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

Recently, the TG-51 protocol for clinical reference dosimetry of external beam radiation therapy using photon beams with nominal energies between 60 Co and 50 MV and electron beams with nominal energies between 4 and 50 MeV, was published (AAPM 1999). The protocol was written by Task Group 51 (TG-51) of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The Canadian Organization of Medical Physicists (COMP) has endorsed and recommended the use of TG-51 for external beam reference dosimetry in Canadian institutions 1 . Parallel to TG-51's development, another protocol has been developed by the International Atomic Energy Agency (IAEA) (Andreo et al 2000), and is close to final acceptance. The IAEA Code of Practice (CoP) has a more general scope, includes low-energy x-rays, protons and heavy particles, and is designed for both technologically advanced and developing countries. Since its introduction, TG-51 has come under some criticism. One such criticism (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-21 parameters, especially the stopping power ratios and the new correction for central electrode'. However, the sharpest criticism to date has come from Professor Pedro 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 photon beam quality index, PDD(10) x , arguing instead for the familiar TPR 20,10 . In reply to this, Dr David Rogers, Group Leader of Ionizing Radiation Standards at the National Research Council of Canada, has mounted a vigorous defence (Rogers 2000). This commentary draws attention to the article in this issue by Andreo, the latest, and probably not the last, published exchange in the debate (Andreo 2000a). As interesting and entertaining as these heated debates are, the reader should be encouraged to plunge beneath the surface and unearth the scientific content in Andreo's article, the articles cited therein and the relevant articles by Rogers and co-authors. Such articles cover important scientific issues with far-reaching implications. In drawing any conclusions we must consider what we should expect from reference standard protocols such as TG-51 and the IAEA/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 or should this leadership come from its client community, the clinics where radiotherapy is delivered? • What should a protocol attempt to do: characterize dose as accurately as possible and accept divergence in absolute dose among the applicants of different protocols? Or, is consistency more important, so that a Gray in the UK and the USA, for example, means the same thing? • Is the development of an international `consensus protocol' 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 points of view is a measure of their deep scientific and personal convictions. Ultimately, the measured response of the rest of community will determine the roles that these protocols will play. Further debate and further research is to be encouraged, for they play important roles in establishing the robustness of a given method. If accuracy in dose determination is paramount, then the emergence of different protocols based upon different methods is highly desirable. Alternatively, if consistency is desirable, we should consider the adoption of a global protocol that may be arrived at by consensus, or determined independently. These are issues that can only be addressed through more discussion. References American Association of Physicists in Medicine (AAPM) 1983 TG-21, a protocol for the determination of absorbed dose from high-energy photon and electron beams Med. Phys. 10 721-71 American Association of Physicists in Medicine (AAPM) 1999 TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams Med. Phys. 26 1847-70 Andreo P 2000a A comparison between calculated and experimental k Q photon beam quality correction factors Phys. Med. Biol. 45 L25-L38 Andreo P 2000b On the beam quality specification of high-energy photons for radiotherapy dosimetry Med. Phys. 27 434-40 Andreo P 2000c Reply to `Comment on `On the beam quality specification of high-energy photons for radiotherapy dosimetry'' Med. Phys. 27 1693-5 Andreo P, Burns D T, Hohlfeld K, Huq M S, Kanai T, Laitano F, Smyth V and Vynckier S 2000 Absorbed dose determination in external beam radiotherapy: an International Code of Practice for dosimetry based on standards of absorbed dose to water IAEA Technical Report Series (published in the name of IAEA, WHO, PAHO and ESTRO) (Vienna: IAEA) at press Khan F M 2000 Comment on `AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams' Med. Phys. 27 445-7 Rogers D W O 2000 Comment on `On the beam quality specification of high-energy photons for radiotherapy dosimetry' Med. Phys. 27 434-40 Schulz R J, Almond P R, Kutcher G, Loevinger R, Nath R, Rogers D W O, Suntharalingham N, Wright K A and Khan F 1986 Clarification of the AAPM Task Group 21 protocol Med. Phys. 13 755-9

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