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and 10% in relative deviations for five key energies within
the treatment area. As an example, FWHM in function of
the air gap for three key energies are reported in Figure
1. Deviations observed are presented in Figure 2.
Agreement achieved in terms of ranges in water is within
0.1 mm in absolute deviation for all the energies
considered.
Conclusion
We extended a preliminary beam model based on a first
predictions at nozzle entrance. The final beam model
describes spot sizes within clinical tolerances of 1
mm/10%, for the treatment area considered. Detailed
validation of this MC beam model is on-going and is based
on beam scattering of the core pencil beam, transverse
dose profiles in the low dose region (nuclear halo),
absolute dose in reference conditions, evaluation of the
delivery of 3D cubes (depth-dose and transverse profiles).
Special emphasis will be given to non-isocentric set-up,
including the use of range shifters
.
EP-1505 Use of Portal dosimetry to monitor treatment
consistency throughout the course of treatment
S. Deshpande
1
, A. Sutar
1
, S. Naidu
1
, M. Vikram
1
, V.
Anand
1
, R. Bajpai
1
, V. Kannan
1
1
P.D. Hinduja National Hospital, oncology, Mumbai, India
Purpose or Objective
Use of portal dosimetry software to check treatment
delivery consistency and to monitor changes in patient
anatomy during course of treatment.
Material and Methods
Varian portal dosimetry software and Electronic Portal
Imaging Device (EPID) aS1200 were used to study
consistency of treatment. Patients undergoing VMAT
treatment were enrolled in this study. Patient plan was
delivered after correcting set up error and transmitted
images were acquired by the EPID aS 1200 during the
treatment. The transmitted dose images were acquired by
EPID after the beam passes through patient. Images were
acquired in continuous mode at source to imager distance
SID = 150cm on the 1,2,3,5,10,15,20,25 fraction number.
Before measuring transmitted dose images cone beam CT
was performed to eliminate any set up error. Day one
transmitted dose images were defined as base line images.
On an average 8 images were acquired during treatment
for each patient. These images were compared with base
line image. Gamma index evaluation was performed with
1mm and 1% parameter using Varian portal dosimetry
software.
Results
For the first five images i.e. up to tenth fraction we got
average gamma index passing 98.3% which is within action
level threshold of 97%. Depending upon the site of
treatment we observed gamma passing percentage varies
during fag end of treatment
Conclusion
Dosimetric measurement during treatment is good tool to
investigate error during the treatment. Portal vision is
mostly used for patient set up and pre treatment QA of
patient. We found that portal dosimetry is useful tool for
checking consistency of treatment delivery and monitoring
changes in patient contours.
EP-1506 Temperature dependent dose readout of
Gafchromic EBT3 and EBT-XD film and clinical relevance
in SRT
K. Buchauer
1
, L. Plasswilm
1
, J. Schiefer
1
1
Kantonsspital St. Gallen, Departement of Radiation
Oncology, St Gallen, Switzerland
Purpose or Objective
Modern radiation therapy modalities regularly produce
SRT/SRS/SBRT plans with highly irregular and steep dose
gradient distributions consisting of many small beam
apertures. Accurate verification of such complex
treatment fields is still challenging and Gafchromic EBT3
and EBT-XD films play a key role as dosimeter with the
highest spatial resolution. Purpose of this work is to
evaluate whether well-known temperature dependences
of former Gafchromic film media are present with EBT3
and EBT-XD film. The observed systematic patterns of
temperature dependence are characterized with respect
to relevance in the pre-treatment verification.
Material and Methods
An Epson V750 pro flatbed scanner was used to perform
scan studies with 125 consecutive scans to purposely warm
up the scanner bed. During all scans two temperatures
probes were used to measure an average scanner bed
temperature. Square film pieces with irradiation dose
form 0 Gy to 64 Gy of 1 cm size were placed in the central
axis of the scanner bed. Evaluation was performed with
the software 'Image J” in all three colour channels in 8100
measurements in total.
Results
Temperature dependent relative transmission (%T)
readout differences known from former type Gafchromic
film media are found to still be present with EBT3 and EBT-
XD film type. Higher temperature results in most cases in
darker film readout. Interestingly, EBT3 red colour
channel changes temperature dependence direction
around 16 Gy irradiation dose, meaning that a higher
temperature results in less dose readout. Figure 1 and
Figure 2 illustrate the relation between the temperature
dependent transmission error and irradiation dose for
EBT3 and EBT-XD respectively.