Prostate brachytherapy treatment using permanent implantation of low-energy (LE) low-dose rate (LDR) sources is successfully and widely applied in Europe. In addition, seeds are used in other tumour sites, such as ophthalmic tumours, implanted temporarily. The calibration issues for LE-LDR photon emitting sources are specific and different from other sources used in brachytherapy. In this report, the BRAPHYQS (BRAchytherapy PHYsics Quality assurance System) working group of GEC-ESTRO, has developed the present recommendations to assure harmonized and high-quality seed calibration in European clinics. There are practical aspects for which a clarification/procedure is needed, including aspects not specifically accounted for in currently existing AAPM and ESTRO societal recommendations. The aim of this report has been to provide a European wide standard in LE-LDR source calibration at end-user level, in order to keep brachytherapy treatments with high safety and quality levels. The recommendations herein reflect the guidance to the ESTRO brachytherapy users and describe the procedures in a clinic or hospital to ensure the correct calibration of LE-LDR seeds.

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Dose measurements at a few cm distance from brachytherapy sources are difficult due to the complexity of positioning the source and detector with the required precision of around 0.1 mm, and due to the uncertainties in detector response caused by energy-dependence, unclear effective point of measurement and volume averaging of detector signal across the extended sensitive volume. Verification of new calculation algorithms in brachytherapy incorporating tissue inhomogeneities, the calibration and relative dosimetry of kV-irradiators used at short source-tissue-distances in intraoperative radiotherapy and also clinical situations when applying these techniques require verification by measurement for which the present codes of practice for dosimetry in radiotherapy are not applicable. A formalism under preparation by the DIN 6803-3 working group describes the determination of absorbed dose to water from measurement by multiplication of calibration and correction factors defined in analogy to the well-known codes of practice:, where is the factor correcting for the radiation quality at the point of measurement and corrects for the volume averaging effect. Values of are given in the work by Chofor et al. (Z.Med.Phys. 26 (2015) 238). Otherwise, small solid detectors can be cross-calibrated to a small ionization chamber at a point in water at around 3-5 cm distance from the source with the chamber's effective point of measurement and determined as proposed by Schönfeld et al. (to be published). DIN 6803-3 will describe these approaches to dose measurement in the source vicinity with acceptable uncertainty, preferentially for detectors with small volume averaging.