Biomaterial Database

Reducing electron contamination for photon beam-quality specification. 1994

X A Li, and D W Rogers

Institute for National Measurement Standards, National Research Council, Canada, Ottawa, Canada.

The percentage depth dose at 10 cm in a 10 x 10-cm2 photon beam at an SSD of 100 cm, %dd(10), is a better beam-quality specifier for radiotherapy beams than the commonly used values of TPR10(20) or nominal accelerating potential. The presence of electron contamination affects the measurement of %dd(10) but can be removed by the use of a 0.1-cm lead filter, which reduces surface dose from contaminant electrons from the accelerator by more than 95% for radiotherapy beams with energies from 60Co to 50 MV. The filter performs best when it is placed immediately below the head. An electron-contamination correction factor is introduced to correct for electron contamination from the filter and air. It converts the %dd(10) which includes the electron contamination with the filter in place [hereafter %dd(10)m], into %dd(10) for just the photons in the filtered beam. The correction factor is a linear function of %dd(10)m for all filtered beams with %dd(10)m > 70%. A small correction for the photon filtering effect converts the pure photon %dd(10) for the filtered beam into that for the unfiltered beam, which can be used to determine stopping-power ratio. Calculations show that the values of water-to-air stopping power ratio in the unfiltered beam are related to the values of %dd(10)m in the filtered beam by a cubic function. The uncertainty of stopping-power ratios in unfiltered beams for the same value of the %dd(10)m is within 0.2% for all beams.

UI	MeSH Term	Description	Entries
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
D010315	Particle Accelerators	Devices which accelerate electrically charged atomic or subatomic particles, such as electrons, protons or ions, to high velocities so they have high kinetic energy.	Betatrons,Linear Accelerators,Accelerator, Linear,Accelerator, Particle,Accelerators, Linear,Accelerators, Particle,Betatron,Linear Accelerator,Particle Accelerator
D011878	Radiotherapy	The use of IONIZING RADIATION to treat malignant NEOPLASMS and some benign conditions.	Radiotherapy, Targeted,Targeted Radiotherapy,Radiation Therapy,Radiation Therapy, Targeted,Radiation Treatment,Targeted Radiation Therapy,Radiation Therapies,Radiation Therapies, Targeted,Radiation Treatments,Radiotherapies,Radiotherapies, Targeted,Targeted Radiation Therapies,Targeted Radiotherapies,Therapies, Radiation,Therapies, Targeted Radiation,Therapy, Radiation,Therapy, Targeted Radiation,Treatment, Radiation
D003037	Cobalt Radioisotopes	Unstable isotopes of cobalt that decay or disintegrate emitting radiation. Co atoms with atomic weights of 54-64, except 59, are radioactive cobalt isotopes.	Radioisotopes, Cobalt
D004583	Electrons	Stable elementary particles having the smallest known negative charge, present in all elements; also called negatrons. Positively charged electrons are called positrons. The numbers, energies and arrangement of electrons around atomic nuclei determine the chemical identities of elements. Beams of electrons are called CATHODE RAYS.	Fast Electrons,Negatrons,Positrons,Electron,Electron, Fast,Electrons, Fast,Fast Electron,Negatron,Positron
D006801	Humans	Members of the species Homo sapiens.	Homo sapiens,Man (Taxonomy),Human,Man, Modern,Modern Man
D017785	Photons	Discrete concentrations of energy, apparently massless elementary particles, that move at the speed of light. They are the unit or quantum of electromagnetic radiation. Photons are emitted when electrons move from one energy state to another. (From Hawley's Condensed Chemical Dictionary, 11th ed)