Dosimetric evaluation of a new design MOSFET detector

1
Dosimetric evaluation of a newdesign MOSFET
detector

• Per H. Halvorsen Stephanie Parker
• University of North Carolina

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Introduction

• Other authors 1-4 have noted the following
  characteristics of the ThomsonNielsen MOSFET
  detectors (TN-RD-502 detectors with
• TN-RD-50 dosimetry system)
• Small modality dependence (lt 5)
• Small energy dependence (lt 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 (lt 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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3
Introduction cont.

• The manufacturer (ThomsonNielsen) 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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4
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
21 E
6 X
290
Average
12 E
18 X
285
10 E
6 E
8 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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5
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

2
295
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
high-sensitivity bias.
5
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Inherent buildup
As a reference, the buildup characteristics of
our Primus accelerators 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, 1010 cm field). All readings
were, therefore, expressed as a percentage of the
dose at the calibration condition.
6
7
Buildup
---- P-P chamber ---- Std MOSFET ----
New MOSFET

• 6MV photon

Depth dose
100
80
60
40
Dose
20
Depth
5.0
1.5
0.1
0.5
3.5
7
8
Buildup
---- P-P chamber ---- Std MOSFET ----
New MOSFET

• 18MV photon

Depth dose
100
80
60
40
Dose
20
Depth
5.0
1.5
0.1
0.5
3.5
8
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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 detectors 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
1.15
Dangle/Dnormal
1.10
1.05
1.00
0.95
Angle relative to normal
0
45
90
180
---- Standard MOSFET ---- New MOSFET
10
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Angular dependence - results

• 18MV photon

1.20
1.15
Dangle/Dnormal
1.10
1.05
1.00
0.95
Angle relative to normal
0
45
90
180
---- Standard MOSFET ---- 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
Mar37(4), 959-964. 2. Francsescon, P., et.al.,
Use of a new type of radiochromic film, a new
parallel-plate 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), 2383-2391. 4.
Scalchi, P., and Francescon, P., Calibration of
a MOSFET detection system for 6MV in-vivo
dosimetry, Int J Radiat Oncol Biol Phys 1998
Mar40(4), 987-993.
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