A comparison of absorbed dose standards for high-energy X-rays

Abstract

AbStracL An indirect comparison of the absorbed dose to water standards of lhe FTB and the NRC was carried out for 18/20 MV x-rays using five ionimtion chambers as m s f e r insmmenls. The absorbed dose standard of the Fl% is based on the total absorption of 5.6 MeV electrons in Fncke solution. The NRC standard uses Wcke solution whose calibration is based on measuremenfs made with a water calorimeter Bnd a calculation of the heat defect For high-energy x-rays. the difference found between the standards of 0.4% is within the expected uncertainties. Tbe comparison was linked to existing %o absorbed dose to warer and air kema standards by comparing measurements of those quantities at the PTB, NRC and BIPM. Agreement was better than 0.78 in all cases. For the i o n i d o n chamberr used in Lhi study, the absorbed dose calibration factors for 18/20 MV x-rays are a b u t 2% lower than those for "CO gamma-rays. . IntroductionThe Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig and the National Research Council of Canada (NRC) in Ottawa have developed primary standards of absorbed dose to water for high-energy radiation. Although both groups are carrying out further developments, this comparison indicates the state of the art at the two laboratories for highenergy photon dosimeby. Section I (x-and gamma-radiation; electrons) of the Comit6 Consultatif pour les & I o n s de Mesure des Rayonnements Ionisants (CCEMRI) (BIPM 1988) recommended that primary standards for absorbed dose be compared in collaboration with the Bureau Intemational des Poids et Mesures (BIPM) so that these comparisons could be linked to the existing BIPM 6oCo reference system. Thus measurements of air kerma and absorbed dose to water for %o were made at the BIPM as well as the NRC and the PTB. The results of these measurements are presented here.As can be seen from section 2, the primary standards of absorbed dose to water for high-energy x-rays are more complex than those of air kerma since auxiliary equipment and procedures are involved. Therefore, primary standards are not transportable and comparison of these standards has to be made using transfer instruments. Since ionization chambers are easily transportable, have good precision and long-term stability, and are connected to directly readable measuring assemblies, they were chosen for use as transfer instruments.

Measured and Monte Carlo calculated kQ factors: Accuracy and comparison

“…Measured k Q factors have been determined in previous publications using water calorimetry [3][4][5][6][7][8] or Fricke dosimetry. [9][10][11][12] However, aside from the work of McEwen, 1 experimental determination of k Q has been limited to only a few chamber types. Additionally, many of the measured values suffer from large uncertainty (in most cases >0.5%) or use Fricke chemical dosimetry, which must be corrected for intrinsic energy dependence.…”

Characterization of a Fricke dosimeter at high energy photon and electron beams used in radiotherapy

“…The variation of G (Fe 3? ) with beam quality for the Fricke dosimeter is small for low-LET radiation [4] and has usually been assumed [5] to be negligible.…”

Experimental determination of fluence correction factors at depths beyond for a Farmer type cylindrical ionization chamber in clinical electron beams

“…The new generation of protocols being developed are based on absorbed dose-to-water standards in photon beams from 60 Co and accelerator beams. 1,[4][5][6][7][8] Application of a dosimetry protocol based on absorbed dose to water standards will considerably reduce the uncertainty of the dose delivered to a patient as the number of steps involved and correction factors applied is reduced. In the absorbed dose-to-water based calibration formalisms that are under consideration, 1 one will start with an absorbed dose-to-water calibration factor of the user's ion chamber in a 60 Co beam, i.e., N D,w 60 Co , and then determine the absorbed dose to water at any other beam quality Q by using the equation…”