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MOSFET device consists of P-type silicon semiconductor substrate with a insulating oxide layer between it and a metal gate. REF
The overall time from setting up to acquiring results is much shorter. However, there is not much difference in it dose accuracy compared to other dosimetry tools in use. (reference 3).
When an ionising radiation is exposed to MOSFET, electron-holes are generated within the silicon dioxide layer. The junction potential produced causes electron to migrate to the gates while holes move towards oxide silicon interface. Some of the holes the holes are trapped causing a negative shift in gate voltage. The voltage at which a shift can be induced is known as the threshold voltage. The threshold voltage causing the shift is directly proportional to the radiation dose absorbed by the metal oxide layer. Temperature does affects the function of the device as it influences the threshold voltage, however this interference by temperature can be combated by the use of two MOSFET one measures the threshold voltage before irradiation and the other after the irradiation and at the same temperature. By so doing a correction factor for temperature will not be needed in the final reading. The device used is the dual bias MOSFET. (Reference 2).
(Rajesh et al) conducted a study to estimate the skin dose of patient being treated with tomotherapy. Two head and neck patients were selected for this study. The first with carcinoma of the nasopharynx with intracranial extension and multiple cervical adenopathy. Second patient was diagnosed with carcinoma of buccal mucosa cavity. The MOSFET detectors were placed on the skin inside the mask with a tape with TLD also placed beside the MOSFET and precaution taken to maintain the same location of the detectors for subsequent fraction in a week. The skin dose was measured for ï¬ve consecutive fractions using both MOSFET and TLD. For the first patient the MOSFET measured the skin dose as 90% while TLD read 92% of the prescription dose (2.2Gy). The variation between skin dose measured with MOSFET and TLD was 2.2%. For the second patient the skin dose measured with MOSFET and TLD was 88% and 86% of the prescription dose, respectively. A variation of 2.3% was observed between skin dose measured with MOSFET and TLD. This result showed there are great similarities between TLD and the MOSFET. The difference in the results was attributed to discrepancy in the tomotherapy treatment planning system used, given a less than accurate value for the skin dose to the patient which when corrected may make the dosimetry readout more accurate. However, the complex nature of setting up and obtaining readout makes the MOSFET a more favourable option. This study though, had a sample of two patients which is rather small and the number of reading taken over the period of the trial is not a good representation of the whole treatment period. A more varied sample should have been used which a sample of two doesn't provide.
The benefits of MOSFET supersedes other solid state dosimetric techniques available, the calibration procedure is simple compared to TLD and also easy to maintenance and operation. The MOSFET is a non-intrusive semiconductor radiation dosimeter which enable direct and simple readout of the dose. This technique has a lot in common with semi conductor diode. The use of metal oxide semiconductor field effect transistors(MOSFET) are being applied in more radiotherapy centres for measuring absorbed dose. This is a very good alternative to the thermoluminescence (TLD) as it eliminates the short coming of (TLD)'s and improves upon the dosimetric techniques being used.
One notable disadvantage of MOSFET dosimetry is its limited life span where the linearity of the detector decreases after a large dose accumulation, hence requiring a larger dose for the same potential difference. However, (peter and butson) found that the clinical semiconductor dosimetry system (CSDS) MOSFET dosimetry system provides an adequate dose assessment at low dose levels, with an investigation conducted using a new MOSFET system realised an increase in the accuracy of low dose readings. This was carried out using a high-energy x-rays produced by a pulsed radiation linear accelerator. It was noticed that by applying this procedure the life span of the MOSFET can be prolonged without affecting the accuracy of results.
The use of a micro-MOSFET as dosimeter in vivo for intraoperative external radiotherapy (IOERT) has proved positive, providing its precision around 4%. (Ciocca et al). With Precision of MOSFET being 0.7% 1SD for and that of diodes 0.05%. the accuracy of MOSFET is lower compared to diodes ( Jornet et al). however it other favourable advantage makes is a better tool than diode also the accuracy of MOSFET although lower than diode is very acceptable for clinical use.
