New Radiotherapy Surface Dose Detector - Assignment Example

MOSFET Dosimetry and its Application in IMRT Name Task Tutor Date MOSFET DOSIMETRY AND ITS APPLICATION IN IMRT Abstract MOSFET dosimeters today are vastly being used in medicine in the field of clinical dosimetry for radiation beams in radiotherapy. This is due to the various properties that they have like small physical size, active areas which are very thin, permanent storage of post-radiation signals, among others. This paper will review the basic structure, properties and use of MOSFET dosimeters in clinical medicine with a bias towards Intensity Modulated Radiotherapy (IMRT) (Ramani, 1994). Introduction This paper will deeply discuss some experimental investigations that have been conducted to prove the various capabilities that have been found in MOSFET. Such include the electrical characteristics, experiments on human samples including the effect of conducting some phantom on breasts and skin. Electrical characteristics of MOSkin detectors and RADFET detectors. In checking the electrical characteristics, we will focus on the response of the transistors which make up the gates. There are linear curves which indicate a low dose that is followed by a saturation region. This is directly proportional to the voltage V(Girr) for an oxide which is lower than 1MV/cm. this means that there is a zero-electric-field. The given slope is the trapped charge’s centroid (Kelleher, 1994). The above graph is the effect of the irradiation-dose (D) on the threshold-voltage-shift Change in voltage (T) is given by D (ex) = 1, 000 A. Voltage influence on the gate The linearity of the curve increases with applied bias. Dose D (L) for the experiment deviates with 5% from the linear part of the curve. The above graph shows the extension of the linear region as compared to the voltage (Girr). The sensitivity of the MOS device in this case the RADFET and the MOSkin detectors increases with an increase in the voltage of the gate (Sarrabayrouse, 1986). This is depicted in the first figure. The chief reason for this is because they depend on the probability that a hole will escape recombination upon the electric field. This variation is founded by the following equation. F (E (ir)) = (1/(1+(SmA/Eir))) This is depicted in the graph below. From this figure, it is clear that under 1MV/cm f (Eir) usually increases very fast the present electric field and on a very slight note less than 1 MV/cm. negatively influenced gate voltage affects the MOS device’s stability. Response of MOSkin and RADFET on various devices Response on medical LINACs In the field of clinical-dosimetry, dose delivery accuracy on medical LINAC curves has to have an accuracy of +/-2%. The current in vivo devices help in relative dosimetry applications. Problems in measurements of surface dose and the TLD is the reason that the thickness of the present over-layer cannot allow the dose depth to be obtained at 0.07mm. Due to the thinness of the MOSFET detectors, it is possible to measure their surface dose for better packaging (Rosenfeld, 2002). This is a comparison of MOSFET with Attix chamber and simulations for MCNP of the depth dose. When the MOSFETs are derived accurately, they can build close to a surface dose of 17%. These measurements apply to MOSFET detectors. RADFETs are mostly applied for the treatment of chest walls. They have a slight difference in the dose percentage due to their thickness. In the LINAC medical treatment, MOSFET detectors take advantage of the RADFET detector in the in vivo dosimetry. Preparation of the Phantom of Head and Breast The need to treat the skin using the intensity modulated radiotherapy (IMRT) arises when the margin between the clinical target volume (CTV) and the planning target volume (PTV) is big in such a way that it can leave space for geometrical uncertainties. Inverse planning of the IMRT can be effected to increase the skin dose. In this situation, the responsible clinician is supposed to draw a CTV that stops just near the surface of the skin thus meaning there is no disease. The uncertainty margin is set up by drawing a PTV at the skin surface. The concept of the PTV thus helps the beam sizes and their arrangements in ensuring that the doses are delivered to the needed CTV (Simon, 2004), it is preferred that one should use a CTV-PTV margin of 