Advanced quality assurance is paramount in treatment delivery during radiation therapy procedures. These quality assurance procedures include proper dose simulation verification to ensure that dose planning systems are working optimally. This is achieved using either of these two methods; in – phantom dosimetry or in – vivo through the use of online dose measurements during treatment.
There exists a wide class of methods in radiation therapy for absorbed dose as well as radiation detectors for radiobiological dosimetry.Therapeutic beams have a radiological property which is characterized using semiconductor detectors. Bomford, 2003). Semiconductor electronic detector application is of great benefit particularly in the application of the recently developed and tested silicon micro dosimeters, thin strip and pixilated dosimeters.
Spectroscopic radiation therapy docimetry methodology is usually related to the measurement of a radiation spectra deposited by single events from primary and secondary radiation generated within a phantom and is usually applied in hadrons’ therapy. Recent developments as exemplified by Monte Carlo based treatment planning verification in hadrons’ therapy can be realized using silicon micro dosimeters and nano-dosimeters.In our discussion we concentrate on the recently developed hybrid pixel detectors, silicon pixel detectors and single strip detectors particularly epitaxial single strip p-type silicon radiation detectors where special emphasis will be made on the characteristics of the silicon diode detector p-type. Literature Review Proper understanding of the target constrains is of great importance in radiation therapy especially in order to facilitate the delivery of a radiation dose to the specific target.This is usually achieved through the use of modern imaging techniques, planning of radiation beams and verification of dosage in a phantom (Djouguela, 2007).
As a result this requires dosimetry with high spatial distribution and the ability to measure both surface and inside doses. In Radiation therapy, the most diodes used for dosimetry are the p-type silicon diodes which are because p-type diodes suffer less sensitivity loss as a function of accumulated dose than n-type and they are also less dose rate dependent.The application of the functional characteristics of the p-type silicon diode detector in dosimetry is mainly observed in relation to relative constancy of their response as a function to time. In this case, according to Horowitz, (2006), the diodes sensitivity decreases as a function of the dose accumulated that is necessary to monitor its sensitivity over time more closely than in Ionization chamber (IC). Discussion Studies have shown that temperature dependence, which is one on the p-type silicon characteristics, is quite moderate with a sensitivity variation with temperature.Semiconductor dosimeters particularly the silicon detectors are an excellent choice because they exhibit a much greater sensitivity (* 18000) than ionizing chambers due to their higher density and an ionization energy which is ten times smaller than available in gas.
(Horowitz, 2006), They also have small dissymmetric volume size thus fulfilling the Brag-Grey cavity theory requirement. As a result the dosimeter can be placed in a confined space within a phantom. This is achieved because the detector exhibits good mechanical stability making it possible to operate in passive mode.The silicon p-type detector has a significant advantage since the mass collision stopping power ration (silicon to water) is almost energy independent for electron energies in conventional MV therapy. The p-type silicon detector has very useful characteristics in dosimetry which is their relatively small active volume and high sensitivity. (Jursinic, 2009).
Unlike ionization chambers (IC), the detectors have a high sensitivity ranging to up to 20-100 times that of ionization chamber.As a result, the high signal to noise ratio is usually very useful in measuring weak signals from leakage or scattered radiation. Another very important characteristic of p-type silicon detectors is its relatively low resistivity. This is because a p-type detector with a low doping level exhibits a non linear dose rate response in a high energy photon beam which contains neutrons where it remains linear after pre-irradiation even when the doping level is increased.
(Grusell, 1993). The linearity is therefore constant at different doping levels.The p-type silicon detector’s response depends on the dose rate and temperature. Direction dependence is another very important characteristic of these diodes whose study results showed that the diode’s response decrease due to an increase in the angle between central beam axis and the symmetry axis of diode because of the shape of this is hemispherical symmetry. (Chao, 2005). This indicates that the hemispherical symmetry of the diode has a relatively strong directional dependence.
Conclusion The p-type silicon diode detectors pose as the best choice of dosimeters used in radiation therapy.This is because they exhibit good working characteristics which are favorable for dosimetry. There characteristics range from; good signal stability which is acceptable in dosimetry, short-term reproducibility which shows a satisfactory standard deviation, tolerable dose linearity, suitable dose rate response, directional dependence in relation to angle correction factor and finally temperature dependence. In conclusion, the total results indicate the p-type silicon diode detector have reliable dosimetric characteristics which are satisfactory for radiation technology.
