What is the active length to be used for a target volume of 4.5 cm treated with an interstitial implant of iridium wire?

In the context of an interstitial implant using iridium wire, the active length refers to the length of the radioactive material that will effectively contribute to the dose delivered to the target volume. When treating a target volume of 4.5 cm, it is important to consider not just the dimensions of that target but also the geometry of the implant and the distribution of the radiation emitted by the sources.

For brachytherapy with iridium wire, the radiation dose typically has a rapid fall-off, which means that the edges of the target volume may not receive adequate coverage if only the exact dimensions of the target are taken into account. To adequately cover the target volume and ensure that the prescribed dose is delivered uniformly to the entirety of the target, a larger active length than the target size is often necessary.

In clinical practice, it is common to add a margin to the target dimension to account for factors such as the geometry of the implant and the fall-off of the radiation dose. This leads to the requirement of a 6.4 cm active length, which allows precise placement of the iridium wire and sufficient overlap in dose distribution to ensure that the entire 4.5 cm target volume is adequately treated.

By utilizing a length of 6.4 cm