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profiles showed qualitatively good agreement between
the gel dosimeter, EBT film and RTP data for all PTVs.
Conclusion
The results indicate that those processes could effectively
evaluate geometric and dosimetric accuracy of brain SRT.
This study using 3D dosimetry system was useful to
validate the 3D dose distributions for patient-specific QA.
EP-1473 Improving the accuracy of dosimetry
verification by non-uniform backscatter correction in
the EPID
Y. Md Radzi
1,2
, R.S. Windle
2
, D.G. Lewis
2
, E. Spezi
1,2
1
Cardiff University, School of Engineering, Cardiff,
United Kingdom
2
Velindre Cancer Centre, Department of Medical Physics,
Cardiff, United Kingdom
Purpose or Objective
Challenges in improving the accuracy of EPID-based
patient dose verification have been widely discussed and
remain a key topic of interest for patient safety, as
exemplified in the UK by the ‘Towards Safer Radiotherapy’
2008 report[1]. In particular, one of which is for every
radiotherapy centre to have protocols for in vivo
dosimetry (IVD) to be used for most patients as
recommended in the Annual Report of the Chief Medical
Officer for 2006 and it is already a legal requirement in
many European Countries [2]. In this presentation, we
report on commissioning and implementation of the
commercially available Dosimetry Check (DC) [3, 4]
system. Particular emphasis has been given to addressing
the significant non-uniform backscatter effect from the
VARIAN aSi-1000 EPID arm [5, 6].
Material and Methods
A backscatter correction matrix was developed by
combination of dosimetric information from a set of
segmented fields sampling on different positions around
the active area of the imager. The matrix was then used
to correct EPID images using MATLAB programming scripts.
The corrected image was created in DICOM format and
exported to Dosimetry Check to read and analyse.
Example treatment fields were generated in our Oncentra
MasterPlan (OMP) Treatment Planning System (TPS), with
several equidistant dose reference points relative to
central axis included. A dose comparison given by DC with
reference to the TPS was recorded in an auto-generated
report. Assessment and comparison undertaken included
the
(i)
asymmetry evaluation of equidistant points before
and after correction being applied with respect to TPS,
(ii)
improvement in segmented IMRT dose profiles after
correction, and
(iii)
OMP-DC pass rate with gamma
criterion 3%/3mm[7], as well as 2-D Gamma Volume
Histogram (GVH) evaluation on outlined PTVs.
Results
(i)
Correction for non-uniform backscatter improved with
overall agreement between fields generated in OMP and
those recorded in DC from within 3% to better than 1%.
(ii)
Agreement between OMP and DC for IMRT dose profiles
with a sample Head & Neck case was improved by
approximately 3% using the correction methodology
(
Table 1
).
(iii)
For gamma comparison of fields in OMP and
DC with 3%/3mm, pass rates were improved from around
80% to around 90% by the correction method. Similarly in
GVH evaluation for the outlined PTVs, pass rate has
increased from around 80% to 90% after correction being
applied.
Conclusion
The correction method implemented herein for the
Dosimetry Check system has proved to be an effective way
to reduce verification inaccuracy caused by backscatter
from the Varian EPID arm and can be used to enhance the
previously established portal verification method for IMRT
using this technology.
EP-1474 Feasibility of dose delivery error detection
by a transmission detector for patient-specific QA
H. Honda
1,2
, K. Kubo
1
, R. Yamamoto
1
, Y. Ishii
1
, H.
Kanzaki
1
, Y. Hamamoto
1
, T. Mochizuki
1
, M. Oita
3
, M.
Sasaki
4
, M. Tominaga
5
, Y. Uto
6
1
Ehime University, Department of Radiological
Technology, Toon, Japan
2
Tokushima University, Graduate School of Advanced
Technology and Science, Tokushima, Japan
3
Okayama University, Department of Radiological
Technology- Graduate School of Health Sciences,
Okayama, Japan
4
Tokushima University Hosipital, Department of
Radiological Technology, Tokushima, Japan
5
Tokushima University, Institute of Health Sciences,
Tokushima, Japan
6
Tokushima University, Institute of Bioscience and
Bioindustry, Tokushima, Japan
Purpose or Objective
Dose delivery error detection of on-line treatments is an
important issue for clinical QA practices. The goal of this
study was to evaluate a feasibility of the delivery error
detection by a new type of on-line transmission detector
compared to a 3D detector in patient-specific QA
measurements for VMAT treatment.
Material and Methods
The Delta
4
Discover system is a transparent, p-type
semiconductor diodes detectors, placed in the accessory
holder of the treatment head. The system measures the