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BEST IN PHYSICS (THERAPY) - An International Code of Practice for the Dosimetry of Small Static Photon Fields

H Palmans

H Palmans1*, R Alfonso2, P Andreo3, R Capote-Noy4, M Saiful Huq5, J Izewska4, J Johansson6, W Kilby7, TR Mackie8, A Meghzifene4, K Rosser9, J Seuntjens10, W Ullrich11, (1) National Physical Laboratory, Teddington, UK,(2) Instituto Nacional de Oncologia y Radiobiologia, La Habana, Cuba ,(3) University of Stockholm, Sweden,(4) International Atomic Energy Agency, Vienna, Austria,(5) University of Pittsburgh Medical Center, Pittsburgh, PA, (6) Elekta Instrument AB, Stockholm, Sweden,(7) Accuray Inc., Sunnyvale, CA, (8) University of Wisconsin, Madison, WI, (9) Royal Marsden Hospital, Sutton, UK, (10) McGill University, Montreal, QC (11) BrainLAB AG, Feldkirchen, Germany

TH-E-BRB-5 Thursday 1:00:00 PM - 2:50:00 PM Room: Ballroom B

Purpose: The increased use of small photon fields in stereotactic and intensity modulated radiotherapy has raised the need for standardizing the dosimetry of such fields using procedures consistent with those for conventional radiotherapy. An international working group, established by the IAEA in collaboration with AAPM and IPEM, is finalising a Code of Practice for the dosimetry of small static photon fields.

Methods: Procedures for reference dosimetry in non-standard machine specific reference (msr) fields are provided following the formalism of Alfonso et al., Med. Phys. 35 (2008). Recommended correction factors are based on experiment data or Monte Carlo simulated data reported in the literature. They are tabulated for suitable detectors for specific machines such as CyberKnife, GammaKnife and TomoTherapy as well as for generic rectangular fields defined by multi-leaf collimators and circular fields defined by cones.

Results: Reference dosimetry using ionization chambers in machines that cannot establish a conventional 10 cm x 10 cm reference field is based on either a direct calibration in the msr field traceable to primary standards or a calibration in a reference field and a generic correction factor or the product of a correction factor for a virtual reference field and a correction factor for the difference between the msr and virtual fields. For the latter method, procedures are provided for determining the beam quality in non-reference conditions. For the measurement of field output factors in small fields, procedures for connecting large field measurements using ionization chambers to small field measurements using high-resolution detectors such as diodes, diamond, liquid ion chambers, organic scintillators and radiochromic film are given.

Conclusions: A new Code of Practice will be published by the IAEA in the near future. Further research to measure or calculate missing data according to well-defined procedures will be strongly encouraged by the publication of this document.

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