Semiconductors diodes are one of the few established means of radiotherapy dosimetry. The feasibility of it use and accuracy has been tested and proven over the years. Semiconductor diodes are usually silicon, p-n junction diodes. Electron holes are created during irradiation of the diode, charge carriers move across the voltage to recombine with electron hole. Current is hence generated which flows in the reverse direction to the diode current flow. The popularity of the semiconductor diode is due mainly to its; high sensitivity with comparison to ionisation chamber as the standard, real-time read out, and simple implementation.
It has a small size, high-quality mechanical stability, absence of external voltage, and instant availability of the measured dose. ( Esser and Mijnheer)
Temperature and pressure adjustment uncertainty of the linear accelerator dose monitor due to leakage in the chamber; incorrect cable connection to the unit that could electronically change the dose monitor for pressure and temperature and inaccuracies in the planning system. (Nilsson et al)
Semiconductor diode is also very useful for taking both entry and exit dose during in vivo dosimetry. (Rodica et al).
However, with all the benefits of the semiconductors it does have it drawbacks
(i) The increase in treatment time due to the time needed to position the diode on the
patient. (ii) Diodes act as a buildup material and thus increase the skin dose (18). (iii) If used for entrance dose measurements, diodes attenuate the primary beam (18). (iv)
For practical reasons, only very limited numbers of diodes are used for simultaneous measurements. (v) In order to determine the midplane dose, both entrance and exit dose measurements have to be performed. EDIT. (kasper et al).
Sensitivity of semiconductor affected by temperature hence, a factor has to be incorporated in it calibration, the temperature variation is also dependent on dose received by the diode. Diodes required for entry dose dosimetry are place on the patient possibly, resulting in temperature change in the diode usually to room temperature and could rise to about 10 degrees C, this has made it necessary to include a correction factor as this effect can alter the sensitivity of the diode. Significant changes in sensitivity do occur after the first irradiation, many diodes are therefore irradiated to overcome initial change in sensitivity before calibration. However, the use of diode for electron dosimetry is not widely accepted and various research and experiment have been conducted regarding the issue. REF
A study conducted by (Lee et al) realised some uncertainties in the measurement of the diode notably, the reproducibility of the target dose measured was just 7%.
(Li C et al quote), suggest "due to the significant reduction of diode system sensitivity with large cumulative dose, a diode system in the present configuration is not fit for monitoring treatment machine dose output, neither should in vivo diode dosimetry measurements be used for electron beams, due to the significant perturbation of depth dose features by the detectors placed on the beam-entrance surface, especially for beam energies less than 9 MeV"
However, (RAVINDRA YAPARPALVI et al) thinks diode dosimetry is an essential tool for verifying accuracy of dose delivered in electron beam treatment but might result in reduction of dose to target volume due to attenuation of beam.
An investigation examined the issues associated with in vivo dosimetry in breast irradiation and the signiï¬cance of error in placement of the diode detectors showed due to the varying contour of the breast throughout the treatment volume and the effect of the use of wedge compensators on the off-axis contribution, a substantial error is possible. (Herbert et al). Also some difficulty in positioning of the diodes on the patient, particularly in the head and neck area.
(Noel et al) conducted a study involving 7519 patients over 5 years were checked by in vivo dosimetry following a protocol to detect any systematic error that may have been missed during other checks performed at the various stages of planning and calculation before the commencement treatment session. The results showed that 79 errors were detected, half of which could have induced a variation of more than 10% in the dose delivered. Except for the breakdown of the cobalt unit used during irradiation, 78 out of 79 errors were of human origin.
These deductions were made from the study; dose delivered to patient by in vivo dosimetry for radiotherapy treatment is an essential aspect of a quality assurance programme and with less difficulty. In vivo dosimetry performed on patients can give us information on the efficiency of a medical accelerator. They mostly arise during the treatment planning step. Mistakes are usually made during data transfer by; recording errors when writing down data and missed data.
