Angular dependence ofH_p(3): horizontal vs vertical rotation

This study investigates how much the conventional quantity value (true value) of the operational quantityH_p(3), defined in International Commission on Radiation Units and Measurements (ICRU) 47 and 51, 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 coefficientsh_pK(3) andh_pD(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 (and maximum beta energies from 1.4 MeV to 3.3 MeV). From the results, the ratio of the values for horizontal and vertical rotations were determined. To complement these, the corresponding ratios for the protection quantity equivalent dose to the lens of the eye,H_lens, defined in International Commission on Radiological Protection 116, were calculated and compared with the results forH_p(3). For photon fields most relevant in lens monitoring, i.e. especially in interventional radiology, it turned out thatH_p(3) is only slightly (up to ∼10%) smaller when rotating around a horizontal instead of the commonly used vertical axis. Larger differences only occur for small photon energies (below ∼20 keV) and large angles of radiation incidence (75°) whereH_p(3) is reduced to practically zero for a horizontal rotation compared to the normal (vertical) rotation. For reference beta-particle fields, the differences between vertical and horizontal rotations are also mostly below 10% - due to the rather broad angular distribution of the beta fields resulting in only small differences of the 'effective' path length 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 forH_p(3). From the author's point of view, these findings do not impose the necessity to update theH_p(3) by distinguishing between the (usual) vertical and a horizontal rotation axis. Nevertheless, the data presented may serve ICRU to judge whether an updated definition of the operational quantityH_p(3) should be considered. If so, International Standardization Organization needs to provideH_p(3) values for horizontal rotations of the cylinder phantom for reference radiation fields and International Electrotechnical Commission needs to extend their dosemeter type-test procedures to horizontal rotations.