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S941
ESTRO 36 2017
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An optimization bolus was added from the body and 12 mm
expansion, defined as mass density 1.0 g/cm
3
. The
prescribed dose was 12 Gy delivered in 6 fractions. The
dose planning was aimed to keep D
95%
> 95% to PTV and
minimizing dose to organs at risk, which was defined as
the rest of the body. An optimization structure was used
to create a tangential irradiation of the skin, minimizing
the dose to normal tissue. We tested the bolus effect of
Neoprene with Gafchromic EBT3 film by irradiating slabs
of 7mm, dry and soaked in water. To verify skin doses, the
phantom with wet suit was irradiated with several 2x2 cm
2
slabs of film taped to the body. The film were evaluated
at least 24 hours after irradiation. Corresponding detector
array measurements (Delta4, Scandidos) were done and
evaluated with gamma analysis. Further, a robustness test
was done by moving the phantom 10 mm in the x, y and z
directions, to evaluate the effect of mispositioning.
Results
Results of planning and robustness tests are presented in
table 1. Measured data fit to depth dose data yields a dose
maximum at 28 mm for Neoprene. Hence, 7 mm is
equivalent to 3 mm thick water bolus and lightly soaked
Neoprene adds another 1.2 mm thickness of water. Delta4
gamma analysis with 2 mm and 3%, global dose, is clinical
acceptable with regards to deliverability (M = 93%, SD =
3%). The verification of 27 film slabs for skin dose gave an
average difference from TPS dose of 4% (SD = 3%), figure
1.
Conclusion
The difference of measured dose compared to TPS, for
both film and Delta4 dosimetry is larger than most types
of targets treated with HT which is to be expected
considering the technique and size of target. The
deliverability is within limit of our clinic action levels
(gamma pass rate < 90%) and neoprene is feasible as bolus.
The benefits, in comparison with reported electron
treatments, are target homogeneity and target coverage
with good immobilization and complete irradiation with
two positions. The higher dose to organs at risk than
reported with electrons needs to be addressed if
acceptable with regards to
toxicity.
EP-1736 Radiation and lasers isocenters coincidence
with ArcCheck phantom
F. Tato de las Cuevas
1
, J. Yuste Lopez
1
1
Hosp. Univ. de Canarias, Medical Physics Dept., Santa
Cruz de Tenerife, Spain
Purpose or Objective
One tool of Machine QA module of ArcCheck phantom (
AC
)
software checks Radiation and Lasers Isocenters
Coincidence (
RLIC
). The purpose of this work is to
evaluate the precision and accuracy of this software tool,
comparing it to the same test made with EPID (Electronic
Portal Imaging Device).
Material and Methods
The LINAC is an Elekta Synergy with Agility MLC and 6 MV
energy.
The RLIC with ArcCheck phantom (
AC
) are obtained
following the instructions of the software manual. The
measurements are done in continuous gantry movement
and for discrete gantry angles. Measurements are made at
9 º collimator angle for a 1x25 cm field. A series of
measurements were made also in 99º to see the MLC
effect, as Agility head has not backup jaws. The AC
displacements from laser isocenter in two directions are
made in order to check software sensitivity.
RLIC are made with EPID, positioning a Bearing Ball (
BB
)
in the lasers isocenter of a 5x5 cm field and acquiring
Images from 0º to 360º gantry angles in 45 º steps. The
radiation center of the squared field and the center of the
BB are calculated with a MATLAB in-house software. BB
center is calculated with sub-pixel accuracy in each
direction, 3 profiles are obtained and fitted to Gaussian
curves, and the mean maximum of the 3 curves is
calculated. Radiation field center is obtained calculating
the 50% pixel value of a vertical and horizontal profile.
The difference between BB center and radiation field
center are computed for each gantry angle for in-plane
and cross-plane directions. The RLIC for EPID
measurements are computed using these values.
Results
The RLIC results obtained with AC for each gantry are
compared with EPID in the first figure. The mean distance
over all gantry angles, for AC (for 9 and 99 º collimator
degrees) and EPID are: 0.3, 0.6 and, 0.7 mm, respectively.
The AC results are just distance (because this phantom is
not capable of give deviation in in-plane direction for each
gantry angle). The results for AC for 9º are higher than for
99º because of the irregular MLC radiation field limit
exposed for 9º to the AC diodes. The RLIC for EPID are
given in in-plane and cross-plane directions, the distance
for each gantry angle is calculated from both directions
and show a bigger mean value than for AC, because of
being calculated in just one direction in this phantom.