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ESTRO 36
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PO-0807 Practical advantages of a transmission
chamber in relative dosimetry of Brainlab conical
applicators
B. Tang
1
, J. Li
1
, S. Kang
1
, P. WANG
1
, L.C. Orlandini
1
1
Sichuan Cancer Hospital, Radiation Oncology, Chengdu,
China
Purpose or Objective
The commissioning of a radiosurgery unit requires the
acquisition of specific detectors able to characterize the
geometry and dosimetry of small fields. The acquisition of
the equipment for absolute dosimetry remains the priority
for the Hospitals, considering that relative measurements
can be performed without a reference chamber using long
acquisition time. The commissioning results therefore in a
tedious procedure. In this study, a new transmission
chamber (Stealth Chamber, IBA Dosimetry) was used as a
reference chamber (RC) in relative dosimetry of Brainlab
cone applicators. The timing of the practical procedure
and dosimetry results with and without the reference
chamber, will be analyzed and compared.
Material and Methods
IBA SFD3G diode detector was used to measure the 6MV
photon beam of a Varian Novalis used with Brainlab cone
applicators. Inline and crossline profiles at different
depths and central axis depths doses (PDDs) were
measured with a motorized water phantom (Blue
phantom, IBA Dosimetry) and OmniPro v7.4 software for
every cone. The measurements were acquired with the
transmission reference chamber positioned on the gantry
head in a continuous mode and without RC in a step by
step mode. The details of the acquisition parameters were
reported in Table 1. The total measurements time for each
procedure was registered.
Table 1 The Acquistion parameters with and without
stealth chamber
Acquisition
parameters/method
No
reference
Stealth
Chamber
Scan mode
step by step continuous
Scan speed
-
5 mm/s
In-scan positioning speed 5 mm/s
-
Positioning speed
10 mm/s
10 mm/s
Acquisition time
5 s
-
Stabilzation time
1 s
0.08 s
Results
Profiles at depth 10 cm for 4/15 mm diameter cones and
the depth doses acquired with the two procedures (Figure
1&Figure 2) shown a good agreement. The total
measurement time registered was 490 seconds for the
PDDs acquisition without RC and only 64 seconds when the
scan mode change from “step by step” to “continuous”
after stealth chamber was in place. The overall
measurement time for 4mm diameter was 575 s and 12 s
without and with RC respectively, 735 s and 17 s for the
15 mm diameter cone.
Figure 1
Figure 2
Conclusion
Traditionally, there is no way of applying a reference
detector when measuring small fields, especially for SRS
Brainlab conical collimators. The lack of reference signal
usually requires to acquire more signals in each measured
point to suppress the linac output fluctuation, which
results into a long measurement procedure. However, by
the introduction of stealth chamber,“continuous mode”
became available to us which substantially shorten the
measurement time while a good agreement between
measurements with and without stealth chamber for both
PDDs and Profiles was still reached. The use of stealth
chamber is a good solution to spare time during small field
dosimetry measurements. This aspect is important during
the commissioning of the stereotactic unit but it becomes
fundamental for the frequently quality control
performed.
PO-0808 Comparison of multi-institutional QA for
VMAT of Nasopharynx with simulated delivery errors
D.I. Thwaites
1
, E.M. Pogson
1
, S. Arumugam
2
, C.R.
Hansen
3
, M. Currie
4
, S. Blake
1
, N. Roberts
5
, M. Carolan
4
,
P. Vial
2
, J. Juresic
2
, C. Ochoa
2
, J. Yakobi
2
, A. Haman
2
, A.
Trtovac
2
, T. Al-Harthi
1
, L. Holloway
2
1
University of Sydney, Institute of Medical Physics-
School of Physics, Camperdown, Australia
2
Liverpool and Macarthur Cancer Treatment Centres,
Medical Physics-Radiation Oncology, Liverpool, Australia
3
Odense University Hospital, Laboratory of Radiation
Physics, Odense, Denmark
4
Illawarra Cancer Care Centre, Medical Physics -
Radiation Oncology, Wollongong, Australia
5
University of Wollongong, Centre of Medical Radiation
Physics, Wollongong, Australia
Purpose or Objective
Quality assurance of individual treatment plans is often
performed using phantom measurement and analysing
acceptable delivery accuracy by gamma analysis with a
required pass rate. Simplifying a complex treatment plan
and measurement into a single number is
problematic. This study evaluates the sensitivity of