Biomaterial Database

A first order approximation of field-size and depth dependence of wedge transmission. 1998

R C Tailor, and D S Followill, and W F Hanson

Department of Radiation Physics, University of Texas M. D. Anderson Cancer Center, Houston 77030, USA. rpc@radonc.mdacc.tmc.edu

The Radiological Physics Center, through its dosimetry review visits to participating institutions, is aware that many institutions ignore the field-size and depth dependence of wedge transmission values. Reference wedge transmission values are normally measured by the Radiological Physics Center for a 10 cm x 10 cm field at the calibration depth of 5 or 7 cm. Recently, additional measurements (1) for a 10 cm x 10 cm field at 20-cm depth and (2) for a 20 cm x 20 cm field at the calibration depth were included. The transmission under these two conditions was compared with that under reference conditions. The relative transmission values for 138 photon beams from 88 separate linear accelerators (4-25 MV) and 60Co units were measured. Our data suggest that the dependence of the wedge transmission on field-size and depth, in the first approximation, depends on the absolute value of the transmission under reference conditions. For wedges with a transmission value greater than 0.65%, field-size dependence and change in depth dose are typically less than 2%. However, for wedges with transmission values less than 0.65%, field-size dependence increases with decreasing reference wedge transmission. The change in wedge transmission with depth is significant (> 2%) only for photon energies less than or equal to 10 MV and can exceed 5% for thick wedges. Failure to include the depth and field-size dependencies of wedge transmission in patient dosimetry calculations can result in significant tumor-dose discrepancies.

UI	MeSH Term	Description	Entries
D009369	Neoplasms	New abnormal growth of tissue. Malignant neoplasms show a greater degree of anaplasia and have the properties of invasion and metastasis, compared to benign neoplasms.	Benign Neoplasm,Cancer,Malignant Neoplasm,Tumor,Tumors,Benign Neoplasms,Malignancy,Malignant Neoplasms,Neoplasia,Neoplasm,Neoplasms, Benign,Cancers,Malignancies,Neoplasias,Neoplasm, Benign,Neoplasm, Malignant,Neoplasms, Malignant
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
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
D011880	Radiotherapy Planning, Computer-Assisted	Computer-assisted mathematical calculations of beam angles, intensities of radiation, and duration of irradiation in radiotherapy.	Computer-Assisted Radiotherapy Planning,Dosimetry Calculations, Computer-Assisted,Planning, Computer-Assisted Radiotherapy,Calculation, Computer-Assisted Dosimetry,Calculations, Computer-Assisted Dosimetry,Computer Assisted Radiotherapy Planning,Computer-Assisted Dosimetry Calculation,Computer-Assisted Dosimetry Calculations,Dosimetry Calculation, Computer-Assisted,Dosimetry Calculations, Computer Assisted,Planning, Computer Assisted Radiotherapy,Radiotherapy Planning, Computer Assisted
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
D012015	Reference Standards	A basis of value established for the measure of quantity, weight, extent or quality, e.g. weight standards, standard solutions, methods, techniques, and procedures used in diagnosis and therapy.	Standard Preparations,Standards, Reference,Preparations, Standard,Standardization,Standards,Preparation, Standard,Reference Standard,Standard Preparation,Standard, Reference
D006801	Humans	Members of the species Homo sapiens.	Homo sapiens,Man (Taxonomy),Human,Man, Modern,Modern Man
D001703	Biophysics	The study of PHYSICAL PHENOMENA and PHYSICAL PROCESSES as applied to living things.	Mechanobiology
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)
D055592	Biophysical Phenomena	The physical characteristics and processes of biological systems.	Biophysical Concepts,Biophysical Processes,Biophysical Phenomenon,Biophysical Process,Biophysical Concept,Concept, Biophysical,Concepts, Biophysical,Phenomena, Biophysical,Phenomenon, Biophysical,Process, Biophysical,Processes, Biophysical