Code of Practice for the Absorbed Dose Determination in High Energy Photon and Electron Beams

Accurate determination of the dose delivered to the tumour in external beam radiotherapy is of primary importance in clinical dosimetry. Until recently, the determination of absorbed dose under reference conditions has been based on the use of ionisation chambers calibrated in terms of air kerma as recommended in codes of practice, such as published in NCS Reports 2 and 5. 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.

In NCS report 18 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 provides the concepts and methods to determine the absorbed dose in high energy photon and electron beams produced by medical linear accelerators and corresponds to the current clinical practice in Belgium and The Netherlands. 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.

The present CoP replaces the codes published in NCS reports 2 and 5 and recommends the use 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 physical concepts outlined in the Code represent a major simplification compared to the previous NCS 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.

https://doi.org/10.25030/ncs-018

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