Author: Indra J. Das, Ahtesham Ullah Khan, Andrew J. White 👨🔬
Affiliation: Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin - Madison, Northwestern Memorial Hospital, Northwestern University Feinberg School of Medicine 🌍
Purpose: Small field dosimetry presents challenges due to lateral disequilibrium and the selection of suitable detectors. Volume averaging and density corrections hamper accurate profile measurements. This study examines the directional dependence (axial versus edge-on) of advanced detectors such as the PTW microDiamond and microSilicon, which are recommended for their minimal required corrections and micron-thin disc shapes.
Methods: PTW microDiamond (60019), microSilicon (60023), and Advanced Markus (34045) detectors were examined for orientational dependence, as the detectors have different dimensions in the axial and edge-on direction. An IBA Blue Phantom 2 scanning water tank was used to mount detectors in both orientations to assess dose perturbation and volume averaging effects. Using a Varian TrueBeam, profiles were measured from square fields ranging from 5 mm to 100 mm in both 6 and 10 MV photon beams. Central axis deviations in small fields were corrected to determine field output factors (FOF) in various orientations.
Results: Profile measurements, in terms of full width at half maximum, showed nearly identical results (±0.01 mm) for axial and edge-on orientations, except for the Advanced Markus detector. However, significant discrepancies were found in FOF measurements for the edge-on orientation of microDiamond and microSilicon detectors, likely due to housing-induced perturbations. The overresponse of the detectors in the edge-on orientation was found to be up to 12% for the 5 mm square field.
Conclusion: While microdetectors are suitable for small field dosimetry, their use in the edge-on orientation for FOF measurements is not recommended. Profile measurements showed no observable advantage in utilizing the edge-on orientation. The Advanced Markus detector is not suitable for small field dosimetry in any orientation due to its size.