ESTRO 36 Abstract Book

S505 ESTRO 36 2017 _______________________________________________________________________________________________

beams (0.4 ± 3.2%). Comparing C4 to C2, log file accuracy is analyzed for MC. LF resulted in lower σ values for 20/22 arcs (-5.4 ± 3.4%) and improved pass rates for 14/18 arcs (1.1 ± 1.4%). Comparing C5 to C2, LF and AC QA techniques are compared. The LF technique yielded decreased σ values for 22/22 arcs (-51 ± 7%) and improved pass rates for 18/18 fields (9.9 ± 3.8%). The LF technique also eliminated systematic AC errors; mean dose errors decreased from 3.2% to 0.1%. For 1/2/3/4° LF-CS control point spacing, 1%/1mm pass rates were 80.0 ± 5.0%, 78.0 ± 4.2%, 74.0 ± 5.1%, and 68.8 ± 5.3%. Plan-CS pass rates were 80.2 ± 4.0%. Figure 2 plots difference in pass rates [(LF-CS vs. AC) minus (Plan-CS vs. AC)] as a function of control point spacing for each arc. Calculation times for CS and MC were 12s per control point and 3 minutes per VMAT arc respectively.

Conclusion For brain metastases stereotactic radiotherapy, Cyberknife with Iris collimator and VersaHD with ExacTrac both allowed compliance to dosimetric criteria. Cyberknife provided higher dose gradients than VersaHD and limited low dose irradiation of healthy tissues. The agreement between calculated dose and measured dose was acceptable for both modalities with mean gamma values lower than 0.5. An investigation will be performed to evaluate the use of low margins (1 mm) with the VersaHD / ExacTrac due to the very low geometric deviations. PO-0920 Utilizing monte carlo for log file-based delivery QA C. Stanhope 1 , D. Drake 1 , M. Alber 2 , M. Sohn 2 , J. Liang 1 , C. Habib 1 , D. Yan 1 1 Beaumont Health System, Radiation Oncology, Royal Oak MI, USA 2 Scientific RT, Munich, Germany Purpose or Objective The purpose of this study is to (1) investigate the feasibility of using Elekta’s R3.2 Log File (LF) Convertor as a standalone technique for patient-specific QA, and (2) assess Scientific RT’s SciMoCa monte carlo (MC) algorithm Eleven clinical, dual-arc VMAT patients [9 H&N, 2 low dose rate brain (35MU/min)] previously planned in Pinnacle and calculated using Adaptive Convolution (CS) were selected for this study. Arcs were delivered on Sun Nuclear’s ArcCHECK (AC) phantom and LF recorded. LF were converted into dicom plan files and calculated using CS and MC. For MC, all LF samples were reconstructed with no increase in calculation time. For CS, plans were reconstructed using 1° control point spacing to decrease computational cost. Original (Plan), LF, and AC doses were compared; statistical distributions (mean ± σ) of percent diode dose error, as well as 1%/1mm gamma pass rates, were calculated and compared for the five comparisons C1 to C5 shown in Table 1. A standard 10% threshold was utilized for both statistical and gamma analyses. Dosimetric degradation due to increased control point spacing (1/2/3/4°) was assessed for CS using 1%/1mm gamma criteria for 4 H&N and 1 brain patient. Delivering a 25x25 arc at various dose rates (35 to 570 MU/min) diode sensitivity dependence on dose rate was quantified. Results In-field diodes under-responded by 1.5±0.4% at 35 MU/min compared to 570 MU/min. Consequently, the four brain fields yielded lower Plan-MC pass rates (44±8%). These arcs were excluded from subsequent gamma analysis. Pass rates and diode dose errors are shown in Table 1. Comparing C2 to C1, MC and CS are compared. MC resulted in decreased σ values for 17/22 arcs (-3.7 ± 6.5%) and increased passing rates for 10/18 for use in said system. Material and Methods

Conclusion MC doses proved more accurate than CS when compared to AC measurement. LF-MC plans yielded superior accuracy and shorter calculation times than LF-CS plans. By cutting out the phantom and comparing LF dose to that of the original plan, systematic error was eliminated and random error greatly reduced. PO-0921 Dose considerations of IGRT using MV projection and MV CBCT on a prototype linear accelerator P. Balter 1 , T. Netherton 1 , Y. Li 1 , P. Nitsch 1 , S. Gao 1 , M. Muruganandham 1 , S. Shaitelman- 1 , S. Frank 1 , S. Hahn 1 , A. Klopp 1 , L. Court 1 1 UT MD Anderson Cancer Center Radiation Physics, Radiation Physics, Houston- TX, USA Purpose or Objective The use of the mega-voltage treatment beam for image- guided patient setup has some potential advantages over kV imaging, especially reduced equipment and QA requirements. One of the challenges that MV imaging introduces is the increase in daily imaging dose. Here we investigate (1) whether the MV imaging dose can be correctly calculated and incorporated into the treatment plan, and (2) the impact of MV imaging dose on the dose to normal tissues such as the lung and heart. Material and Methods MV imaging dose to the lung, heart and other soft tissue was measured using an ion chamber in anthropomorphic thorax phantom (CIRS), and compared with dose calculated in the TPS (Eclipse) for orthogonal MV-MV imaging fields and MV CBCT images using a prototype linear accelerator, each with a low-dose and high-quality mode (total 4 modes). The impact of the imaging