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implant in a few seconds to the same level of accuracy as
a detailed MC calculation.
Conclusion
RayStretch
is a robust method for heterogeneity
corrections in prostate BT supported on TG-43 data. Its
compatibility with commercial TPSs and its high
calculation speed makes it feasible for use in clinical
settings for improving treatment quality. It will allow in a
second phase of this project, its use during intraoperative
ultrasound planning.
EP-1501 Field-by-field and composite plan pseudo-3D
verification of IMRT techniques with radiochromic film
T. Hanušová
1
, I. Horáková
2
, I. Koniarová
2
1
Faculty of Nuclear Sciences and Physical Engineering-
Czech Technical University in Prague, Department of
Dosimetry and Application of Ionizing Radiation, Prague,
Czech Republic
2
National Radiation Protection Institute, Section of
medical exposures, Prague, Czech Republic
Purpose or Objective
The purpose of this study was to compare field-by-field
pre-treatment verification of IMRT dose distributions,
which is often performed clinically, to a pseudo-3D
method that verifies the global plan dose distribution in
several transversal, coronal and sagital planes.
Material and Methods
Sheets
of EBT3 film were placed into an IMRT cube
phantom into 5 transversal, 5 coronal and 5 sagital planes
close to the isocenter. The phantom was irradiated in this
setup with six IMRT step-and-shoot treatment plans. These
included one head-and-neck case and five pelvic cases.
Two of these plans had not previously met the clinical
tolerance criteria and were not used for treatment. Dose
distributions obtained with film were compared to
predicted dose distributions in OmniPro I’mRT software.
Gamma pass rates were obtained for 3 %/3 mm criteria.
The same IMRT plans were then measured field-by-field
with one EBT3 film sheet placed in the isocentric coronal
plane in an RW3 slab phantom, with gantry at 0° for all
fields. Again, gamma pass rates were obtained. Finally,
the results were compared to clinically performed
verification. This was done with a PTW seven29 detector
placed in the isocentric coronal plane in an RW3 slab
phantom and each field of the plan was tested with gantry
at 0°. Gamma pass rates for the PTW array measurements
were obtained in VeriSoft with the same criteria
3 %/3 mm. Treatment planning was performed in XiO
version 4.80 and plan delivery was carried out on a
Siemens Artiste linear accelerator at the Thomayer
Hospital in Prague.
Results
EBT3 film gave higher gamma pass rates than the PTW
seven29 array for field-by-field measurements for all
patients. If all fields of each plan were averaged out, the
average gamma score for both film and PTW detector was
above the clinical tolerance limit of 90 % for all plans. This
was not true for the composite plan measurements with
film. While a certain plan met the tolerance limit if
measured field-by-field, it could fail to meet the
tolerance limit when the global plan dose distribution was
measured. Moreover, for a given plan, different gamma
score values could be seen with film for the three
directions tested. While in some directions the plan met
the clinical tolerance limit of a 90 % gamma score, it could
fail to meet the limit in others. These findings can be
influenced by film directional dependence, but this is
supposed to be negligible.
Conclusion
Field-by-field pre-treatment verification of IMRT dose
distributions, both with radiochromic film and an array of
ion chambers, gave higher gamma scores than if the global
plan dose distribution was measured in a pseudo-3D
manner. Field-by-field measurements might be
insufficient to detect potential plan errors. More complex
investigations are recommended at least when new IMRT
techniques are being established in the clinic.
EP-1502 Dosimetric assessment of brass bolus using
radiochromic film
P. Lonski
1
, L. Walton
2
, N. Anderson
2
, J. Lydon
1
, T. Kron
1
,
B. Chesson
2
, R. Prabhakar
1
1
Peter MacCallum Cancer Centre, Physical Sciences,
Melbourne, Australia
2
Peter MacCallum Cancer Centre, Radiation Therapy
Services, Melbourne, Australia
Purpose or Objective
Brass bolus is a new type of bolus designed to enhance
surface dose in radiotherapy. Manufacturers claim the
impact on other radiation beam characteristics is
negligible. The main advantage is the mesh-like grid of
brass links can conform to complex patient contours which
compared to conventional bolus reduces air gaps between
bolus and patient skin. This study aims assess the
dosimetric impact of brass bolus on surface dose in
megavoltage photon beams.
Material and Methods
Brass bolus (Radiation Products Design Inc) with a nominal
thickness of 1.5mm was used on a tissue equivalent slab
phantom (RMI solid water). It was used as single layer and
folded over in 2 or 4 layers. Radiochromic film (EBT3) was
used to assess surface dose and dose variation on the
phantom for a 6 MV and 18MV photon beams (Varian 21iX).
Surface dose was measured with and without the brass
bolus which was placed on top of the film. A photo of the
brass is shown in Figure 1 (a) alongside a 50 mm section of
steel ruler. A film calibration curve was derived by
exposing samples from the same sheet to various known
doses under reference conditions. Film was scanned 12
hours post exposure and manually analysed using ImageJ
and MS Excel software.
Results
Surface dose measured using film in the absence of bolus
was 20 % of dose at d-max for a 6 MV beam in a 10 cm x
10 cm jaw-defined square field. Surface dose with a single
layer of the brass bolus increased to an average of 57 % of
dose at d-max (1.5cm). The mesh-like structure of the
brass resulted in a dose enhancement pattern which was
non-uniform across the film, as shown for a 1 cm x 1 cm
square region in Figure 1 (b), which shows the peaks and
troughs resulting from the mesh. The maximum dose
(peaks) was 62 % and the minimum (troughs) was 53 % of
dose at d-max under reference conditions. Increasing the
number of layers of bolus increased the surface dose.