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Beyond the Alfonso Formalism for Small Field Dosimetry: A New Metric, New Detectors, Or Both?

T Underwood

T.S.A. Underwood1*, H.C. Winter2, M.A. Hill1, J.D. Fenwick3, (1) Gray Institute for Radiation Oncology and Biology, University of Oxford, (2) Oxford University Hospitals NHS Trust, (3) Department of Oncology, University of Oxford

TU-C-108-3 Tuesday 10:30AM - 12:30PM Room: 108

The Alfonso et al formalism recommends the use of correction factors to report point-doses under small field conditions. This work considers whether the requirement for small field point-dose correction factors may be eliminated via (i) an extension of the Alfonso formalism based upon a new integral dose metric and/or (ii) novel mass-density-compensation-based detector design strategies.

We have developed a mathematical definition of dose area product (DAP) for small detectors used to integrate dose inside and outside the boundaries of a radiation field. Here we test the resulting theorem of 'inter-detector DAP invariance' directly using Monte Carlo (MC) simulations and indirectly using linac experiments. Additionally, MC simulations are used to explore the possibility of mass-density-based compensation for a series of hypothetical, spherical detectors (of diameter 0.1 cm and 0.2 cm). Cavities of greater than unit mass-density are modified using shells of less than unit mass-density and vice-versa. A similar MC-based methodology is used to investigate the potential of modifying the designs of two real radiotherapy detectors: PTW PinPoint 31006 and PTW Diamond 60003.

The results demonstrate that the application of conventional (detector-specific) large-field calibration factors leads to accurate measurement of the dose area product (integral dose) when small dosimeters are used to integrate dose over enough positions. The simulations of modified detectors also indicate that, if a detector cavity is small enough, it should also be possible to use mass-density-compensation to manipulate the small to large field detector response ratio so that it matches that obtained for a "point-like" water-structure (under a variety of irradiation conditions).

This work presents two novel methods by which the requirement for small-field point-dose correction factors may be reduced or eliminated: (i) an integral-dose-based reformulation of the Alfonso formulation and (ii) a detector re-design strategy based upon mass-density compensation.

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