ESTRO 36 Abstract Book

520 / 1082

S505 ESTRO 36 2017
_______________________________________________________________________________________________
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. Stanhope1, D. Drake1, M. Alber2, M. Sohn2, J. Liang1, C.
Habib1, D. Yan1
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
for use in said system.
Material and Methods
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
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
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. Balter1, T. Netherton1, Y. Li1, P. Nitsch1, S. Gao1, M.
Muruganandham1, S. Shaitelman-1, S. Frank1, S. Hahn1, A.
Klopp1, L. Court1
1UT 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