Best Medical Canada - Frequently Asked Questions

frequently asked questions

Q. What is a MOSFET dosimeter?

A. Metal Oxide Semiconductor Field Effect Transistor:
The MOSFET dosimeter is an electronic device which integrates the radiation dose. It is a silicon chip 1 mm square (active area 0.2 mm x 0.2 mm!), which is located under a black epoxy bulb. The bulb is located at the end of a flexible cable. The MOSFET dosimeter is sensitive to radiation and one of its electrical properties ("threshold voltage") is permanently changed by the absorption of radiation. The voltage parameter is a function of the absorbed dose. The threshold voltage is measured by the Reader by passing a current down the cable to the MOSFET dosimeter chip and measuring and recording this voltage on the chip. This voltage increases with any increase in the absorbed dose. The Bias Supply provides a constant bias voltage on the MOSFET during irradiation.

Q. What is a microMOSFET?

A. The standard width of a MOSFET flex is 2mm. A microMOSFET is only 1mm in width. It is the exact same MOSFET chip and therefore has the same properties as the standard MOSFET, but it is packaged in a much smaller package. The microMOSFET is ideal for small fields, Brachytherapy and Radiosurgery applications.

Q. How do I use it with a patient?

A. Just "ZERO" the dosimeters, tape them to the patient (and add buildup if required), and follow the READ procedures after irradiation. Tape the dosimeter(s) in position to measure the beam and, if required, tape them to other organs to measure the scatter dose (e.g. to eye, thyroid, gonads).

Q. What are the technical advantages of using MOSFET dosimeters?

A1. Small size. Ease of use and patient acceptance.
A2. No angular dependence / Isotropic - Response of ±2% for 360°. This makes the dosimeter ideal for IMRT & Brachytherapy.
A3. Use one MOSFET dosimeter for all energies and modalities; photons and electrons.
A4. Minimum "build-up" on the MOSFET dosimeter, resulting in a "skin" equivalent dosimeter.
A5. There are no dose rate or temperature effects.
A6. Only one Reader is required to measure up to twenty dosimeters.

Q. Can the MOSFET dosimeter be cleaned?

A. Yes. You can clean it with alcohol between uses.

Q. Do I need buildup on the dosimeters?

A. If you want to measure the dose at D_max, then you should add the appropriate buildup (e.g. 1.5 cm of bolus for 6 MV photons) to the dosimeters.

Many physicists use our hemispherical buildup caps or flexible bolus material (e.g. "Superflab") cut to 2cm diameter and up to 2 cm thick, depending on photon beam energy.

If you need to measure the surface or entrance dose, do not add buildup. You may not need buildup for electron irradiations.

Q. What is the angular dependence of the dosimeter?

A. The MOSFET dosimeter has a response of +/- 2% over 360°.

Q. Can I use these systems with a computer?

A. Yes. The mobile system is entirely software driven. It is designed to completely and remotely control the reader. For the Portable Dosimeter System, the software that is included is used to provide optional data export. Dosimetry measurements are available at the touch of a button and display on the integrated LCD, without the need for software or a computer.

Q. Does the color on the dosimeters indicate different energies?

A. No. Different colors have been placed on each end of the dosimeters so that one can differentiate between them (e.g.: positions on a patient). This is most useful in cases of TBI treatments.

Q. Can I use the MOSFETs in water?

A. Yes.

Q. Can I use the MOSFETs for intracavitary dose measurements?

A. Yes, as long as the MOSFETs are placed inside a closed end catheter.

Q. What is the lifetime of the dosimeters?

A. 20,000mV.

Click here to view the List of Publications on MOSFET Dosimetry (PDF)