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 • Ion chamber reliability in air or water
• Ion chamber long-term stability
• Ion chamber accuracy
• Ion chamber reproducibility every time
Features
• Central axis chamber
• Waterproof and vented
• Simultaneously checks symmetry, flatness, light field vs. radiation field coincidence, field size, beam center, penumbra, constancy, and integrated total dose.
• Real-time dose maps for service and setup
• No electronics in or near the beam
• Windows® applications software
• Calibrated by Global Calibration Laboratory
• Range: 50 to 500 cGy/min
• No external power is needed at detector location
• Dynamic wedge contour mapping
• Daily, weekly, and yearly checks as per TG-40
• Use with watertank for depth dose calculations
• Microsoft® Access database compatible
• Microsoft Excel data export
The Thebes® II (Therapy Beam Evaluation System) consists of a linear ion chamber array, electrometer, communicator, THEBES II Level II Contour Manager software, wall mount power supply, interconnecting cable, and carrying case. An acrylic base plate that holds the ion chamber array and build up plates is also provided. The linear ion chamber array is permanently connected to the electrometer by a 1.5 m shielded multiconductor cable to eliminate radiation damage to the electrometer.
The THEBES II communicator provides power and the communication interface for the THEBES II electrometer.
THEBES II Level II Contour Manager Software is a Windows® based beam analysis software for daily and monthly accelerator QA
Applications
The THEBES II is used to perform linear accelerator quality assurance using air ion chamber technology for dose measurement, avoiding the ion transport issue of the competing liquid chamber technology and the radiation damage issues of diode detectors. Two detector arrays are offered: Model 7020 with 47 ion chambers on a 0.5 cm pitch (23.42 cm total active length) and Model 7040 with 47 ion chambers on a 1 cm pitch (46.88 cm total active length). Both arrays are waterproof.
SPECIFICATIONS
LINEAR ION CHAMBER ARRAY
Detector Type: Ionization chambers, waterproof and vented
Dimensions: 7020 6.2 W x 30.2 D x 3.7 cm H
7040 6.2 W x 53.5 D x 3.7 cm H
Active Area: 7020 1.0 x 23.42 cm (23 cm center to center)
7040 0.88 x 46.88 cm (46 cm center to center)
Detector Spacing: 7020 0.5 cm - 7040 1.0 cm
Inherent Buildup: 0.5 cm polystyrene, .02 cm polycarbonate
Inherent Backscatter Without Mounting Plate: 0.3 cm acrylic
Inherent Backscatter With Mounting Plate: 0.3 cm acrylic, 1.0 cm acrylic
Radiation Detected:
Photons: 60Co to 25 MV Electrons: 6 to 25 MeV
Beam Limits
Maximum Dose/Pulse: 12.5 mGy per pulse
Maximum Pulse Rate: 1000 pulses per second
Maximum Continuous Dose Rate: 500 cGy/min
Ion Chamber Dimensions
7020: 0.42 W x 0.95 D x 0.50 cm H
7040: 0.88 W x 0.88 D x 0.50 cm H
Ion Chamber Nominal Volume: 7020: 0.17 cm3 7040: 0.36 cm3
Ion Chamber Alignment: ± 0.3 mm chamber outline on top of array in all axes
Nominal Bias Voltage: -300V
Weight: 0.75 lb
ELECTROMETER
Number of Channels: 48
Amplifiers: Non-multiplexed, low leakage, mosfet operational amplifier
Array Scan Time: 1.1 ms
Frame Rate: 5 frames/sec
Status Indicators: 4
Dimensions: 10.9 W x 21.6 D x 3.2 cm H
COMMUNICATOR
Computer Interface: RS-232, DB-9 connector
Power Requirements: 12 VDC, 1 A
Dimensions: 4.2 W x 8.8 D x 2.0 cm H
SYSTEM
Power Requirements
Input: 120 VAC, 60 Hz, 22 W - Output: 12 VDC, 1 A
Computer Requirements
Computer: IBM® compatible PC, Intel® Pentium® 90 or higher with at least one unused COM port
Operating System: Microsoft Windows 98, ME®, XP®, 2000
Hard Disk Space: 64 MB of available space
Calibration:
Accuracy: 2% Reproducibility: 1%
Long Term Stability: 1% Linearity: 1%
Weight: 21 lb
Accessories Supplied with either 7020 or 7040:
AC Adapter: 110 VAC, 12 VDC 1000 mA,
Holding Assembly
Communication Cable: 75 ft
Terminator: 120 ohm
Communicator
1 cm and 2.5 cm Buildup Plates
THEBES II LEVEL II SOFTWARE
Contour Manger Version 2.0
• Varian, Siemens, IEC calculation methods for flatness and symmetry.
Features
• Field size factor measurement
• Percent depth dose curve
• Wedge angle measurement
• Wedge factor measurement
The THEBES II Level II software expands the already powerful THEBES II Contour Manager analysis package. It contains three main features: DICOM-RT, vendor based calculations and dynamic wedge protocol analysis. There are no additional system requirements to upgrade an existing THEBES II, and this is an option for existing THEBES II users to purchase.
THEBES II with Level II is well suited for daily and monthly QA of any medical linear accelerator. This includes open fields, static wedge, dynamic wedge and enhanced dynamic wedge analysis. Also, because the chambers are vented and waterproof and the central chamber can be given an ADCL traceable calibration, THEBES II can be used for annual QA in a water tank as well as commissioning activities.
SPECIFICATIONS
DICOM: Allows the user to export an exposure profile (integrated dose) to a DICOM enabled device. The user may open an exposure, and then export the dose data to a DICOM compatible file.
New Calculations: Includes calculations from various vendors for symmetry and flatness.
Flatness: Percent value, calculated per Siemens or Varian method.
Mirror Symmetry: (Varian) Percent value, the maximum symmetry error in the specified field region.
Area Symmetry: (Siemens, IEC) Percent value, the difference between the area under the right half of the beam curve and area under the left half.
Percent Depth Dose: Percent depth dose is a graph of output vs. depth in water. After each exposure the chamber array is moved to a different depth and another exposure is taken. The results are graphed and saved for trend analysis.
Wedge Angle - Dynamic Wedge: Calculates the angle of the profile using the following equation:
Wedge Angle = tan-1 ((Ddose/width) / (Ddose/depth))
Field Size Factor: (Central Axis Dose (fs1) / Central Axis Dose (fs2)) Field size (fs) is the only change.
Wedge Factor - Dynamic Wedge: (Central Axis Dose (1) / Central Axis Dose (2)). The first exposure is open. The second exposure contains a wedge.
Enhanced Scale Factors: The user has the ability to modify (calibrate) the output of the THEBES II hardware through the use of scale factors (correction factors).
Calculation Window Customization: Allows the user to customize the data in the calculation window similar to Windows Explorer. Allows the user to display any detail about the exposure (i.e. name, collimator settings, calculations, SSD, just whatever).
ZOOM: Provides an obvious ZOOM function.
Disable Individual Channels: User may disable any channel of the THEBES II hardware in case a channel is not working properly.
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