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Absorbed dose to water is the quantity of interest to specify the amount of radiation to be used in radiotherapy and has the advantage that it can be measured more directly than the quantity air kerma. Advances in radiation dosimetry concepts and the development of primary measurement standards based on absorbed dose to water over the last decades offer the possibility to calibrate ionisation chambers directly in terms of absorbed dose to water. Absorbed dose standards have an uncertainty of less than 1% (1 sd) in photon beams up to 25 MV, resulting in improved accuracy in clinical reference dosimetry. Nowadays several international and national Codes of Practice (CoP) for external beam radiotherapy based on absorbed dose standards have been published and adopted in many countries worldwide.In this report a Code of Practice is presented for the dosimetry in high energy photon and electron beams based on absorbed dose to water standards for 60Co reference beams. The CoP has been written by a Subcommittee of the Netherlands Commission on Radiation Dosimetry (NCS) and corresponds to the current clinical practice in Belgium and The Netherlands. The CoP provides the concepts and methods to determine the absorbed dose in high energy photon and electron beams produced by medical linear accelerators. The CoP covers the reference dosimetry in static, open photon beams with nominal energies between 1 and 25 MV and in static, open electron beams with nominal energies between 4 and 25 MeV produced by conventional linear accelerators. Treatment modalities as intensity modulated therapy (IMRT), stereotactic radiotherapy, robotic radiotherapy, helical tomotherapy, etc. are beyond the scope of this report.Important features are the simple basic concepts to determine absorbed dose to water under reference conditions in a water phantom and the introduction of a single, chamber dependent correction factor taking into account all effects dependent of the radiation beam quality. For photon beams these beam quality correction factors are based on experimental data, partly measured in selected clinical accelerator beams in Belgium and The Netherlands using a portable water calorimeter.The present CoP recommends a limited number of ionisation chambers for reference dosimetry in radiotherapy, but the physical concepts outlined in the Code represent a major simplification compared to the previous Codes based on the concept of air kerma employing Bragg-Gray or Spencer-Attix cavity theory. This CoP uses updated information compared to the codes presented in TRS-398 and AAPM TG-51.This Code of Practice replaces the protocols based on the concept of air kerma calibration coefficients described in NCS reports 2 and 5.

NCS Report 18: Code of practice for the absorbed dose determination in high energy photon and electron beams