Transcript Quality Assurance of 3D Planning & Treatment processes

Dosimetric evaluation of a new
design MOSFET detector
Per H. Halvorsen* & Stephanie Parker
University of North Carolina
Introduction
Other authors 1-4 have noted the following characteristics of the
Thomson&Nielsen MOSFET detectors (TN-RD-502 detectors with
TN-RD-50 dosimetry system)
Small modality dependence (< 5%)
Small energy dependence (< 5%). { At very low energies (e.g. 33KeV),
a pronounced over-response is evident; its magnitude (factor of 4.2)
is greater than TLD (1.2) but less than diodes (7.7). }
High reproducibility (< 3%)
No angular dependence for electron beams
Very small angular dependence for photon beams at angles of 135,
but an over-response beyond these angles, reaching a maximum of
approximately 18% and 13% at 180 to the normal, for 6MV and 18MV
photon beams respectively.
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Introduction cont.
The manufacturer (Thomson&Nielsen) has recently introduced a redesigned
MOSFET detector, called the “Isotropic MOSFET”. When used with the
TN-RD-50 dosimetry system, the manufacturer claims a significant
reduction in the photon anisotropy described above.
We have measured the dosimetric characteristics of this new detector,
and compared the characteristics with those of the current design
(TN-RD-502).
Initially, the detectors’ energy dependence and inherent buildup were
evaluated, to ensure that the beneficial aspects of the current design
have not been compromised in order to reduce the anisotropy.
Next, a series of measurements were conducted to evaluate the angular
dependence of the new design. The same measurements were
performed for the current design, and the results compared.
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Energy & modality dependence
For a 100 cGy irradiation at the calibration condition (10x10 field,
isocenter at depth 5.0 cm for photons, dmax with 100 SSD for electrons),
we obtained the following signal (in mV) with the new MOSFETs:
+ 3%
15 E
295
290
21 E
6X
12 E
285
Average
18 X
6E
8E
10 E
- 3%
Our measured calibration factors show a total range of 3.6% for all energies
and both modalities; an average value would give a ±2% uncertainty,
nearly identical to that of the current design. Repeated measurements
with different detectors have shown a ±1.5% level of reproducibility.
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Reproducibility
For a 100 cGy irradiation at the calibration condition (10x10 field,
isocenter at depth 5.0 cm for photons), we obtained the following signal
(in mV) with the new MOSFETs, using a high-sensitivity bias:
295
+ 2%
290
Average
285
- 2%
----
Std MOSFET
----
New MOSFET
These measurements (repeated with different detectors) show a ±1.5% level
of reproducibility, consistent with the current design detectors and highsensitivity bias.
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Inherent buildup
As a reference, the buildup characteristics of our Primus
accelerator’s 6 and 18 MV photon beams were measured with an
Exradin A-11 parallel-plate ionization chamber, in a virtual-water
phantom. Measurements were made with the following thicknesses
of virtual water above the detector:
0.0, 0.2, 0.3, 0.5, 0.7, 1.0, 1.5, and 5.0.
The effective depth of measurement (proximal electrode surface, at
0.05 g/cm2 depth) was accounted for when plotting the buildup
curve, and a curve-fit was used to extrapolate to depth 0.00 cm.
Our calibration geometry for the MOSFET detectors is identical to
that used for the dose calibration of the accelerator (at isocenter,
depth 5.0 cm, 10*10 cm field).
All readings were, therefore, expressed as a percentage of the dose
at the calibration condition.
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Buildup
6MV photon
----
P-P chamber
----
Std MOSFET
----
New MOSFET
Depth dose
100
80
60
40
20
Depth
0.1
0.5
1.5
3.5
5.0
7
Buildup
18MV photon
----
P-P chamber
----
Std MOSFET
----
New MOSFET
Depth dose
100
80
60
40
20
Depth
0.1
0.5
1.5
3.5
5.0
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Angular dependence
The MOSFET detectors were placed, flat side “up”, in a virtual water
phantom with a 0.5 cm thick sheet of bolus material under the
detector. This eliminated the slight air-gap which would otherwise
result from placing the detector directly between two slabs of
virtual water. The detectors were placed such that the depth to
the center of the detector was 5.0 cm from the front, side and rear.
Both detector designs were irradiated in this geometry.
The detectors were irradiated with a fixed field size and distance
(detector at isocenter), and at angles of 0°, 90 ° and 180 ° relative
to the normal to the detector’s flat surface. For an intermediate,
45 ° measurement, the measured dose was normalized back to
5.0 cm depth by the measured isocentric fractional dose (TRR).
Sample data are shown below.
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Angular dependence - results
6MV photon:
1.20
Dangle/Dnormal
1.15
1.10
1.05
1.00
0.95
Angle relative to normal
0°
45°
---- Standard MOSFET
90°
180°
---- New MOSFET
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Angular dependence - results
18MV photon:
1.20
Dangle/Dnormal
1.15
1.10
1.05
1.00
0.95
Angle relative to normal
0°
45°
---- Standard MOSFET
90°
180°
---- New MOSFET
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Conclusion
Our evaluations have confirmed the findings of other investigators
regarding the standard MOSFET detector system (see page 2).
Our analysis of the new detector design shows no change in many
important characteristics (energy & field size dependence,
reproducibility).
We have found the new MOSFET detector design to have an angular
dependence of 3% or less over the full 360° range.
With this detector redesign, resulting in minimal angular
dependence, we conclude that this system is a reliable and
efficient in-vivo dosimetry system, and well suited for quality
assurance in our IMRT program.
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References
1. Ramani, R., et.al., “Clinical dosimetry using MOSFETs”, Int J Radiat Oncol Biol
Phys 1997 Mar:37(4), 959-964.
2. Francsescon, P., et.al., “Use of a new type of radiochromic film, a new parallelplate micro-chamber, MOSFETs, and TLD800 microcubes in the dosimetry of
small beams”, Med Phys 1998 25(4), 503-511.
3. Edwards, C.R., et.al., “The response of a MOSFET, p-type semiconductor and
LiF TLD to quasi-monoenergetic X-Rays”, Phys Med Biol 1997 42(12), 23832391.
4. Scalchi, P., and Francescon, P., “Calibration of a MOSFET detection system
for 6MV in-vivo dosimetry”, Int J Radiat Oncol Biol Phys 1998 Mar:40(4),
987-993.
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