Dose errors due to charge storage in electron irradiated plastic phantoms

Journal Article · · Med. Phys.; (United States)

DOI:https://doi.org/10.1118/1.595601· OSTI ID:6189042

Galbraith, D M; Rawlinson, J A; Munro, P

Commercial plastics used for radiation dosimetry are good electrical insulators. Used in electron beams, these insulators store charge and produce internal electric fields large enough to measurably alter the electron dose distribution in the plastic. The reading per monitor unit from a cylindrical ion chamber imbedded in a polymethylmethacrylate (PMMA) or polystyrene phantom will increase with accumulated electron dose, the increase being detectable after about 20 Gy of 6-MeV electrons. The magnitude of the effect also depends on the type of the plastic, the thickness of the plastic, the wall thickness of the detector, the diameter and depth of the hole in the plastic, the energy of the electron beam, and the dose rate used. Effects of charge buildup have been documented elsewhere for very low energy electrons at extremely high doses and dose rates. Here we draw attention to the charging effects in plastics at the dose levels encountered in therapy dosimetry where ion chamber or other dosimeter readings may easily increase by 5% to 10% and where a phantom, once charged, will also affect subsequent readings taken in /sup 60/Co beams and high-energy electron and x-ray beams for periods of several days to many months. It is recommended that conducting plastic phantoms replace PMMA and polystyrene phantoms in radiation dosimetry.

• Research Organization: Ontario Cancer Inst., Toronto, Canada
• OSTI ID: 6189042
• Journal Information: Med. Phys.; (United States), Journal Name: Med. Phys.; (United States) Vol. 11:2; ISSN MPHYA
• Country of Publication: United States
• Language: English

Related Subjects

61 RADIATION PROTECTION AND DOSIMETRY
655003* -- Medical Physics-- Dosimetry
CHARGE COLLECTION
DOSIMETRY
ELECTRIC CHARGES
ELECTRON DOSIMETRY
ELECTRONS
ELEMENTARY PARTICLES
ENERGY RANGE
ESTERS
FERMIONS
IONIZATION CHAMBERS
LEPTONS
MATERIALS
MEASURING INSTRUMENTS
MEDICINE
MEV RANGE
MEV RANGE 01-10
MOCKUP
NUCLEAR MEDICINE
ORGANIC COMPOUNDS
ORGANIC POLYMERS
PETROCHEMICALS
PETROLEUM PRODUCTS
PHANTOMS
PLASTICS
PMMA
POLYACRYLATES
POLYMERS
POLYVINYLS
RADIATION DETECTORS
RADIOLOGY
RADIOTHERAPY
STRUCTURAL MODELS
SYNTHETIC MATERIALS
THERAPY