title: Absorbed dose to water reference dosimetry using solid phantoms in the context of absorbed-dose protocols.
abstract: For reasons of phantom material reproducibility, the absorbed dose protocols of the American Association of Physicists in Medicine (AAPM) (TG-51) and the International Atomic Energy Agency (IAEA) (TRS-398) have made the use of liquid water as a phantom material for reference dosimetry mandatory. In this work we provide a formal framework for the measurement of absorbed dose to water using ionization chambers calibrated in terms of absorbed dose to water but irradiated in solid phantoms. Such a framework is useful when there is a desire to put dose measurements using solid phantoms on an absolute basis. Putting solid phantom measurements on an absolute basis has distinct advantages in verification measurements and quality assurance. We introduce a phantom dose conversion factor that converts a measurement made in a solid phantom and analyzed using an absorbed dose calibration protocol into absorbed dose to water under reference conditions. We provide techniques to measure and calculate the dose transfer from solid phantom to water. For an Exradin A12 ionization chamber, we measured and calculated the phantom dose conversion factor for six Solid Water phantoms and for a single Lucite phantom for photon energies between 60Co and 18 MV photons. For Solid Water of certified grade, the difference between measured and calculated factors varied between 0.0% and 0.7% with the average dose conversion factor being low by 0.4% compared with the calculation whereas for Lucite, the agreement was within 0.2% for the one phantom examined. The composition of commercial plastic phantoms and their homogeneity may not always be reproducible and consistent with assumed composition. By comparing measured and calculated phantom conversion factors, our work provides methods to verify the consistency of a given plastic for the purpose of clinical reference dosimetry.
authors.affiliation: Department of Medical Physics, McGill University Health Centre, 1650 Cedar av., Montreal H3G 1A4, Canada. jseuntjens@medphys.mcgill.ca
authors.forename: Jan
authors.initials: J
authors.lastname: Seuntjens
authors.forename: Marina
authors.initials: M
authors.lastname: Olivares
authors.forename: Michael
authors.initials: M
authors.lastname: Evans
authors.forename: Ervin
authors.initials: E
authors.lastname: Podgorsak
mesh_terms: D000465:Algorithms; D002138:Calibration; D003037:Cobalt Radioisotopes; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D017785:Photons; D011829:Radiation Dosage; D012015:Reference Standards; D014867:Water
chemical_list: D003037:Cobalt Radioisotopes; D014867:Water