Cavity theory-Radiation physics

Cavity theory.. Radiotherapy..

I explained about Bragg-gray, Spencer attix and Burlin theory..
In future I'll try to explain this with some more points. So wait for the updation.

I referred Radiation oncology (IAEA) book and
Introduction to Radiological Physics and Radiation Dosimetry by Frank Herbert Attix book

1. Cavity theory KATHIRAVAN E 2016245011 M.SC MEDICAL PHYSICS,ANNA UNIVERSITY

2. introduction… To measure the absorbed dose in a medium introduce a dosimeter ( Chamber+electrometer ) Cavity sizes are referred to as small, intermediate or large in comparison with the ranges of secondary charged particles produced by photons in the cavity medium.

3. Types… For small size cavity Bragg Gray cavity theory Spencer Attix cavity theory For intermediate size cavity Burlin cavity theory

4. Basis…

5. Under assumption c the energy thus lost by the particles remains in the foil as energy imparted. Hence the absorbed dose in the foil can be gotten by dividing Equation by the mass per unit area of the foil

6. The Bragg- Gray cavity theory The Bragg- Gray (B-G) cavity theory was the first cavity theory developed to provide a relationship between absorbed dose in a dosimeter and the absorbed dose in the medium containing the dosimeter.

7. By Ignoring Backscatter

8. Gray in particular identified the probe as a gas-filled cavity, whencethe name “cavity theory”. The simplest such theory is called the Bragg- Gray (BG) theory, and its mathematical statement, referred to as the Bragg- Gray relation, will be developed next.

9. FIRST condition for application of the Bragg- Gray cavity theory is: the cavity must be small when compared with the range of charged particles incident on it so that its presence does not perturb the fluence of charged particles in the medium The result of condition is that the electron fluences are the same and equal to the equilibrium fluence established in the surrounding medium. This condition can only be valid in regions of CPE or TCPE . In addition, the presence of a cavity always causes some degree of fluence perturbation that requires the introduction of a fluence perturbation correction factor.

10. Scatter & fluence PERTURBATION… For heavy charged particles (either primary, or secondary to a neutron field),which undergo little scattering, this B-G condition is not seriously challenged so long as the cavity is very small in comparison with the range of the particles. However, for electrons even such a small cavity may be significantly perturbing unless the medium g is sufficiently close to w in atomic number.

11. second B-G condition the absorbed dose in the cavity is deposited solely by charged particles crossing it, i.e., photon interactions in the cavity are assumed negligible and thus ignored. Condition (2) implies that all electrons depositing the dose inside the cavity are produced outside the cavity and completely cross the cavity. Therefore, no secondary electrons are produced inside the cavity and no electrons stop within the cavity

12. Under the terms of the two B-G conditions For a differential energy distribution , (particles per cm2 MeV) the appropriate average mass collision stopping power in the cavity medium g is

13. for a thin layer of wall material w that may be inserted in place of g,

14. Combining Eqs . (10.4) and (10.5) gives for the ratio ofabsorbed dose in w to that in g, which is the B-G relation in terms of absorbed dose in the cavity:

15. If the medium g occupying the cavity is a gas in which a charge Q(of either sign) is produced by the radiation, can be expressed (in grays ) in terms of that charge as is the mean energy spent per unit charge produced (J/C

16. we obtain the B-G relation expressed in terms of cavity Ionization: This equation allows one to calculate the absorbed dose in the medium immediately surrounding a B-G cavity, on the basis of thecharge produced in the cavity gas,provided that the appropriate values of m, ,and are known

17. Conclusion of bragg gray cavity theory the cavity size is not explicitly taken into account in the Bragg- Gray cavity theory, the fulfillment of the two Bragg- Gray conditions will depend on the cavity size which is based on the range of the electrons in the cavity medium, the cavity medium, and electron energy.

18. Spencer attix cavity theory The Bragg- Gray cavity theory does not take into account the creation of secondary (delta) electrons generated as a result of the slowing down of the primary electrons in the cavity

19. B-g and s-a theory… Monte Carlo calculations have shown that the difference between the Spencer– Attix and Bragg– Gray cavity theories is non-negligible yet generally not very significant . Since collision stopping powers for different media show similar trends as a function of particle energy, their ratio for the two media is a very slowly varying function with energy.

20. Considerations in the application of cavity theory to ionization chamber calibration and dosimetry protocols

21. Taking into account all further small perturbations, the dose in the medium is determined with a thin-walled ionization chamber in a high energy photon or electron beam by:

22. Large cavities in photon beams

23. For a large cavity the ratio of dose cavity to medium is calculated as the ratio of the collision kerma in the cavity to the medium and is therefore equal to the ratio of the average mass-energy absorption coefficients, cavity to medium: where the mass-energy absorption coefficients have been averaged over the photon fluence spectra in the medium (numerator) and in the cavity gas (denominator).

24. Burlin cavity theory Burlin extended the Bragg- Gray and Spencer- Attix cavity theories to cavities of intermediate dimensions by introducing the large cavity limit to the Spencer- Attix equation using a weighting technique . This was introduced on a purely phenomenological basis. He provided a formalism to calculate the value of the weighting parameter.

25. Conditions to apply the Burlin theory The surrounding medium and the cavity medium are homogeneous ; A homogeneous photon field exists everywhere throughout the medium and the cavity Charged particle equilibrium exists at all points in the medium and the cavity that are further than the maximum electron range from the cavity boundary The equilibrium spectra of secondary electrons generated in the medium and the cavity are the same

26. How to get the weighting parameter d in this theory?

27. Any questions..? if(yes) { Ask me, We can discuss ; } else { Let me say Thank You…! ; }