10mm. Scanning of the phantom on CT and planning using PINNACLE TPS, skin dose determination. The following is a diagram that was borrowed from Simon (2004) that depicts a phantom shape that can be used for head and breast radiotherapy. The needed plans are evaluated by a summation of the quadratic functions that are appropriate for the targets and the organs that are at risk. For the phantoms in the figure which in our case we will assume to be phantoms for the head and the breast, a commercial treatment is applied. In this case the paper has directed the use of PINNACLE TPS which results to the production of a plan for the IMRT. For the clinical skin dose determination, the plan is in form of an IMRT that is forward planned. It has conformal plan fields, top up fields which are towards the PTV which have a low dose (Butson, 1986). The weights of the segments are optimized by the variation of the weights of the segments for the purpose of minimizing the objective functions which are subject to a preset constraint. Analysis of the set plan is first done by calculating the PTV objective functions. Through assumptions, the plan which has the lower objectives gets the more optimal pair. In the second case, the CTV is calculated for a range of shifts which belong to the iso-center. This method is simplified due ton the relativity if the CTV to the iso-center and vise versa. This can be concluded in the below figure. The above figure shows the DVH to PTV plans for the plans. Both of them are good and acceptable plans. Experimental measurements of skin doses with MOSkin, RADFET and radiographic film during IMRT and comparison with TPS. A MOSFET dosimeter is of great importance in the measurements of dose distributions within regions where the gradient of the given dose is higher and where electronic disequilibrium exists. This packaging is however not critical where electronic equilibrium exists. If tissue measurements for photon fields with an energy range of 0.05 to 1.25MeV were done, then the packaging requirements will differ depending on how the dosimeter has been placed. This means that enhancing optimization in the response of MOSFET energy for an equivalent tissue kerma does not guarantee an equivalent tissue response the device if the device is worn on the body (Rosenfield, 1995).TPSs are applied in the calculation of doses administered to the body organ with the assumption that the organ is homogeneous water equivalent medium whose size is infinite. RADFET and MOSkin are mainly useful in the measurement of absolute doses. The measurement puts into consideration the effect that empty/ air filled body cavities would have on the amount of dosage administered (Cutting and DearNaley, 2000). The dose measured using TPS is higher than that measured using RADFET and MOSkin thus the heterogeneity of in the volume of a body organ therefore highly affects the administered dose in comparison to the calculated dose. References Butson, Martin et al. (1996) New Radiotherapy Surface Dose Detector. Med. Phys, 23(5):655-657. Cutting C. and DearNaley, D. (2000) Intensity Modulated Therapy: A Critical Review. The British Journal of Radiology, 73 (2000):459-469. Freeman, Roger and Holmes, Andrew (1978) A Simple Model for Predicting Radiation Effects in MOS Devices. IEEE Transactions on Nuclear Science, 25(6):1216:1225. Gladstone, Lu et al, (1994) Miniature MOSFET Radiation Dosimeter Probe. Medical Physics, 21(11):1721-1728. Kelleher, A. et al. (1994) Investigation to the Re-use of PMOS Dosimeters. IEEE Transactions on Nuclear Science, 41(3):445-451. Ramani, Ramaseshan et al. (1997) Clinical Dosimetry Using Mosfets. Int. J. Radiology Oncology Biol. Phys., 37(4):959-964). Rosenfeld, Anatoly et al. (1995) MOSFET Dosimeters: The Role of Encapsulation on Dosimetric Characteristics in Mixed Gamma-Neutron and Megavoltage X-ray Fields.IEEE Transactions on Nuclear Science, 42(6):1870:1877. Rosenfield, A. (2002) MOSFET Dosimetry on Modern Radiation Oncology Modalities. Radiation Protection Dosimetry, 101(1-4):393-398. Sarrabayrouse G, Bellaouar A. and Rossel P. (1986) Electrical Properties of MOS Radiation Dosimeters. Revue Phys. Appl. 21(1986):283-287. Thomas, Simon and Hoole, Andrew (2004) The Effect of Optimization on Surface Dose in Intensity Modulated Radiotherapy (IMRT). Phys. Med. Biol. 49(2004):4919- 4928. Read More