9 Reporting in Brachytherapy: Dose and Volume Specification

Reporting in Brachytherapy 173

6.2.3 Central plane To give the necessary information about the dose or dose rate distribution, isodose curves must be calculated in at least one chosen plane. If only one plane is used for isodose calculation, the “ central plane of the implant “ should be chosen. In order to assess the dose distribution in other areas of the implant, multiple planes for isodose calculation can be chosen, either parallel or perpendicular to the central plane. In source patterns in which the source lines are straight, parallel, of equal length and with their centres lying in a plane perpendicular to the direction of the source lines, this plane is defined as the central plane (see Fig 6.4A and B) (45). In an actual implant, all source lines may not necessarily be straight, parallel, and of equal length. In such cases, the central plane should be chosen perpendicular to the main direction of the source lines and passing through the estimated centre of the implant (see Fig 6.4.C). For more complex implants, it may be necessary to subdivide the target volume into two or more subvolumes for dose evaluation. In this event, a central plane may be defined for each of these subvolumes (Fig 6.7). The calculation of dose distributions in multiple planes throughout the target volume shows that a variation of a few millimetres in the position of the central plane is not critical.

Fig 6.7 : Central planes in a complex implant. It is sometimes necessary to plan the treatment in terms of two or more sub- volumes. In the example shown, where all source lines are not of equal length, two central planes are identified: (a) for the long source lines and (b) for the shortest ones. Two Mean Central Doses are determined in the two sub-volumes D ma and D mb respectively. Open circles are the intersections of the sources with the central planes, and closed circles are the points where the local minimum doses are calculated. (from ICRU Report 58, [45]).