Vai 1 , M. Ciocca 1 1 Fondazione CNAO, Medical Physics, PAVIA, Italy Purpose or Objective In order to physically characterize the particle beams clinically used in hadrontherapy, the integrated depth- dose-distributions (IDDs) are measured and implemented as basic input data for commissioning of commercial TPSs, usually compared against Monte Carlo (MC) simulations. The IDDs curves are typically acquired as a ratio of readings of two large parallel-plate ionization chambers mounted on a variable depth water column detector. The purpose of this work is the evaluation, in terms of ion recombination and polarity effect, of the dosimetric correction for both proton and carbon ion curves as a function of depth in water, hence of LET variation. Material and Methods Dose-averaged LET values were calculated with a MC code for some selected IDDs. Several points were investigated: the beam entrance, the initial slope, the Bragg peak position, the distal falloff and the practical range position. The charge produced was measured with the plane- parallel Bragg ionization chamber mounted on the PTW Peakfinder by delivering a fixed number of particles, with the maximum current level available. For each measurement, the field chamber was alternatively supplied with +400, +200, +100, -400 Volt. Ion recombination and polarity correction factors were then punctually calculated. Three different beam energies, representative of the clinical commissioned range, were investigated for this study, for both protons and carbon ions. The IDD k s, corrected for the aforementioned factors, were then compared against MC simulations, for the same experimental setup. Results Ion recombination correction factors were both LET and energy dependent, ranging from 1.000 to 1.035 with uncertainties ≤ 0.5% for carbon ions, while they were found to be nearly negligible for protons. Moreover, no corrections need to be applied due to polarity effect, being < 0.5% along all the measured IDDs for both particle types. Corrected IDDs (IDD k s) showed a better agreement than uncorrected curves when compared to MC In this study, correction factors due the ion recombination and polarity effect were estimated and applied along the pristine IDDs for protons and carbon ions. The IDD k s were then compared against MC simulations showing a very good agreement. All results confirmed that corrections cannot be completely neglected. EP-1757 Comparison of geometrical accuracy of different SRS delivery systems with end-to-end tests G. Stelczer 1,2 , D. Szegedi 1 , T. Major 1,3 , C. Polgár 1,3 , C. Pesznyák 1,2 1 National Institute of Oncology, Center of Radiotherapy, Budapest, Hungary ; 2 Budapest University of Technology and Economics, Institute of Nuclear Techniques, Budapest, Hungary ; 3 Semmelweis University, Department of Oncology, Budapest, Hungary simulations. Conclusion

Results Results: the end-to-end test method dedicated to CK system was implemented also on the TB system successfully. Both systems’ spatial accuracy was far below the tolerances. The average targeting errors in vertical, longitudinal and lateral directions for CK and TB in mm were 0,14±0,22; 0,13±0,18; -0,07±0,18 and -0,15±0,22; - 0,24±0,14; -0,29±0,18, respectively. The average total targeting errors in mm for CK and TB were 0,36±0,09 and 0,48±0,13. Average total targeting errors with TB regarding coplanar and non-coplanar techniques were similar, with the values of 0,43±0,14 mm and 0,53±0,13 mm respectively.

Conclusion Conclusion: the AAPM TG135 report recommended end-to- end test with the head phantom of CK is also feasible on