Radaition and Dosematry Essay Example

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    Physics
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  • Level:
    Masters
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Radiation and dosimetry 5

Radiation and Dosimetry

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19/05/2011

Absorbed dose based protocol is one of the international recognized protocols for determining the absorbed dose in water for megavoltage photon beams. TRS-398 protocol is this kind of dosimetry protocol. The dose in water phantom (formalization) is determined by DwQ(zref)=MkqNDCo, where DwQ(zref) is the dose in the users beam quality Q at reference location zref. kq is the quantity conversion factor that changed the absorbed dose calibration coefficient in a 60Co beam to that in arbitrary beam of quality Q. NDCo is the absorbed dose to water factor for cobalt as given by the SSDL and M is the corrected chamber reading. The measured quantities requires some corrections this include temperature and pressure. It is corrected by the formula kTp = P0/P (T + 273.2)/ (T0 + 273.2) with pressure (P) (in kPa) and temperature (T) (in oC) (Andreo et al, 2004). The recombination of the ions in the chamber is also corrected by the formula ks = ((V1/V2)2 — 1)/ ((V1/V2)2 — (M1/M2)). The electrometer reading is also corrected

as shown by the formula MQ = Mraw kTP kelec kpol ks

where the MQ and Mraw are the corrected and the raw reading;

kTP and ks are the temperature, pressure and recombination correction;

kelec a factor allowing for separate calibration of the electrometer and the kpol = (M+ + M )/ 2M is the polarity correction with M being the reading at normal polarity. In practical, the PTW small water phantom is filled with water up to the correct depth. The temperature is allowed to equilibrate for more than an hour. The phantom is then leveled and the chamber inserted. The measurement depth in water should be 5cm; the chamber position together with the geometric centre of the chamber is at the measurement depth.

Absorbed dose to air inside the ionization chamber is given as

Dair= Q/Mair X (w/e) air

Where Q is the charge of the water phantom

Mair is the mass of the air in the ionization chamber

(w/e) air is the mean energy expanded in air per ion pair formed, it is usually taken as

Mass of air = density of air at S.T.P X volume (1.293 kg/m3 X 1×10-6m3) = 1.293X10-6kg

Dair =50 X 10-9/1.293 X 10-6 X 33.97

1698.5/1.293= 1.313X106 j/kg-1

Absorbed dose to water at the same point without the ionization chamber is given as

Dwater = Dair X Sw,a

Where Sw,a is the product of the stopping power ratio for PTW 23323 micro = 1.119 (Andreo et al, 2004).

Dwater = 1.313 X 106 X 1.119

1.469 X 106j/kg

The factors that might affect the measurement include in practice, correction factors are required to because the ion chamber materials are not perfect Bragg-Gray cavity. In reality there will be some perturbation of Φ and therefore a perturbation correction factor is introduced. Bragg-Gray cavity principle assumes that there are negligible photon interactions, delta electrons and brehmsstrahlung production in the cavity. The product of technical implementation dependent correction is assumed to have no effect on the overall results.

The difference between the Bragg-Gray principle and Spencer Attix is that Bragg-Gray principle cannot fully describe the phenomenon of ionization in the cavity while Spencer Attix show full analysis of the electron spectrum in the wall ought to take into account accumulation of primary electrons with higher energy (Seuntjens & Duane, 2009). The dose to medium by use of Spencer Attix theory differs with the Bragg-Gray theory as cavity size decreases and Z of the medium increases. The setback of the Bragg-Gray principle is that it doesn’t consider the fact that the secondary electrons may not be in charged particle equilibrium hence deviating from the experimental measures. This was improved by the Spencer Attix by accounting for secondary electrons above its threshold Δ, this bring in a quantity that characterizes the size of the cavity. In Bragg-Gray principle, the dose to the medium Dmed is related to the dose in the cavity Dcav by the given formula

Dmed = Dcav (S/ ρ) medcav

Where (S/ ρ) medcav is the ratio of the unrestricted mass collision stopping powers

The Spencer Attix relation between the dose to the medium and the dose to the cavity is given by the relation

Dmed = Dcav (LΔ/ ρ) medcav

Where (LΔ/ ρ) medcav is the ratio of the mean restricted mass stopping power of the medium to that of the cavity; the Spencer Attix stopping power ratios is given by Sm, a= Dmed / Dcav

References

Vial Phil. (2011). Dosimetric principles, radiation dosimetry. Institute of medical physics.

Sydney: university of Sydney.

Andreo Pedro et al. (2004). Absorbed dose determination in external beam radiotherapy.

Retrieved on 5/9/2011 from http://www-naweb.iaea.org/nahu/dmrp/pdf_files/CoPV11b.pdf.

Seuntjens Jan & Duane Simon. (2009). Photon absorbed dose standards, IOP publishing. McGill

University: Toronto.