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Wide Energy Hemispherical Brass Build-Up Cap for MOSFET Dosimeters.
Wide Energy Hemispherical Brass Build-Up Cap for MOSFET Dosimeters.
Hemispherical Build-up Caps
MOSFET dosimeters have only 0.8 mm inherent build-up, which allows for flexibility in measuring surface dose as well as dose at Dmax. In order to measure dose at Dmax, build-up is required. This custom designed Build-up Cap for use with MOSFET Dosimeters allows for depth dose measurements over a wide range of energies.
Full Photon Range & 15 MeV - 18 MeV Electrons
Brass Build-up Caps - 0.635 cm Radius
In order to maintain the isotropic characteristics of the MOSFET dosimeter (± 2% for 360°) and allow for one calibration factor for all energies and modalities, it is recommended that a hemispherical build-up cap be used. This small brass hemisphere is specially grooved for precise placement of the MOSFET and maintains the characteristics which distinguish these devices from other types of dosimeters.
The brass build-up cap is very lightweight (only 4g) and small (radius: 0.635 cm), which makes it ideal for placement on patients.
This build-up cap may be affixed to the MOSFET for the duration of its life, i.e. 200 doses. Using just one build-up cap for all Photons and some Electron energies makes the dosimeters easier to use.
Why Brass?
Brass is a metal alloy containing mainly copper and zinc compounds. Due to its high density (8.5g/cm3) and to its high Z number (Z~30), it provides the minimal amount of metal needed to achieve full build-up at Dmax for a range of photon energies (4, 6, 10 and 18MV) and some electron energies (15-18MeV)* at a very practical size.
MOSFET Correction Factors (CR) Under Brass Build-Up Caps
To directly correct for dose readings at Dmax, the system’s software allows for Correction Factor (CR) values to be entered which then convert MOSFET response to dose.
These CRs vary between 0.8 and 1.1 and are Linac and calibration set-up dependent. They must be determined for any new MOSFET/Cap combination. These CRs may be stored in the system’s software dose measurement template.
For example, two sets of CRs were obtained with a Siemens Mevatron Linac at 6 and 18MV photon energies, with a nominal dose of 200 cGy at Dmax in water and 10 x 10 cm2 field size (100 cm Source Axis Distance):See Below
Typical Correction Factors for Brass Build-up Caps:
• 6 MV Energy - Correction Factor “CR”: 1.10
• 18 MV Energy - Correction Factor “CR”: 0.84
Note: The Correction Factor “CR” is the value used in the system software, along with the Calibration Factor “CF” to convert MOSFET response to dose. (Please refer to Operators’ Manuals for further details).
Handling and Cleaning
Brass build-up caps are easily attached to the MOSFET. Circular adhesive patches are provided with the caps to fasten the MOSFET dosimeter to the build-up cap for the duration of the lifetime of the dosimeter. The cap/MOSFET is then adhered to the patient’s skin using paper tape.
Cleaning - use rubbing alcohol or alcohol swabs.
Custom Build-Up Caps
Build-up caps for MOSFET dosimeters can also be custom made per customers specifications.
*Note: For low electron energies, it is suggested that no build-up is used. However, if desired, one can use the 1.5 cm radius tissue equivalent build-up cap.
