Measurement of the tissue to A-150 tissue equivalent plastic kerma ratio at two p(66)Be neutron therapy facilities.
The ICRU tissue to A-150 tissue equivalent plastic kerma ratio is needed for neutron therapy dosimetry. The current ICRU protocol for neutron dosimetry recommends using a common conversion factor of 0.95 at all high-energy neutron therapy facilities. In an effort to determine facility specific ICRU tissue to A-150 plastic kerma ratios, an experimental approach was pursued. Four low pressure proportional counters that differed in wall materials (i.e. A-150, carbon, zirconium and zirconium-oxide) were used as dosimeters and integral kerma ratios were determined directly in the clinical beam. Measurements were performed at two p(66)Be facilities: iThemba LABS near Cape Town and Fermilab near Chicago. At the iThemba facility the clinical neutron beam is routinely filtered by a flattening and hardening filter combination. The influence of beam filtration on the kerma ratio was evaluated. Using two recent gas-to-wall dose conversion factor (r(m,g) value) evaluations a mean ICRU tissue to A-150 plastic kerma ratio of 0.93 +/- 0.05 was determined for the clinical beam at iThemba LABS. The respective value for the Fermilab beam is 0.95 +/- 0.05. The experimentally determined ICRU tissue to A-150 plastic kerma ratios for the two clinical beams are in agreement with theoretical evaluations. Beam filtration reduces the kerma ratio by 3 +/- 2%.
Langen
K M
KM
iThemba LABS, Somerset West 7129, South Africa. Langen@radonc17.ucsf.edu
Binnss
P J
PJ
Schreuder
A N
AN
Lennox
A J
AJ
Deluca
P M
PM
D055592:Biophysical Phenomena; D001703:Biophysics; D005214:Fast Neutrons / Q000627:therapeutic use*; D006801:Humans; D009369:Neoplasms / Q000532:radiotherapy; D019047:Phantoms, Imaging; D010969:Plastics; D011874:Radiometry / Q000295:instrumentation; D011880:Radiotherapy Planning, Computer-Assisted / Q000706:statistics & numerical data*
D010969:Plastics; C036927:A 150 (plastic)