The use of small fields in radiotherapy techniques has increased substantially, in particular in stereotactic treatments and large uniform or nonuniform fields that are composed of small fields such as for intensity modulated radiation therapy (IMRT). This has been facilitated by the increased availability of standard and add-on multileaf collimators and a variety of new treatment units. For these fields, dosimetric errors have become considerably larger than in conventional beams mostly due to two reasons; (i) the reference conditions recommended by conventional Codes of Practice (CoPs) cannot be established in some machines and (ii) the measurement of absorbed dose to water in composite fields is not standardized. In order to develop standardized recommendations for dosimetry procedures and detectors, an international working group on reference dosimetry of small and nonstandard fields has been established by the International Atomic Energy Agency (IAEA) in cooperation with the American Association of Physicists in Medicine (AAPM) Therapy Physics Committee. This paper outlines a new formalism for the dosimetry of small and composite fields with the intention to extend recommendations given in conventional CoPs for clinical reference dosimetry based on absorbed dose to water. This formalism introduces the concept of two new intermediate calibration fields: (i) a static machine-specific reference field for those modalities that cannot establish conventional reference conditions and (ii) a plan-class specific reference field closer to the patient-specific clinical fields thereby facilitating standardization of composite field dosimetry. Prior to progressing with developing a CoP or other form of recommendation, the members of this IAEA working group welcome comments from the international medical physics community on the formalism presented here.

Purpose A joint IAEA/AAPM international working group has developed a Code of Practice (CoP) for the dosimetry of small static fields used in external megavoltage photon beam radiotherapy, published by the IAEA as TRS‐483. This summary paper introduces and outlines the main aspects of the CoP. Methods IAEA TRS‐483 is a condensation of the wide range of different approaches that have been described in the literature for the reference dosimetry of radiotherapy machines with nominal accelerating potential up to 10 MV that cannot establish the conventional 10 cm × 10 cm reference field, and for the determination of field output factors for relative dosimetry in small static photon fields. The formalism used is based on that developed by Alfonso et al. [Med Phys. 2008;35:5179–5186] for this modality. Results Three introductory sections describe the rationale and context of the CoP, the clinical use of small photon fields, and the physics of small‐field dosimetry. In the fourth section, definitions of terms that are specific to the CoP (as compared to previous CoPs for broad‐beam reference dosimetry, such as IAEA TRS‐398 and AAPM TG‐51) are given; this section includes a list of the symbols and equivalences between IAEA and AAPM nomenclature to facilitate the practical implementation of the CoP by end users of IAEA TRS‐398 and AAPM TG‐51. The fifth section summarizes the equations and procedures that are recommended in the CoP and the sixth section provides an overview of the methods used to derive the data provided in IAEA TRS‐483. Conclusions This is the first time an international Code of Practice for the dosimetry of small photon fields based on comprehensive data and correction factors has been published. This joint IAEA/AAPM CoP will ensure consistent reference dosimetry traceable to the international System of Units and enable common and internationally harmonized procedures to be followed by radiotherapy centers worldwide for the dosimetry of small static megavoltage photon fields.

primer for health physicists Hcollh Phyr. 4 8 717-33 Udale M 1988 A Monte Carlo investigation of surface doses for broad electron beams Phya. Mcd. Bial. 33 939-54 Waihel E and Grosswendt B 1983 Spatial energy dissipation profiles, W values, backscatter coefficients, and ranges for low-energy electrons in methane Nucl. Instrum. Methods A 2 1 1 487-98 Webb S 1979 The ahsorbed dose in the vicinity of an interface between two media irradiated by a 6oCo source E d. J. Radio(. 5 2 962-7 Webb S and Fox R A 1979 The dose in water s u r r o n n d h ooint isotrooic ~amma-rav emitters B r i f .-.. I J. Rodiol. 52 482-4-1980 Verification bv Monte Carlo methods of a ~o w e r law tissue-air ratio alaorithm for inhom-geneily corrections in photon beam dose calculation P h y s M i d. Bid. 25 225-40

The present work provides a consistent and specific set of data for the output correction factors of a broad set of detectors in a Varian Clinac iX 6 MV accelerator and contributes to improving the understanding of the physics of small photon beams. The correction factors cannot in general be neglected for any detector and, as expected, their magnitude increases with decreasing field size. Due to the reduced number of clinical accelerator types currently available, it is suggested that detector output correction factors be given specifically for linac models and field sizes, rather than for a beam quality specifier that necessarily varies with the accelerator type and field size due to the different electron spot dimensions and photon collimation systems used by each accelerator model.