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

26. 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.

27. CONSIDERATIONS IN THE APPLICATION OF
CAVITY THEORY TO IONIZATION CHAMBER
CALIBRATION AND DOSIMETRY PROTOCOLS

29. • 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:

30. LARGE CAVITIES IN PHOTON BEAMS

31. • 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).

32. 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.

34. 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

35. HOW TO GET THE WEIGHTING PARAMETER
D IN THIS THEORY?

38. ANY QUESTIONS..?
if(yes)
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Ask me, We can discuss;
}
else
{
Let me say Thank You…!;
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