444 / 1082
S429
ESTRO 36 2017
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Purpose or Objective
Current clinical portal dosimetry from Varian evaluates
dose using calibrated units (CU). This work assesses the
quality of the first Varian implementation of the MAASTRO
algorithm for pre-treatment absolute portal dosimetry (in
Gy) of 6X-FF fields.
Material and Methods
To achieve the proposed goal, a comparison was made
between the gamma analysis results obtained using both
Varian’s clinical portal dosimetry (PDIP v10.0.28) and the
MAASTRO algorithm [1] implementation made available
for the authors in the Portal Dosimetry (PD) application
accessible through the Varian Citrix Research Environment
(CRE). For this study, 10 breast IMRT breast plans and 10
VMAT prostate plans were chosen from the patients’
database. In total, 71 IMRT fields and 40 VMAT arcs were
compiled for analysis. Each plan was recalculated with
gantry zero on a water-equivalent slab phantom, for later
comparison of absolute dose at 5cm depth. Verification
plans were created for irradiation with 6X-FF beams at the
Varian Edge LINAC in order to measure the doses at the
Electronic Portal Imaging Device (EPID) level. For each
field/arc, the measured doses and the calculated doses
were compared by gamma analysis in CU for PDIP and in
absolute dose values (Gy) for the PD system on the CRE.
[1] Nijsten SM et al, 'A global calibration model for a-Si
EPIDs used for transit dosimetry”, Med. Phys. 34(10):
3872-84, 2007
Results
Table 1 presents the summary of the gamma analysis
results obtained in the comparison between the measured
dose at the EPID and the calculated dose using the
MAASTRO algorithm implementation and PDIP. The results
show that the analyzed IMRT plans using the MAASTRO
algorithm obtained, on average, a higher gamma pass
rate, lower mean gamma values and lower dose
differences than while using PDIP. The same is observed
for VMAT plans. Figure 1 shows the graphical comparison
between the gamma passing rate obtained using the
MAASTRO algorithm and PDIP, where the black circles
represent the comparison of the gamma passing rates for
IMRT plans (averaged over all beams) and the open
triangles represent the comparison of the gamma passing
rates for VMAT plans (averaged over all arcs). One can see
that the gamma pass rate obtained using the MAASTRO
algorithm is consistently higher than the one obtained
using PDIP.
Table
1
Figure
1
Conclusion
It is shown that the MAASTRO algorithm implementation
for gamma analysis based on absolute dose comparison is
reliable and provides very good results for both types of
plans tested. When compared with the results obtained
with PDIP v10.0.28, the MAASTRO algorithm presents at
least as good results for the pre-treatment portal
dosimetry as the currently available PDIP, while reporting
absolute dose results, making it a viable, and even
desirable, alternative.
PO-0811 Monte Carlo simulation of peripheral dose for
Gamma Knife treatments
B. Sanchez-Nieto
1
, E. Doerner
1
, A.M. Cardona
1
, F. Bova
2
1
Pontificia U-dad Catolica de Chile, Insitute of Physics,
Santiago, Chile
2
Gamma knife Chile, Radiotherapy, Santiago, Chile
Purpose or Objective
Induction of second cancers after external beam
radiotherapy (RT) is associated to the dose deposited
outside the treatment field (Peripheral Dose -PD)
[1]
. New
advances in radiation oncology have increased the survival
of patients beyond the period of latency of the occurrence
of secondary cancer (> 5 years), so that the estimation of
PDs has become particularly relevant. Commercial
treatment planning systems present a great uncertainty in
the dose calculation outside the treatment field
(differences up to 50%)
[2]
; therefore, alternative
methodologies for estimation of PD to radiosensitive
organs are needed. There are previous studies
[3,4]
applicable to external RT with linear accelerators.
However, no such a model exists for Gamma Knife. The
aim of this study was to estimate the peripheral dose
associated to radiosurgery treatments using Monte Carlo
(MC) and experimental measurements with TLDs.
Material and Methods
A Leksell Gamma Knife 4C radiosurgery equipment was
modeled using the set of C++ libraries Egspp, part of the
MC platform EGSnrc. The model includes the entire set of
201 Cobalt-60 sources, along with their respective beam
channel. Validation was performed by comparing profiles
and dose deposited in depth during irradiation of the Lucy
QA Phantom with all sources opened. Then, the photon
spectrum and absorbed dose were calculated and
measured with TLD-100 pairs, for the plan above, at 14
points of a pseudo-anthropomorphic phantom. TLD-100
had calibration factors for 6 MV nominal energy. TLD
readings were corrected by an energy response correction
factor due to the change in response from the 6 MV
calibration beam to the softened spectrum at the
measurement points.
Results
The simulated geometry was tested using a raytracing
method, included in Egspp, which allowed visualization of
geometrical details to be compared with the available