Abstract Book

S1018

ESTRO 37

delivery, for accurate 3D reconstructions of the daily delivered dose. As such, a log-file and Monte Carlo (MC) based independent dose calculation for PBS-PT has been developed. Material and Methods The MC model, based on TOPAS 3.0.p1 (Geant 4.10.02.p01), has been commissioned and tuned to the beam data of our PBS-PT gantry. Each pencil beam of a plan is characterized by its energy, energy spread, angular and spatial distribution, and number of protons. A pre-absorber (if required) is modelled as a component in the beam. MC based calculations have then been included in the Independent Dose Calculation (IDC) system developed by Meier et al. such that TPS data, machine control (MCF) and treatment log files (LF) (spot positions and proton numbers based on measured parameters during delivery) can all be used as input to the analytical TPS or MC calculation engines. Using the terminology of Meier et al, all 4 levels of independence for PBS-PT dose calculations are thus supported by this Bragg peak ranges and widths between measured and MC data fit to within 0.1 mm/0.2 mm respectively for individual pencil beams, with range shifts in water due to the pre-absorber fitting to within 0.2 mm. Energy dependent beam widths in air match to within 0.1 mm to measurements. Independently calculated dose distributions for an example clinical case are shown in figure 1. a) shows the analytical TPS dose, calculated using the ray casting approach (TPS), b) the analytically reconstructed dose from machine control files (TPS-MCF), c) analytically reconstructed dose from log-files (TPS-LF) and d) MC reconstructed dose from log-files (MC-LF). Dose difference distributions are shown in figure 2. Dose to 100, 89 and 79% of voxels agree to within +/- 2% of TPS dose for the TPS-MCF, TPS-LF and MC-LF reconstructions respectively. Good agreement with the TPS-MCF and TPS- LF calculations indicate that TPS data has been transferred to, and delivered by, the gantry correctly (TPS-MCF and TPS-LF reconstructions respectively), whereas the MC-LF reconstructed dose provides an accurate estimate of the actually delivered dose for subsequent dose accumulation as part of the DAPT process. system. Results

In particular, the heart and thyroid mean doses and lung V20 were significantly reduced using AP engine. Accordingly, AP provides similar, if not lower, complication risks (Table and Figure).

The median number of monitor units was slightly reduced by AP (3.5%). Hands-on planning time decreased on average by a factor of 3 by AP. Conclusion Pinnacle 3 AP module was able to limit heart and thyroid complication risks, producing clinically acceptable HL plans with stable quality without additional user input. Overall AP-ARC provided better results in terms of OAR sparing when compared with AP-BF. Of note, given the high interpatient PTV (size and position) variability, it was not possible to create a standard HL AP optimization list. EP-1881 Log-file based Monte Carlo simulations for proton pencil beam scanning (PBS) C. Winterhalter 1 , G. Meier 1 , S. Safai 1 , D. Oxley 1 , A. Bolsi 1 , A. Fredh 1 , D.C. Weber 1 , A. Lomax 1 1 Paul Scherrer Institute, Center for Proton Therapy, Villigen PSI, Switzerland Purpose or Objective Daily adaptive proton therapy (DAPT) maximises the accuracy and effectiveness of PBS proton therapy (PBS- PT). Independent dose calculations (Meier et al, PMB, 60, 2015) are a critical tool in DAPT for plan specific QA and, when combined with logged information from treatment

Conclusion A MC tool for PBS-PT has been included in a level 4 IDC system for estimating both delivery and calculational uncertainties as part of a PBS-PT DAPT regime. The accumulation of reconstructed daily distributions from MC based log file reconstructions will provide accurate estimates of the actually delivered dose, and by being