2017 AAPM Annual Meeting
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Session Title: Developments in Clinical Reference Dosimetry
Question 1: There are several reasons for updating the TG-51 protocol. Which of the following is NOT one of them?
Reference:McEwen et al., “Addendum to the AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon beams,” Med. Phys. 41:041501, 2014.
Choice A:There have been advances in reference dosimetry since publication of TG-51.
Choice B:The update prescribes a new, improved formalism for beam calibration.
Choice C:Accurate Monte Carlo based kQ factors are now available.
Choice D:TG-51 deliberately avoided uncertainties.
Question 2: One of the main reasons for recommending the use of cylindrical chambers for reference dosimetry of electron beams, even with energy less than 6 MeV, is:
Reference:Muir and Rogers, “Monte Carlo calculations of electron beam quality conversion factors for several ion chamber types,” Med. Phys. 41:111701, 2014.
Choice A:Cylindrical chambers are not as stable as plane-parallel chambers.
Choice B:Perturbation corrections for cylindrical chambers are variable for similar chambers.
Choice C:Beam quality conversion factors, kQ, are now well known from Monte Carlo calculations and include Pgr, simplifying the calibration procedure.
Choice D:Clinical medical physicists are not comfortable using cylindrical chambers for electron beam reference dosimetry.
Question 3: What will be the change in the TG-21 determination of absorbed dose to water following the recommendations of ICRU 90?
Reference:The International Commission on Radiation Units and Measurements, “Key data for ionizing radiation dosimetry: measurement standards and applications,” Journal of the ICRU, volume 14, no. 1, ICRU report 90 (2016).
Choice A:No change.
Choice B:Less than 0.5%.
Choice C:Approximately 0.7%.
Choice D:More than 1%.
Choice E:TG-21 (Air kerma-based) dose determination is no longer recommended.
Question 4: Why is the dose to tissue different than the dose to water?
Reference:The physics of Radiology. Johns and Cunnigham. Charles C Tomas publishing, Springfield Il. 1983.
Choice A:Because they have different densities.
Choice B:Because they have different chemical compositions.
Choice C:Because they exist in different states.
Choice D:Because tissue is at a higher temperature than typical water tank water.
Question 5: A 1% correction between dose to water and dose to tissue:
Reference:IROC Houston, (2010, September 20) Water or tissue – revisited. Accessible at http://rpc.mdanderson.org/RPC/newsletter/September%202010%20water_4C1EF9.pdf
Choice A:Has been consistently applied by the medical physics community for photon beams.
Choice B:Has been consistently applied by the medical physics community for electron beams.
Choice C:Has been inconsistently applied by the medical physics community.
Choice D:Is never necessary.
Choice E:Is not necessary for electron beams.
Question 6: Collapsed Cone Convolution photon algorithm as described by Ahnesjo and Aspardakis1 is expected to approximate dose to tissue because:
Reference:Ahnesjo A, Aspradakis MM. Dose calculations for external photon beams in radiotherapy. Phys Med Biol. 1999;44(11):R99-155. 2. Wieslander E, Knöös T. Dose perturbation in the presence of metallic implants: treatment planning system versus Monte Carlo simulations. Phys Med Biol. 2003;48(20):3295.
Choice A:The energy deposition kernel used for C/S is assigned the tissue material and density.
Choice B:The mass energy absorption coefficient for TERMA calculation is based on tissue radiological properties.
Choice C:The superposition/convolution equation models the dose distribution in 3 dimensions by taking the local density into account for lateral dose spread calculations.
Choice D:The TERMA decrease with depth is accounted for by using CT# to electron density conversion.
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