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showed a 20% decrease in survival when exposed to
Trastuzumab alone, but a combined treatment with
radiation did not yield the expected decrease in survival,
indicating an antagonistic interaction.
Conclusion
Our results show that before starting clinical trials, the
combination of radiation therapy and combined targeting
agents needs to be closely examined for each sub-type
under consideration. The assumption that a combination
of treatments will result in a synergistic response is clearly
not always true.
Acknowledgements
We acknowledge funding from the Sydney Breast Cancer
Foundation
PO-0919 Stereotactic radiotherapy for brain
metastases : Cyberknife versus VersaHD / ExacTrac
M. Perdrieux
1
, M. Celeste
1
, I. Lecouillard
1
, E. Nouhaud
1
,
C. Blay
1
, F. Jouyaux
1
, N. Delaby
1
, J. Bellec
1
, C. Lafond
1
1
Centre Eugène Marquis, Radiotherapy, Rennes CEDEX,
France
Purpose or Objective
The aim of this study was to compare dosimetric and
geometric performances of the CyberKnife (Accuray) and
VersaHD (Elekta) with the ExacTrac system (BrainLab) in
stereotactic radiotherapy for brain metastases.
Material and Methods
This study was conducted on 10 patients for Cyberknife M6
v10.6 with Iris collimator and VersaHD equipped with
ExacTrac v6.1 and the Frameless system (BrainLab). The
prescribed dose was 27 Gy in 3 fractions with 1mm margin
between CTV and PTV for both modalities. The dosimetric
study was also conducted with 2 mm margin for VersaHD
plans in accordance to our clinical practices.
Plans have been computed for CyberKnife with non-
isocentric non-coplanar beams generated by inverse
optimization on Multiplan v5.3 (Accuray) with the
RayTracing dose calculation algorithm. For VersaHD, 4
non-coplanar arcs (VMAT) have been generated b y inverse
optimization on Pinnacle v9.10 (Philips ) with the
Adaptative Convolution algorithm. For each case, plans
were normalized to obtain the same PTV co verage at +/-
0.2 %.
The comparison was based on the brain volume outside
PTV receiving 23.1 Gy. The volume of isodoses 6 Gy, 2.7
Gy and 1 Gy have been reported as well as the Paddick’s
Gradient Index to characterize the dose gradient around
PTV and the spread of low doses.
Quality controls have been performed with Gafchromic
EBT3 films (Ashland) and with an ionization chamber
(Pinpoint 31014 /PTW) in an anthropomorphic phantom
(STEEV/CIRS). The measured dose with film has been
compared to the calculated dose according to the gamma
index method with a 3% (local) / 2 mm criteria (analytical
threshold : 30% of the maximum dose). The geometric shift
between the measured and calculated dose distribution
has been also reported.
Results
Table 1 shows that dosimetric criteria for plan validation
were reached for both modalities and both margins.
Compared to VersaHD, dose gradients obtained with
Cyberknife were greater and lower volumes of healthy
tissue received doses below 6 Gy.
Ionization chamber measurements showed mean
differences with the calculated dose of 2.53% and 0.03%
for Cyberknife and VersaHD respectively. The mean value
of the gamma index was 0.42 for the Cyberknife and 0.38
for the VersaHD. The mean geometric shifts between the
measured and calculated dose distributions were 0.87 mm
and 0.84 mm for Cyberknife and VersaHD 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
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