Commentary: The controversy between the IAEA Code of Practiceand the TG-51 protocol

Abstract

Recently, the TG-51 protocol for clinical reference dosimetry of external beamradiation therapy using photon beams with nominal energies between 60Coand 50 MV and electron beams with nominal energies between 4 and 50 MeV, waspublished (AAPM 1999). The protocol was written by Task Group 51 (TG-51) of theRadiation Therapy Committee of the American Association of Physicists inMedicine (AAPM) and has been formally approved by the AAPM for clinical use.The Canadian Organization of Medical Physicists (COMP) has endorsed andrecommended the use of TG-51 for external beam reference dosimetry in Canadianinstitutions1. Parallel to TG-51'sdevelopment, another protocol has been developed by the International AtomicEnergy Agency (IAEA) (Andreo et al 2000), and is close tofinal acceptance. TheIAEA Code of Practice (CoP) has a more general scope, includes low-energyx-rays, protons and heavy particles, and is designed for both technologicallyadvanced and developing countries.Since its introduction, TG-51 has come under some criticism. One suchcriticism (Khan 2000) proclaimed that the former AAPM protocol,TG-21 (AAPM 1983, Schulz et al 1986), is adequate since `one could easily update the TG-21parameters, especially the stopping power ratios and the new correction forcentral electrode'. However, the sharpest criticism to date has come fromProfessorPedro Andreo, the current Head of the Dosimetry and Medical Radiation Physics Section of the IAEA (Andreo 2000b,c), who takes issue with TG-51's photonbeam quality index, PDD(10)x, arguing instead for the familiarTPR20,10. In reply to this, Dr David Rogers, Group Leader of IonizingRadiation Standards at the National Research Council of Canada, has mounted avigorous defence (Rogers 2000).This commentary draws attention to the article in this issue by Andreo, thelatest, and probably not the last, published exchange in the debate (Andreo2000a). Asinteresting and entertaining as these heated debates are, the reader should beencouraged to plunge beneath the surface and unearth the scientific content inAndreo's article, the articles cited therein and the relevant articlesby Rogers and co-authors. Such articles cover importantscientific issues with far-reachingimplications. In drawing any conclusions we must consider what we shouldexpect from reference standard protocols such as TG-51 and theIAEA/CoP. For example:• Should we adopt the protocol that has more universal applicability?• Should we adopt the protocol that is more practical to implement?• Should Standards' Laboratories lead the development of protocols orshould this leadership come from its client community, the clinics whereradiotherapy is delivered?• What should a protocol attempt to do: characterize dose as accurately aspossible and accept divergence in absolute dose among the applicants ofdifferent protocols? Or, is consistency more important, so that a Gray in theUK and the USA, for example, means the same thing?• Is the development of an international `consensusprotocol' desirable?If it is, should the IAEA/CoP fulfil this role? Does TG-51 fulfil this role?Or, should one be developed?The enthusiasm with which the protagonists in this debate advocate their pointsof view is a measure of their deep scientific and personal convictions.Ultimately, the measured response of the rest of community will determine theroles that these protocols will play. Further debate and further research is tobe encouraged, for they play important roles in establishing the robustness ofa given method. If accuracy in dose determination is paramount,then the emergence of different protocols based upon different methods ishighly desirable. Alternatively, if consistency is desirable, we shouldconsider the adoption of a global protocol that may be arrived at by consensus,or determined independently. These are issues that can only be addressed throughmore discussion.ReferencesAmerican Association of Physicists in Medicine (AAPM) 1983TG-21, a protocol for the determination of absorbed dose fromhigh-energy photon and electron beamsMed. Phys. 10 721-71American Association of Physicists in Medicine (AAPM) 1999TG-51 protocol for clinical reference dosimetry of high-energyphoton and electron beamsMed. Phys. 26 1847-70 Andreo P 2000aA comparison between calculated and experimental kQ photon beamquality correction factorsPhys. Med. Biol. 45 L25-L38 Andreo P 2000bOn the beam quality specification of high-energy photons forradiotherapy dosimetryMed. Phys. 27 434-40Andreo P 2000cReply to `Comment on `On the beam quality specification ofhigh-energy photons for radiotherapy dosimetry''Med. Phys. 27 1693-5Andreo P, Burns D T, Hohlfeld K, Huq M S, Kanai T, Laitano F, Smyth V andVynckier S 2000Absorbed dose determination in external beam radiotherapy: anInternational Code of Practice for dosimetry based on standards ofabsorbed dose to waterIAEA Technical Report Series (published in the name of IAEA, WHO,PAHO and ESTRO) (Vienna: IAEA) at pressKhan F M 2000Comment on `AAPM's TG-51 protocol for clinical reference dosimetryof high-energy photon and electron beams' Med. Phys. 27 445-7Rogers D W O 2000Comment on `On the beam quality specification of high-energyphotons for radiotherapy dosimetry' Med. Phys. 27 434-40Schulz R J, Almond P R, Kutcher G, Loevinger R, Nath R, Rogers D W O,Suntharalingham N, Wright K A and Khan F 1986Clarification of the AAPM Task Group 21 protocolMed. Phys. 13 755-9