Biomaterial Database

Bragg-Gray theory and ion chamber dosimetry for photon beams. 1991

C M Ma, and A E Nahum

Joint Department of Physics, Royal Marsden Hospital and Institute of Cancer Research, Sutton, Surrey, UK.

The validity of the Bragg-Gray cavity theory in photon radiation dosimetry for photon energies from 10 keV to 10 MeV has been investigated quantitatively in this paper. A crucial assumption for Bragg-Gray cavity theory to be valid is that the dose from photon interactions in the detector material is negligible. The ratio, Fair, of the absorbed dose resulting from photon interactions in an air cavity, DPA(air), to that in air under the condition of charged-particle equilibrium, DCPE(air), has been used as a parameter to determine if the air cavity can be classified as a Bragg-Gray cavity. Monte Carlo calculated results show that, for monoenergetic photon beams of energies above about 220 keV, the dose ratio, Fair, is smaller than 0.05 for an air cavity of 6 mm thickness and 6 mm diameter in vacuum. Furthermore, it is shown that the Burlin general cavity theory seriously overestimates the departure from Bragg-Gray behaviour. For clinical photon beams the dose ratio, Fair, is 0.29 for a 150 kVp beam and 0.27 for a 240 kVp beam compared to 0.006 for a 60 Co beam if the cavity is placed at a depth of 5 cm in water. This study confirms that typical air-filled ionization chambers cannot be considered to be Bragg-Gray cavities for low- and medium-energy photon radiation.

UI	MeSH Term	Description	Entries
D008962	Models, Theoretical	Theoretical representations that simulate the behavior or activity of systems, processes, or phenomena. They include the use of mathematical equations, computers, and other electronic equipment.	Experimental Model,Experimental Models,Mathematical Model,Model, Experimental,Models (Theoretical),Models, Experimental,Models, Theoretic,Theoretical Study,Mathematical Models,Model (Theoretical),Model, Mathematical,Model, Theoretical,Models, Mathematical,Studies, Theoretical,Study, Theoretical,Theoretical Model,Theoretical Models,Theoretical Studies
D009010	Monte Carlo Method	In statistics, a technique for numerically approximating the solution of a mathematical problem by studying the distribution of some random variable, often generated by a computer. The name alludes to the randomness characteristic of the games of chance played at the gambling casinos in Monte Carlo. (From Random House Unabridged Dictionary, 2d ed, 1993)	Method, Monte Carlo
D011874	Radiometry	The measurement of radiation by photography, as in x-ray film and film badge, by Geiger-Mueller tube, and by SCINTILLATION COUNTING.	Geiger-Mueller Counters,Nuclear Track Detection,Radiation Dosimetry,Dosimetry, Radiation,Geiger Counter,Geiger-Mueller Counter Tube,Geiger-Mueller Probe,Geiger-Mueller Tube,Radiation Counter,Counter Tube, Geiger-Mueller,Counter Tubes, Geiger-Mueller,Counter, Geiger,Counter, Radiation,Counters, Geiger,Counters, Geiger-Mueller,Counters, Radiation,Detection, Nuclear Track,Dosimetries, Radiation,Geiger Counters,Geiger Mueller Counter Tube,Geiger Mueller Counters,Geiger Mueller Probe,Geiger Mueller Tube,Geiger-Mueller Counter Tubes,Geiger-Mueller Probes,Geiger-Mueller Tubes,Probe, Geiger-Mueller,Probes, Geiger-Mueller,Radiation Counters,Radiation Dosimetries,Tube, Geiger-Mueller,Tube, Geiger-Mueller Counter,Tubes, Geiger-Mueller,Tubes, Geiger-Mueller Counter
D011879	Radiotherapy Dosage	The total amount of radiation absorbed by tissues as a result of radiotherapy.	Dosage, Radiotherapy,Dosages, Radiotherapy,Radiotherapy Dosages
D011882	Radiotherapy, High-Energy	Radiotherapy using high-energy (megavolt or higher) ionizing radiation. Types of radiation include gamma rays, produced by a radioisotope within a teletherapy unit; x-rays, electrons, protons, alpha particles (helium ions) and heavy charged ions, produced by particle acceleration; and neutrons and pi-mesons (pions), produced as secondary particles following bombardment of a target with a primary particle.	Megavolt Radiotherapy,High-Energy Radiotherapy,Radiotherapy, Megavolt,High Energy Radiotherapy,Radiotherapy, High Energy
D003198	Computer Simulation	Computer-based representation of physical systems and phenomena such as chemical processes.	Computational Modeling,Computational Modelling,Computer Models,In silico Modeling,In silico Models,In silico Simulation,Models, Computer,Computerized Models,Computer Model,Computer Simulations,Computerized Model,In silico Model,Model, Computer,Model, Computerized,Model, In silico,Modeling, Computational,Modeling, In silico,Modelling, Computational,Simulation, Computer,Simulation, In silico,Simulations, Computer
D006801	Humans	Members of the species Homo sapiens.	Homo sapiens,Man (Taxonomy),Human,Man, Modern,Modern Man