9 Reporting in Brachytherapy: Dose and Volume Specification

Reporting in Brachytherapy 171

The Paris System of implant planning was developed mainly with iridium-192 wire sources (20,80,81,82). The sources are of equal linear activity, parallel, placed at equal distances, and arranged in such a way that their centres are in the same plane perpendicular to the direction of the lines (Fig 3A and B). This plane, called the ” central plane “ is the midplane of the application (Fig 6.4 A, B and C).

Fig 6.5A. and B : Dose planning for implants with iridium-192 wires contained in two parallel planes, following the Paris system. Examples of a breast implant in two planes. (A) The 7 wires are equidistant and arranged in triangles (length of the wires 7 cm for the upper row and 8 cm for the lower row), linear activity 52 MBq cm -1 , application time 43.32 h for a reference dose of 20 Gy. (B) The 6 wires are equidistant and arranged in squares (length of the wires 6 cm for the upper row and 7cm for the lower row), linear activity 52 MBq cm -1 , application time 42.91h for a reference dose of 20 Gy. (From Wambersie and Battermann [115]) If the volume to be treated is large, more than one plane containing wires is used. Again, equidistance of the radioactive lines is required. This means that their intersections with the central plane are arranged according to the apices of equilateral triangles or squares (Fig 6.4B and 6.5 A and B). This regular distribution of the wires results in a slight overdosage at the centre of the target volume. The dose rate at a point in the middle of a group of sources is called the basal dose rate (BD). This BD is always calculated from the position of the sources in the central plane and is the minimum dose rate between a pair or group of sources. The value of the isodose curves are expressed as a percentage of the BD. The reference dose rate is derived from the BD and is equal to 85% of the BD. It is used for calculating the total treatment time of the implant. Because the ends of the active wires are not crossed, as in the Manchester System, the active sources should be 20 - 30% longer than the target volume at both ends. The minimum thickness of a treated volume is 50 - 60% of the source separation for single planes and 120 - 150% for 2 planes.