Measurement of Absorbed Dose (5)

1. Measurement of Absorbed Dose (5) 參考資料：1. The Physics of Radiation Therapy.Faiz M. Khan2. Introduction to Radiological Physics and Radiation Dosimetry. Frank H. Attix

2. The Bragg-Gray cavity theory • Chamber Volume • The quantity Jgcan be determined for a chamber of known volume or known mass of air in the cavity if the chamber is connected to a charge-measuring device. • However, the chamber volume is usually not known to an acceptable accuracy. • Indirect method of measuring • Jairis to make use of the exposure calibration of the chamber for Co-60 γray beam

3. The Bragg-Gray cavity theory • Chamber Volume • Suppose the chamber with this build-up cap is exposed in free air to a Co-60 beam and that a transient electronic equilibrium exists at the center of the chamber. • assume initially that the chamber wall and the build-up cap are composed of the same material (wall).

4. The Bragg-Gray cavity theory • Chamber Volume • if the chamber (plus the build-up cap) is replaced by a homogeneous mass of wall material with outer dimensions equal to that of the cap,

5. The Bragg-Gray cavity theory • Chamber Volume • where is the ratio of electron fluence at the reference point P (center of the cavity) with chamber cavity filled with wall material to that with the cavity filled with air. • This correction is applied to the Bragg-Gray relation to account for change in electron fluence

6. The Bragg-Gray cavity theory • Chamber Volume • ψ in the above equation can be replaced by Ψ , provided a transient electron equilibrium exists throughout the region of the wall

7. The Bragg-Gray cavity theory • Chamber Volume • If Dair is the absorbed dose to air that would exist at the reference point with the chamber removed and under conditions of transient electronic equilibrium in air,

8. The Bragg-Gray cavity theory • Chamber Volume

9. The Bragg-Gray cavity theory • Chamber Volume • Also Dair (under conditions of transient electronic equilibrium in air) can be calculated from exposure measurement in a Co-60 beam with a chamber plus build-up cap,

10. The Bragg-Gray cavity theory • Chamber Volume • where k is the charge per unit mass produced in air per unit exposure ( 2.58 x 10-4 C kg-1R-1 ) • M is the chamber reading normalized to standard atmospheric, 22℃, 760 mmHg • Aion is the correction for ionization recombination under calibration conditions, • Pion is the ionization recombination correction for the present measurement.

11. The Bragg-Gray cavity theory • Chamber Volume

12. The Bragg-Gray cavity theory • Chamber Volume

13. The Bragg-Gray cavity theory • Chamber Volume • consider a more realistic situation in which the chamber wall and build-up cap are of different materials.

14. The Bragg-Gray cavity theory • Chamber Volume

15. The Bragg-Gray cavity theory • Effective Point of Measurement • Plane Parallel Chambers • Since the front plane • (toward the source) of the • air cavity is flat and is • exposed to a uniform fluence • of electrons, the point of • measurement is at • the front surface • of the cavity.

16. The Bragg-Gray cavity theory • Effective Point of Measurement • Cylindrical Chambers • Electrons (from an electron beam or generated by photons) transversing a cylindrical chamber of internal radius r will enter the sensitive volume of the chamber (air cavity) at different distances from the center of the chamber.

17. The Bragg-Gray cavity theory • Effective Point of Measurement • Cylindrical Chambers • Dutreix and Dutreix showed that theoretically the point of measurement for a cylindrical chamber in a unidirectional beam is displaced by 0.85r from the center and toward the source. • the effective point of measurement is influenced by the number of electrons entering through a surface area ds at A of the chamber and the track length of these electrons in the cavity.

18. The Bragg-Gray cavity theory • Effective Point of Measurement • Cylindrical Chambers • Dutreix and Dutreix :