These mistakes can be serious if they are not detected at the initial stage of treatment of treatment since they could be systematic. In vivo dosimetry is a very efficient technique for detecting errors that can occur during
the course of a radiotherapy treatment. The use of silicon diodes, allows the direct measurement of results hence, an immediate action can be taken to alleviate errors detected during the of treatment of the patient. Most of the detected errors were down to human and analytical error of the results leading to the suspicion that random errors were common in radiotherapy. In order to root out some of these systematic errors computerized record and verify system were incorporated in their quality assurance programme. Even with these measures in place errors can still occur hence the importance of using in vivo dosimetry in order to validate the dose delivery.
A study conducted by (Lanson et al) to investigate the need for in vivo dosimetry during radiotherapy and to also assess the systematic and random errors found during in vivo dosimetry. The study took place 1993 and 1997, using semiconductor diode for dose involving 378 patients being treated at various sites.
The outcome of the study showed the need for in vivo dosimetry patient receiving radiotherapy. This procedure will require a set protocol which will also help eliminate systematic error in calculation of dose or dose delivery. Error was recorded in about 9% of the cases which is intolerable. They established that the lack of in vivo dosimetry errors may occur due to undetected systematic errors. The correction of these errors will lead to improvement in treatment. For these errors to be eliminated an accurate in vivo dosimetry is required with Stable and accurate measurement. Calibration of the dosimetry tool and frequent quality assurance of the tool is also essential to ensure its functioning at optimum. For these errors to be eliminated an accurate in vivo dosimetry is required with stable and accurate measurement. Calibration of the dosimetry tool and frequent quality assurance of the tool is also essential to ensure its functioning at optimum.
In vivo dosimetry during conformal radiotherapy Requirements for and Â®ndings of a routine procedure J.H. Lanson*, M. Essers
1 , G.J. Meijer, A.W.H. Minken, G.J. Uiterwaal, B.J. Mijnheer
TLD's exploits the properties of certain materials to store a fraction of energy they absorb. In radiotherapy the TLD used is Lithuim Fluoride doped with Magnesium and titanium (LiF:MgTi). In this sense the dosimetry energy released by ionisation radiation is absorbed by TLD and released in the form of optical radiation when excited electron return to the ground state after absorbing energy. Electrons absorb sufficient energy to escape from their lattices, a transition occurs to return the electron to it ground state. This process is known as the thermoluminescence. The energy released in the form of photon on return to it ground state is detectable by a photomultiplier tube. The output of the photomultiplier is proportional to the energy originally absorbed. The luminescence of the TLD is directly proportional to the dose accumulated by the TLD material. ( mangili et al). TLD's do not have signiï¬cant directional dependence and does not require temperature and pressure corrections. (Izewska and Rajan).
The use of TLD requires annealing of the device which involves the heating of the TLD for a prolonged time at temperature greater than one the reading was made and then cooling at room temperature, this procedure is required to remove residual signal thereby ensuring optimum sensitivity, this is to ensure TLD has exactly the same properties before irradiation. (A.J.J. Bos).
The main advantages of diode dosimetry are their instantaneous response and their ease of use by the therapists. (Doracy et al)
A study conducted by (Gustavo L. Barbi et al) using 45 thermoluminescent dosimeters (TLD) divided into two batches a cobalt -60 unit was used to irradiate the TLD's. 11 head and neck patients were randomly selected to take part in the study.
The expected dose was defined as the dose at the depth of dose maximum and was calculated manually from the prescribed tumor dose (Van Dam and Marinello, 1994). The first batch was found to be within -/+1.5% of the expected dose and the second batch -/+1.6% of the expected manually calculated dose. The inaccuracy was attributed to discrepancy in the dosimetric system. It was concluded that the
thermoluminescent dosimetric system for performing in vivo entrance dose measurements in external photon beam radiotherapy presented good results.
