9 Reporting in Brachytherapy: Dose and Volume Specification

Reporting in Brachytherapy 167

The TRAK, corresponds in terms of the “modern“ SI units, to the „historical“ quantity mg.h. It implies that the extensive and long standing clinical experience of the use of mg.h can be exploited for today’s protocols and studies. (3) The doses to all organs, and thus the integral dose to the patient, are directly proportional to the TRAK. (4) In addition, the use of TRAK provides, as a first approximation, an indication of the absorbed doses delivered during treatment at distances from the sources down to 20 - 10 cm (i.e., in the pelvis or abdomen). The dose at 10 cm from the centre of the sources is roughly 100 times higher than the TRAK. It is indeed easy to verify that when the distance of a point P from the centre of the volume (C) occupied by the sources is larger than 2.5 times the largest dimension of that volume, the dose rate obtained at P from the actual distribution of the sources differs by less that 4% from that obtained by assuming that all the sources are located at C (20). However, the TRAK does not allow one to derive, even approximately, the absorbed dose in the immediate vicinity of the sources (i.e., in the tumour or target volume). (5) The TRAK, or the sum of the RAKR of all sources, can serve as a useful index for radiation protection of the personnel and nursing staff in charge of the patient (kerma -or dose- rate at 1 meter from the patient, neglecting, as a first approximation, the attenuation and scattering phenomena). 4.3 Additional specification of photon sources used for intraluminal applications In addition to the RAKR and TRAK, for photon sources used in intraluminal brachytherapy, the following recommendation is made (48). For intraluminal brachytherapy applications, it is also recommended to report the dose rate (and dose) at 10 mm from the source axis at the centre of the source (section 7, p. 181). This recommendation is partly justified by the fact that several authors have reported their data using the above source specification. 4.4 Specification of beta-ray sources used for endovascular brachytherapy The following recommendation is made for beta-ray sources used in intravascular brachytherapy (48). The intensity of the beta-ray emitting sources should be specified in terms of the Reference Absorbed Dose Rate at a distance of 2 mm from the source centre (axis). NB : As can be seen from the recommendations above, all sources for brachytherapy applications are specified in terms of their “output” (dose rate) at reference distances and/or in different conditions. The quantity “activity” is used only for regulatory and protection purposes.