5.208.
Paragraph 3.167(a) of GSR Part 3 [3] establishes the requirements for the calibration of sources giving rise to medical exposure. For radiation therapy, all external beam medical radiological equipment and brachytherapy sources used in the radiation therapy facility should be calibrated, as follows:
Medical radiological equipment for external beam radiotherapy should be calibrated in terms of radiation quality or energy and either absorbed dose or absorbed dose rate at a predefined distance under specified conditions; the recommended quantity is absorbed dose to water [316, 336]. The calibrations should be performed for at least the clinically used energies and qualities.
Sealed sources used for brachytherapy should be calibrated in terms of reference air kerma rate in air or an equivalent quantity as recommended by the ICRU, at a specified distance, for a specified date [316].
Internationally or nationally accepted calibration protocols should be used. Examples of such protocols include Refs [324, 336–342].
For brachytherapy, a distinction can be made between removable and permanent implants. For removable implants, each source should be calibrated individually. For permanent implants when a large number of sources are being used, a representative sample may be assessed, for example 10% of the sources [339–342].
Particular attention should be given to the calibration of sources used for special radiation therapy procedures (e.g. radiosurgery, IORT, SRT, tomotherapy and total body irradiation) which may necessitate adaptation of the existing international codes of practice and may introduce additional uncertainties associated with making measurements in non-reference conditions. A particular consideration is small field dosimetry, and guidance is given in Refs [343, 344].
Imaging devices used in the radiation therapy process, such as conventional simulators, CT scanners, CBCT, fluoroscopy, radiography and hybrid imaging systems (PET–CT and SPECT–CT) should be calibrated following the relevant recommendations in paras 3.201–3.205 and 4.197–4.202. Guidance for MV imaging devices is given in Refs [345, 346].
5.209.
Paragraphs 3.154(d) and 3.167 of GSR Part 3 [3] require that the responsibility for calibration in radiation therapy be placed on the medical physicist, with either direct fulfilment or by supervision. Correct calibration in radiation therapy is fundamental and, with increasing complexity in technology and software, the direct presence and involvement of the medical physicist is essential. For the imaging devices used in the radiation therapy process, a medical physicist with competence in diagnostic radiology and image guided interventional procedures, or in nuclear medicine, should be involved as appropriate (see also paras 5.219–5.227 on patient dosimetry and 5.228–5.247 on the programme of quality assurance for medical exposures).
5.210.
Paragraph 3.167(b) of GSR Part 3 [3] specifies when such calibrations are required to be carried out. In addition to the initial calibration prior to clinical use and calibration after major maintenance or upgrade, periodic calibrations are required to be carried out. The intervals for these calibrations may differ, depending on the type of source and unit. For example, linacs should be calibrated at least yearly. These intervals will be specified by the regulatory body in each State, under advice from the professional bodies when appropriate. Constancy tests are addressed in paras 5.228–5.247.
5.211.
Paragraph 3.167(c) of GSR Part 3 [3] requires independent verification of the calibration of radiation therapy equipment prior to clinical use, because miscalibration of a radiation therapy source can result in inappropriate treatment involving many patients and can lead to very serious consequences. Independent verification ideally means verification by a different, independent medical physicist using different dosimetry equipment. However, other options, such as verification by a second medical physicist or only verification using a second set of equipment, or use of a remote dosimetry audit (e.g. IAEA/WHO thermoluminescent dosimetry postal dose audit service), could be acceptable. In checking for compliance, the regulatory body should be aware of the limitations of local resources, but nevertheless some form of independent verification should take place.
5.212.
The licensee of the radiation therapy facility should ensure that independent verification of the calibration of all radiation therapy equipment is performed through participation in a national, regional or international programme. A period of two years is recommended for the intervals between independent verifications of calibration. One of the simplest mechanisms for independent verifications of external beam calibration or physical dosimetry is participation in an IAEA/WHO thermoluminescent dosimetry postal dose quality audit. The regulatory body should encourage licensees to participate in this or similar programmes.
5.213.
Sealed sources used for external beam radiotherapy and brachytherapy will also have a calibration certificate provided by the manufacturer, in accordance with Ref. [307] or its national equivalent standards. While important, this does not replace the calibrations required by para. 3.167 of GSR Part 3 [3] and described in paras 5.208–5.212.
5.214.
New brachytherapy sources should be calibrated and differences of more than 5% from the manufacturer’s certified reference air kerma rate should be investigated. The source should not be used for patient treatment until such differences have been investigated and resolved. Further guidance on resolving differences in calibrations is given in Ref. [341].