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ESTRO 36
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Calculated and measured doses are the average doses of
symmetrical angles from 180º. Reference dose without
Fraxion was the average dose at 0º, 90º, and 270º. 100 MU
were delivered at each angle. All measured doses were
compared with the ones calculated with Monaco.
To measure the skin dose and the dose distribution in the
Build-up region, several radiochromic Films EBT3 were
placed at linac CAX between the slabs of a RW3 phantom
placed over Fraxion (SSD= 90 cm) and read using FilmQA
Pro software. Films were situated at the surface, 0.5 cm,
1.5cm depth and the linac isocenter. 200 MU were
delivered for 10x10 and 5x5 open field sizes and 0º gantry
angle. Once irradiated and removed, another set of films
were placed under the phantom, in contact with Fraxion,
and at 0.5 cm and 1.5 cm from Fraxion, as well as at the
linac isocenter. Additional films were located 1 cm away
from CAX as in this section Fraxion is wider. Same field
sizes and MU at 180º were employed.
Results
Table 1 shows the comparison between measured and
calculated transmitted dose with and without Fraxion in
the calculation. Measurements show a 1% attenuation for
180º gantry angle as stated on the Fraxion manual, but this
attenuation can be as high as 5 % (5x5 open field) or 6 %
(10x10 open field) for 150º gantry angle, as with this
angle, the beam traverses the thickest part of the Fraxion.
If Fraxion is not included in the calculation, Monaco
calculation can result in a 7 % difference between
measured and calculated doses, while with Fraxion in the
calculation, the maximum difference is 1.5% (10x10,
150º).
Table 2 shows the evaluated skin dose increment caused
by Fraxion, and compares calculated and scanned values.
Fraxion increases 3.8 times the surface dose, and by 17%
at 0.5 cm depth, which can be calculated by Monaco with
a difference lower than 1% if Fraxion is included in the
calculation.
Conclusion
It has been shown that the attenuation varies with gantry
angle. The inclusion of Fraxion in Monaco improves the
calculation from 7% difference to 1% in the worst case
(150º, 5x5 open field), furthermore, the skin dose
increment and the dose in the build-up region are
correctly calculated.
EP-1498 IMRT and VMAT commissioning for Versa HD
linear accelerator using AAPM TG-119
S. Sharma
1
, V. Subramani
1
, P. Kumar
1
, S. Bhaskar
1
, S.
Pathy
1
, S. Thulkar
1
, M. Sairem
1
, A. Binjola
1
, P. Agarwal
1
,
N. Dhayanethi
1
, P. Kumar
1
, S. Chander
1
1
All India Institute of Medical Sciences, Radiation
Oncology, New Delhi, India
Purpose or Objective
The purpose of the study is to evaluate the end to end
commissioning accuracy of intensity modulated radiation
therapy (IMRT) and volumetric modulated arc therapy
(VMAT) for Versa HD linear accelerator using AAPM TG-119
protocol.
Material and Methods
Phantom with contoured structure set was downloaded
from AAPM website provided with the TG119 report and
above structure sets were used as the patient for all plans
created in the study. IMRT [step and shoot (SMLC) and
dynamic (DMLC)] and VMAT plans were created for TG119
test cases. All the plans were generated using Monaco 5.1
treatment planning system (TPS) for Elekta Versa HD
(Crawley UK) linear accelerator. All plans were created
using 7-9 beams for IMRT (as per TG119) and single arc for
VMAT for energy 6MV, 6MV-FFF & 10MV-FFF (FFF-
Flattening filter free). Prescription and planning goals
were as kept as per TG119. For point dose measurement
CC01
(0.01cc) ion chamber was used and measurements
were carried out as per TG119 specified points in high and
low dose gradient regions. Point dose difference was
calculated as ratio of difference between measured and
planned dose with prescription dose. Similarly for planar
dose measurement I'matriXX (IBA, Scanditronix Wellhofer,
Germany) along with multicube-lite phantom was used
and measurement plane was kept at 11cm depth. Planned
and measured dose planes were compared using gamma
index criteria (dose difference/distance to agreement) of
3%/3mm and 2%/2mm. All measurements were performed
by keeping phantom on couch at gantry angle zero.
Confidence limit calculation was done as specified in
TG119.