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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
technical drawings. Difference of just 3.5 % was obtained
for the experimental and calculated FWHM. Mean energies
calculated from peripheral photon spectra energies
ranged from 0.242 MeV in the mediastinum to 0.171 MeV
in the pelvis. Based on these results, an average energy
correction factor of 10% was applied to TLD readings. A
maximum of 15% difference between calculated and
measured peripheral doses were obtained.
Conclusion
The use of Egspp allowed us to model accurately the
Gamma Knife. The level of detail achieved in the modeled
geometry is essential for the peripheral dose calculation,
since it is dependent on the radiation leakage. The
agreement between simulations and measurements is
good, with higher discrepancies observed in the points
located on the limbs of the phantom. A MC methodology
for peripheral dose characterization has been validated
and therefore, different scenarios regarding patient size
and/or beam geometry can be estimated for future
references.
References
[1] Radiother Oncol 2012;10(5):122-126
[2] JACFMP, vol 14, N2, 2013
[3] Phys. Med. Biol. 57 (2012) 6167–6191
[4] Biomed. Phys. Eng. Express 1 (2015) 045205
PO-0812 Dosimetric impact of using Acuros algorithm
for stereotactic lung and spine treatments
L. Vieillevigne
1,2
, T. Younes
1,2
, A. Tournier
1
, P. Graff
Cailleaud
1
, C. Massabeau
1
, J.M. Bachaud
1
, R. Ferrand
1,2
1
Institut Claudius Regaud- Institut Universitaire du
Cancer de Toulouse Oncopole, Radiophysique, Toulouse,
France
2
Centre de Recherche et de Cancérologie de Toulouse
CRCT- UMR1037 INSERM - Université Toulouse 3,
Radiophysique- équipe 15, Toulouse, France
Purpose or Objective
The main aim was to assess the dosimetric impact of
calculating with the Acuros (AXB) algorithm instead of
Anisotropic Analytical Algorithm (AAA) for stereotactic
(SBRT) lung and spine cancer treatments.
Material and Methods
Ten stereotactic lung patients and ten stereotactic spine
patients were selected to investigate the dosimetric
impact of using AXB instead of AAA. Dynamic conformal
arc was used for SBRT lung patients with a prescription of
50 to 55 Gy in 3 or 5 fractions to the 80% isodose. For the
SBRT spine patients, Rapid Arc plans were prepared and
27 Gy was prescribed in 3 fractions to the PTV median
dose. The plans were recalculated with the AXB algorithm
by using the same beam settings and monitor units as the
AAA. Two dose reporting modes of AXB, dose to medium
(Dm) and dose to water (Dw) were studied. Relative dose
differences between algorithms were calculated for PTV
(D98%, D95%, D50% and D2%) and for organs at risk (D2%
and mean dose for the ipsilateral lung, the spinal cord or
the cauda equina).
Results
For the 10 SBRT lung patients, the mean lung density was
around 0.18 g/cm
3
which corresponded to a normal lung
tissue. The dosimetric impact on PTV dose (D98%, D95%,
D50%) using AXB instead of AAA was quite small with a
maximum underdosage up to 3.1% for D98%. Larger
differences were obtained on D2% with a maximum
deviation of 7.79%. The average difference on the mean
ipsilateral lung dose was 0.22% and 0.44% for AXBDm and
AXBDw, respectively. AXBDm and AXBDw presented similar
results.
For the 10 SBRT spine patients, large relative dose
disagreement of up to -5.02% for the D98% of the PTV was
observed with AXBDm. On average, for the D50% of the
PTV, AXBDm revealed a relative underdosage of -2.36%,
whereas AXBDw lead to a relative overdosage of +1.64%.
Concerning the spinal cord or the cauda equina, the
average mean dose was reduced up to -6.93% and up to -