Angular dependence of H p (3): horizontal vs. vertical rotation

This study investigates how much the conventional quantity value (true value) of the operational quantity H p (3) changes for oblique radiation incidence when a rotation around the horizontal instead of the commonly used vertical axis of the cylinder phantom is performed. For this, conversion coefficients h p K (3) and h p D (3), for photons and betas, respectively, were calculated for both vertical (usual) and horizontal rotations of the cylinder phantom for mono-energetic photons from 2 keV to 10 MeV and for reference beta-particle fields with mean energies from 0.54 MeV to 1.2 MeV. From the results, the ratio of the values for horizontal and vertical rotations were determined. To complement these, the corresponding ratios for the equivalent dose to the lens of the eye, H lens , were calculated and compared with the results for H p (3).For mono-energetic photons, it turned out that H p (3) is significantly smaller when a rotation around the horizontal instead of the commonly used vertical axis is performed -as photon fields are assumed to propagate (nearly) parallel and, therefore, need to penetrate a significantly longer pathlength in the cylinder phantom when rotated around the horizontal axis; the effect is largest for small photon energies and large angles of radiation incidence where H p (3) is reduced to practically zero for the horizontal rotation compared to the normal (vertical) rotation. For reference beta-particle fields, however, the differences between vertical and horizontal rotations are mostly below 10 % -due to the rather broad angular distribution of beta fields resulting in only small differences of the "effective" pathlength in the cylinder phantom. In contrast, the differences of the dose to the lens of the eye, H lens , are rather large for both photons and betas and always significantly larger than for H p (3).These findings may serve ICRU (International Commission on Radiation Units and Measurements) to judge whether an updated definition of the operational quantity H p (3) shall be considered. If so, ISO (International Standardization Organization) needs to provide H p (3) values for horizontal rotations for reference radiation fields and IEC (International Electrotechnical Commission) needs to extend their dosemeter type-test procedures to horizontal rotations.