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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
dose by the accelerator directly and evaluates the dose
delivery error to the plan. We have used True-Beam
(Varian) with 10 MV photon beams and a QA plan which
underwent prostate VMAT. For the QA plan, we prepared
seven modified MLC files in which MLC positions were
manually shifted between 0 mm to 3 mm, respectively.
Then, the dose delivery errors were measured and
analyzed dose deviation, distance-to-agreement (DTA)
and gamma index by the Delta4 Discover system as well as
to the Delta
4
3D dosimetry system. All measurements were
compared against the points that received less than
appropriate percentage dose (<10%) were excluded from
the gamma index calculation.
Results
The results of the Delta4 Discover system and the Delta
4
3D dosimetry system using all available points for the
gamma calculation, 95.7±5.9% and 97.8±3.5% of them
passed the criteria (3%/2mmDTA/Th10%), respectively.
The two systems also had high correlations of dose
deviation and DTA, permitting the routine verification of
VMAT patient-specific QA plan as well as permanent in-
vivo dosimetry during the patient’s treatment course.
Conclusion
In this study, we have found that there was high
correlation between the pass rate and the intentional dose
delivery error, as respect to dose deviation, DTA, and
gamma index of the Delta
4
Discover system and the Delta
4
3D dosimetry system. It was suggested that the dosimetric
verification system under investigation could be useful for
routine patient specific QA.
EP-1475 RBE estimation of different Brachytherapy
sources based on micro- and nanodosimetry
M. Bug
1
, T. Schneider
2
1
Phys. Techn. Bundesanstalt PTB, 6.5 Radiation Effects,
Braunschweig, Germany
2
Phys. Techn. Bundesanstalt PTB, 6.3 Radiation
Protection Dosimetry, Braunschweig, Germany
Purpose or Objective
Depth-dependent RBE values of typical photon-emitting
Brachytherapy (BT)-sources were determined by a
microdosimetric and a nanodosimetric approach. The
microdosimetric approach considers a biological endpoint
while the nanodosimetric approach is entirely based on
the track structure, given by the interactions of the
photons and secondary electrons. The track structure
characterizes the radiation quality on the nanometric
scale.
Material and Methods
Within a cylindrical water phantom, isotropically emitting
BT-sources were positioned 4 cm below the surface.
Studied were Co-60 and Ir-192 representing high-energy
photon-emitting sources, I-125 being a low-energy photon-
emitting source, and Intrabeam
®
- and Axxent
®
-devices as
examples for electronic BT X-ray sources (EBX). Resulting
photon spectra were calculated at several points along the
cylindrical axis within cylindrical voxels of 0.5 mm depth
and 2 mm radius up to a depth of 10 cm.
The
microdosimetric
calculations of RBE are based on
yield coefficients α
dic,
representing the linear component
of the dose-effect relationship for the dicentric
chromosome aberration yield after an irradiation with
monoenergetic photons. The RBE for a given source in a
given point was determined by convoluting the respective
spectrum with the function α
dic
(E), obtained previously by
microdosimetric calculations.
The same depth-dependent photon spectra were used to
determine
nanodosimetric
quantities by Geant4-DNA
calculations. For each initial photon track, target volumes
in size of one DNA convolution which experienced at least
4 ionizations (F4) were identified. Such quantities were
previously shown to describe the DSB yield. Based on the
average minimum distance between these volumes within
each photon track, the RBE was estimated by
normalization to the distance for Co-60 at 0.125 mm
depth.