The results obtained demonstrated the value of thermoluminescent dosimetry as a treatment dose verification method and its applicability as a dosimetry tool in radiotherapy. (Alessandro et al)
A research was conducted by ( Amor Duch et al) to measure dose distributed during total body irradiation using thermoluminescence dosimetry. A dose of 2.25Gy was given twice a day for three days. The TLD system described was proven to be suitable for in vivo dosimetry in total body irradiations within an uncertainty of 2% even in heterogeneous areas of the body and when lung shielding blocks are applied. However, to attain this level of accuracy, it is necessary to use individual calibration factors, a temporal sensitivity control and calibration conditions as close as possible to irradiation conditions. The results the difï¬culty of calibrating an in vivo dosimetry system when lung shielding blocks are in place, due to the difï¬culty in obtaining reference dose measurements in the tissue. The calibration experiments indicated that in a high dose gradient region, standard calibration is not valid and therefore errors in dose assessment from the reference ionization chamber can be made. Moreover, it is shown that TLDs in an anthropomorphic phantom can be a useful to test any non-standard therapeutical technique. A better knowledge of the radiation beam and of the detector response would be necessary to ascertain the best calibration set-up.
TLD's have proved to be very practical in complicated geometries where best use could be made of the advantages of TLD dosimeters such as their stand alone character and their small physical size (Tomas et al). TLD fast read out time and ability to be used in more complicated geometry is an advantage over semiconductors
Dosimetric accuracy at the level of 5%, is understood today to represent a tolerance of deviation between the response of tumour and healthy tissue, is postulated in dosimetric protocols (TRS 398, 2000).
A clinically acceptable precision in dose measurement of diodes is achieved when the necessary factors influencing the diode response are quantified against an ionization chamber having a traceable calibration (Doracy et al). however,
when larger rotational errors are not taken into account, it leads to a reduced accuracy in dose measurement. (Remeijer et al). There has been greater interest in phantom TLD dosimetry outside the treatment ï¬eld, to determine the undesirable doses in radiotherapy, primarily for estimation of risk of secondary cancers (Harrison, 2007).
3.1.4 Veridose diode dosimetry
A study was conducted by (Ali et al) to investigate
The use of veridose diode as an alternative for semiconductor diode, the results of the study showed study was similarity in result with measurement taken by calibrate ion chamber simultaneously.
The design of these diodes used are made to allow for easy patient set up, as it has a hemispherical shape, which is easy to place on the skin of patients.
Build up materials was attached to the diode; the importance of this is to enable the diode measure the beam at the point of maximum dose. Various thickness of materials are used for individual diodes with respect to the beam energy to be measured. The veridose system can also be used for electron beams and also requires build up material for various electron energies.
The aim of this study was evaluate the linearity, accuracy, reproducibility, energy dependence and orientation of the system.
The energy absorbed by both photons and electron diodes were measured to determine the energy dependence of veridose.
Dose absorbed by the veridose diode was compared to the dose measured by the ionisation chamber in water phantom. The result showed a difference of less than 4% and 3% for both electron and photon diodes respectively compared to the ion chamber.
A 12.5% dose perturbation was recorded for veridose diodes compared to the 25% dose perturbation in semiconductor diodes at 6Mev electron beam
Doses of 10cGy showed less than 5% nonlinearity, conversely, nonlinearity increased with decreasing dose of less thant 10cGy in the electron diode. Hence requires cautious calibration for dose below 10cGy
The outcome showed less than 2% change for beams between 6-20MV. Other diodes had a 30% change with beam energies between 6-20MV
They suggested that different diode be used for various beam energies levels.
It was realised that the veridose diode is a good alternative for semiconductor diodes as it, improves on their overall benefits
Veridose is hence an acceptable device for clinical use as it exhibits high linearity, reproducibility and very good accuracy. It compatibility with linear accelerator was also established
Literature on this technique is however far and between and more research need to be done to confirm it viability as a clinical tool.