Comparison of the ionometric and the calorimetric determination of the absorbed dose to water at Co-60 gamma radiation

The basis of dosimetry of high-energy photon and electron radiation in radiotherapy is the absorbed dose to water, D_W, at Co-60 gamma radiation. PTB’s primary standard for the realisation of the unit of D_W at Co-60 is a water calorimeter [1]. The relative standard measurement uncertainty of D_W is 0.20 %. An alternative method is to determine D_W by means of waterproof, air-filled graphite cavity ionisation chambers [2], for which literature indicates a relative standard measurement uncertainty of 0.3 % [3]. In order to calculate D_W from the measured ionisation chamber charge, two steps are, in principle, necessary:

1. Calculation of the absorbed dose to air in the cavity of the chamber, D_CAV, by means of the W value, i.e. the mean energy for the generation of an ion pair in air.
2. Calculation of D_W from D_CAV using the Bragg-Gray cavity theory which is mainly based on the calculated ratio of the mean electron stopping power of graphite to air.

At present, both the correctness of the W value for air [4] and the correctness of the stopping power of electrons in graphite are under discussion [5, 6]. The aim of this work was to obtain further information on the quantities mentioned above as being "under discussion" by comparing the results for D_W, obtained by means of ionometry and calorimetry, with lowest possible uncertainties.

Two cavity ionisation chambers, which are normally used as PTB primary standards for the realisation of the unit of air kerma, served as measuring chambers. For these, a waterproof coating of Plexiglas was made. The absorbed dose rate to water for Co-60 gamma radiation  was thus determined by means of the relation  , where  is the absorbed dose rate to air in the cavity of the cavity ionisation chamber, I_CAV is the ionisation current of the cavity ionisation chamber measured at the reference point in the water phantom, m_CAV is the mass of the dry air contained in the cavity of the cavity ionisation chamber, W is the mean energy for generating one ion pair in air, e is the elementary charge, and  is a factor to convert  into which was calculated by means of Monte Carlo (MC) methods. f was calculated - by means of the EGSnrc MC code system - as the ratio of the absorbed dose to water D_W at the reference point in the reference depth of a water phantom to the deposited energy D_CAV in the cavity of the cavity ionisation chamber at the same place in the water phantom. According to the cavity theory, f also includes the ratio of the mean electron stopping powers of graphite to air. The results were compared with the values for   which were measured in the same Co-60 radiation field by means of PTB’s water calorimeter [1]. The dose rates determined by means of the ionisation chambers were significantly higher - by (1.4±0.4%) - than those determined by means of the water calorimeter.

The only explanation for this discrepancy is probably that a wrong W value or wrong values of the electron stopping power in graphite have been assumed. Assuming that the W value indicated in literature is correct [4], it was possible to resolve the discrepancy when in the Monte Carlo programme used to calculate the conversion factor f, the values of the electron stopping power in graphite were changed. Other authors having performed complementary experiments have reported similar results [5, 6]. The values for the stopping power of electrons in graphite are of fundamental significance for standard dosimetry since they are directly used for the realisation of the unit of air kerma for gamma radiation, the realisation of the unit of absorbed dose to water at Co-60 gamma radiation by means of graphite cavity ionisation chambers, or for the realisation of the unit of absorbed dose to water for high-energy electron radiation by means of graphite calorimeters.

Literature

1. Krauss, A.:
   The PTB water calorimeter for the absolute determination of absorbed dose to water in ⁶⁰Co radiation,
   Metrologia 43 (2006) 259 - 271
2. Boutillon, M.; Perroche, A.-M.:
   Ionometric determination of absorbed dose to water for cobalt-60 gamma rays,
   Phys. Med. Biol. 38 (1993) 439 - 454
3. Allisy-Roberts, P. J.; Burns, D.T.; Kessler, C.:
   Measuring conditions used for the calibration of ionization chambers at the BIPM,
   Report BIPM-07/06 (2007)
4. Wang, L. L. W.; Rogers, D. W.:
   The replacement correction factor for the BIPM flat cavity ion chamber and the value of W/e,
   Med. Phys. 35 (2008) 4410 - 4416
5. Bichsel, H.; Hiraoka, T.:
   Energy loss of 70 MeV protons in elements,
   Nucl. Instr. Meth. B66 (1992) 345-351
6. Büermann, L.; Hilgers, G.:
   Significant discrepancies in air kerma rates measured with free-air and cavity ionization chambers,
   Nucl. Instr. Meth. A580 (2007) 477 - 480