Measurement of the tissue to A-150 tissue equivalent plastic kerma ratio at two p(66)Be neutron therapy facilities

Abstract

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%.

AAPM TG158: Measurement and calculation of doses outside the treated volume from external‐beam radiation therapy

“…TEPC can also be used for determination of absorbed dose as well as calculation of radiation quality factors, kerma factors, and equivalent doses. The wall of this type of detector can be made of various materials, for example, for the determination of kerma factors . TEPC detectors are usually calibrated with an internal alpha particle source, 244 Cm, where the energy deposited in the cavity, expressed in keV/μm, can be obtained from the stopping power of the alphas and the counter diameter .…”

“…The wall of this type of detector can be made of various materials, for example, for the determination of kerma factors. [243][244][245] TEPC detectors are usually calibrated with an internal alpha particle source, 244 Cm, where the energy deposited in the cavity, expressed in keV/ lm, can be obtained from the stopping power of the alphas and the counter diameter. 242,[246][247][248] This calibration is generally the principal source of uncertainty in this type of measurement, suffering from a poor resolution of the calibration peak as well as uncertainties associated with the stopping power value and mean chord length.…”