9 Reporting in Brachytherapy: Dose and Volume Specification

166 Reporting in Brachytherapy

4.1.2 Units The quantity reference air kerma rate is expressed in Gy s -1 at one meter, or a multiple of this unit (in a convenient way, e.g. for low dose-rate brachytherapy, in microgray per hour, µ Gy h -1 , at 1 m). 4.1.3 Energy cut-off, δ The energy cut-off is intended to exclude low-energy or contaminant photons (e.g., characteristic x rays originating in the outer layers of steel or titanium source cladding) that can significantly increase RAKR without contributing significantly to absorbed dose at distances greater than about 1 mm in tissue. 4.1.4 Air kerma rate constant Γ δ The relation between RAKR of a given source and other quantities used to specify the radioactive sources in brachytherapy is based on the physical quantity „air kerma rate constant“,  δ (14,45,46). is the kerma rate, for a point source, at a reference distance of one meter, per unit activity, due to photons of energy greater than δ, in the „in vacuo“ conditions defined above. For the gamma energies emitted by the radionuclides used in brachytherapy, one may consider that the numerical values for dose and kerma are equal. Γ δ is expressed in Gy per second at one meter, or multiples. Some numerical values are given in Table 2.2 in Chapter 2 / p. 26 [ICRU, Report 58 (45)]. 4.2 Total Reference Air Kerma (TRAK) 4.2.1 Definition The Total Reference Air Kerma (TRAK) is the sum of the products of the Reference Air Kerma Rate and the irradiation time for each source. 4.2.2 Practical application of the TRAK The TRAK is an important quantity which should be reported for all brachytherapy applications, for the following reasons : (1) It is an unambiguous quantity that is simple to calculate (on condition that the strengths of the sources are expressed in RAKR (4.1). (2) The conversion of the quantity mg.h to the TRAK is easy and straightforward : 1 mg.h radium equivalent (0.5 mm Pt filtration) corresponds to 7.2 µ Gy at 1 m. For a gamma emitting radionuclides, Γ δ