ESTRO 36 Abstract Book

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S783 ESTRO 36 2017
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The calculation models the spatial response of the IQM
chamber and the fluence transmitted through the
individual collimating elements. The chamber response is
modelled as a 2D map. The fluence from the machine is
divided into 2 components: a point source at the target
and an extended source at the flattening filter, referred
to as the primary and extended source, respectively. The
primary source is characterized by a radial intensity
profile and is attenuated through the jaws and multileaf
collimator. Transmission is calculated for a 2D array
matching the chamber response map, and area averaged
fluence is calculated for moving collimating elements
during beam delivery. The extended source is modeled as
a Gaussian distributed source with a Compton angular
intensity distribution. The contribution of the Gaussian
source to each element in the fluence array is raytraced
through the collimation to obtain the area averaged
fluence. An element-wise multiplication of the chamber
response map with the primary and extended source
fluence is summed to generate the predicted signal,
modified by factors reflecting the chamber volume, the
intensity of the primary and extended sources and change
in machine output with field aperture. The model has
been implemented for Varian and Elekta treatment units,
with calculations and measurements compared for
clinically relevant fields.
Results
Parameters for the model were determined from a series
of rectangular field measurements with the IQM chamber
combined with ion chamber measurements. Iterative
optimization of parameter values to match rectangular
field IQM measurement were performed. Similar
techniques were used to extract normalization
parameters.
The agreement between the calculated and measured
signals on a Varian TrueBeam unit for over 300 different
IMRT field segments from Prostate and Head & Neck plans
show 99% of segments agree within ±5%; 95% within ±3%.
Similar results were seen for an Elekta Agility unit in a
sample of over 400 different IMRT field segments, with
97% of segments agreeing within ±5% and 91% within
±3%.Conclusion
A 2-source calculation model has been implemented for an
area-fluence monitor designed for on-line patient QA.
EP-1483 Pre-Treatment QA of MLC plans on a
CyberKnife M6 using a liquid ion chamber array.
L. Masi 1, R. Doro 1, O. Blanck2, S. Calusi3, I. Bonucci 4, S.
Cipressi4, V. Di Cataldo4 , L. Livi5
1 IFCA, Medical Physics, Firenze, Italy
2 Saphir Radiosurgery Center, Medical Physics, Frankfurt/
Gustrow, Germany
3 University of Florence, Department of Clinical and
Experimental Biomedical Sciences "Mario Serio", Firenze,
Italy
4 IFCA, Radiation Oncology, Firenze, Italy
5 Azienda Ospedaliera Universitaria Careggi,
Radiotherapy Unit, Firenze, Italy
Purpose or Objective
CyberKnife MLC plans require accurate patient-specific
QA. The purpose of this study is to validate the use of a
liquid ion chamber array for Delivery Quality Assurance
(DQA) of robotic MLC plans, using several test scenarios
and routine patient plans and comparing results to film
dosimetry.
Material and Methods
Five preliminary sensitivity test scenarios were created
from a baseline plan modifying each MLC segment by
introducing increasing shifts in leaves positions (0.5 mm -
2 mm). The baseline and test plans were delivered to an
Octavius 1000SRS array (PTW) as well as to EBT3 films. An
average correction was applied to 1000SRS results to
account for the response dependence on source-detector-
distance (SDD) [O. Blanck . Phys Med 2016]. The same
five test plans were delivered a second time to the
1000SRS re-orienting all beams perpendicularly to the
array (nominal position) to eliminate SDD and angle
dependence. As a second step 40 clinical MLC plans
optimized for various treatment sites (liver, spine,
prostate) were delivered to the liquid ion chamber array
for patient-specific QA using both the clinical beam
orientations and the beam “nominal position”. For the
latter only a subset of segments(18-21) was selected.
Finally, for 15 out of 40 clinical plans a film-based DQA
was also performed. All results were analyzed using (2%,
2mm),( 3%, 1mm) and (2%, 1mm) gamma index criteria [O.
Blanck Phys Med 2016].
Results
The pass-rate reductions from the baseline,obtained
delivering the five test plans, are shown in fig.1 for (3%,
1mm) gamma criteria. The Octavius 1000SRS showed a
good sensitivity to simulated delivery errors with pass-rate
reductions increasing from 1.7% to a maximum of 43% with
increasing leaves shifts (0.5 mm - 2 mm). Similar
sensitivity was observed when the beams were re-oriented
in the nominal position geometry. The pass-rate
reductions observed with films showed a more irregular
trend, and the maximum reduction was 16%. The average
pass-rates obtained over clinical plans are shown in fig.2,
for the three gamma index criteria. The mean values
obtained by the 1000 SRS array, using both the clinical and
nominal beam geometry, and by film-dosimetry are all
above 92%, when using 3%, 1mm criteria. Differences
among the mean pass-rates observed for the three
measurement modalities were not statistically significant
(p> 0.1, t-test)
Conclusion
The results confirm that the 1000SRS array is reliable for
pre-treatment QA of CyberKnife MLC plans. The test
scenarios highlighted a higher sensitivity to small leaves
shifts than what observed by film dosimetry. The gamma