Biomaterial Database

Doses near the surface in high-energy x-ray beams. 1995

B E Bjärngard, and P Vadash, and T Zhu

Department of Radiation Oncology, Roger Williams Cancer Center, Providence, Rhode Island 02908, USA.

In an irradiation with a high-energy x-ray beam, the absorbed dose near the surface is the combined result of incident contaminating electrons and phantom-generated electrons. We describe an experimental method to characterize these processes under conditions of longitudinal electron disequilibrium but lateral equilibrium. The equilibrium dose at large depths is extrapolated back towards the surface and compared with measured doses. The extrapolation uses an expression that is based on Monte Carlo-calculated kerma values. The technique was applied to a 6-MV and a 25-MV x-ray beam. The dose from phantom-generated electrons increased exponentially with depth from zero at the surface. The dose from contaminating electrons decreased rapidly with depth with an attenuation coefficient that was approximately equal to the corresponding coefficient for the increase of dose from phantom-generated electrons. The surface dose from contaminating electrons increased linearly with the side of the square field at 6 MV but an error-function agreed better with the data at 25 MV.

UI	MeSH Term	Description	Entries
D008433	Mathematics	The deductive study of shape, quantity, and dependence. (From McGraw-Hill Dictionary of Scientific and Technical Terms, 6th ed)	Mathematic
D008961	Models, Structural	A representation, generally small in scale, to show the structure, construction, or appearance of something. (From Random House Unabridged Dictionary, 2d ed)	Model, Structural,Structural Model,Structural Models
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
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
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