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What is a MOSFET dosimeter? |
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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 below the black epoxy bulb at the end of the 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 parameter that is a function of absorbed dose is this voltage. 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 increase in absorbed dose. The Bias Supply provides a constant bias voltage on the MOSFET during irradiation. |
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What are the technical advantages of using MOSFET dosimeters? |
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- Small size. Ease of use and patient acceptance.
- Minimum “build-up” on the MOSFET dosimeter, resulting in a “skin” equivalent dosimeter.
- Same MOSFET dosimeter is useful for both photon and electron modalities of LINACs.
- Same MOSFET dosimeter is used for mixed electron and photon fields.
- No dose rate or temperature effects.
- Only one Reader is required to measure up to four bias supplies and up to twenty dosimeters. One Reader can easily serve four treatment rooms at once.
- No angular dependence.
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What should I do before I use the system? |
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Read the Operator’s Manual. For calibrating the dosimeters, read p. 11. |
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When should I use the different Bias Supplies? |
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The standard Bias Supply (TN-RD-20) is used at doses > 100 cGy. For many beam measurements of 200 cGy or TBIs at 500 cGy this is the Bias Supply of choice. The High Sensitivity Bias Supply (TN-RD-19) was developed to give a higher sensitivity (approx. 3 times higher) compared with the standard Bias Supply. This gives better reproducibility for doses < 100 cGy. This is used, for example, for entrance dose or scatter dose measurements. See Technical Note T2 Reproducibility using model TN-RD-19 High Sensitivity Bias Supply”. |
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Q. |
Won’t the High Sensitivity Bias Supply use up the MOSFET dosimeters faster? |
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Yes. Some physicists, however, use them for ALL applications. If, however, you are concerned about the life of the dosimeter, set aside a High Sensitivity Bias Supply and some dosimeters for your low dose measurements (e.g. scatter doses) only. This way you will get the optimum life from your dosimeters. People use the High Sensitivity Bias Supply for doses above 100 cGy when they want to obtain better reproducibility. |
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Q. |
How often do I need to calibrate the dosimeters? |
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Once before you use the dosimeters. Then we recommend calibrating them halfway through the life of the dosimeters. Some physicists have used MOSFETs long enough that they are happy to calibrate their dosimeters only once at the beginning. |
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Q. |
What is the life of the dosimeters? |
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20,000 mV. In our experience, the dosimeters are not used up as quickly as people expect. Because we now have a considerable number of users, we have been able to drop the price of the dosimeters. |
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Q. |
How do I use it with a patient? |
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Just ZERO the dosimeters, tape them to the patient, and follow the READ procedures after irradiation. Tape a dosimeter(s) in position to measure the beam and, if required, tape to other organs to measure scatter dose (e.g. to eye, thyroid, gonads). |
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Q. |
What should I use to tape the MOSFET to the patient? |
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Surgical paper tape is best.
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Q. |
Can the MOSFET dosimeter be cleaned ? |
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Yes. You can clean with alcohol between uses. |
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Q. |
What do I do with the dosimeters before I use them for the first time? |
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Leave the dosimeters connected to the bias supply overnight prior to use in order to stabilize them. Once you have connected the dosimeters to the Bias Supply, leave them permanently connected until you have to change them. |
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Q. |
Do I need build-up on the dosimeters? |
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If you want to measure the dose at Dmax, then you should add the appropriate build-up (e.g. 1.5 cm of bolus for 6 MV photons) to the dosimeters. Many physicists use flexible bolus material (e.g. “Superflab”) cut to 2cm diameter and up to 2 cm thick, depending on photon beam energy. (See page 11 of the Operator’s Manual for more details.) It is also possible to use small hemispherical build-up caps made from various materials which are available for different energies. If you need to measure surface or entrance dose, do not add build-up. *N.B. You may not need build-up for electron irradiations. |
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Q. |
What is the angular dependence of the dosimeter? |
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The new Isotropic MOSFET dosimeter has good angular response, +/- 2% over a full 3600. It is now available. The model number is TN-502-RDI. |
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Q. |
What is the recommended procedure for using the dosimeters? |
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Step 1 - With the Reader, Bias Supply and MOSFET Dosimeters all connected press “ZERO” button and hold down for a few seconds. (Note: The Reader measures the voltage on each MOSFET dosimeter and stores it in memory. Check that the Reader display shows that the dosimeters are “zeroed”).
Step 2 – Disconnect the Bias Supply (with MOSFET dosimeters still connected), take to the patient, and tape the MOSFET dosimeter to the patient before irradiating the patient.
Step 3 – Take Bias Supply and MOSFET dosimeters back to Reader, after irradiation connect to Reader and press “READ” for each dosimeter in sequence 1,2,3,4,5.
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How many times do I push the “READ” button after irradiating the dosimeters? |
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Push the “READ” button ONLY ONCE after irradiating the dosimeters. You can repeat the “READ” after a few minutes, but do not push “READ” every few seconds or you will “disturb” the MOSFET and get a false reading. |
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Can I use this system with a computer? |
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Yes you can. Software is available in order to provide a connection from the “Reader” to a spreadsheet in your PC or Laptop. A printer is also available. |
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Q. |
Does the color on the dosimeters indicate different energies? |
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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.
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