PTV margins should not be used to compensate for uncertainties in 3D image guided intracavitary brachytherapy

The planning target volume (PTV) concept has been created within the context of external beam radiotherapy (EBRT). It would be ideal to have a similar approach in brachytherapy (BT) to compensate for uncertainties. However, the BT and EBRT dose distributions are profoundly different, and the role of a PTV concept in BT needs a distinct discussion. The EBRT PTV concept is based on expanding the dose distribution into a homogeneous plateau reaching beyond the CTV. According to basic BT physics, there are significant dose gradients around radioactive source positions, and it is impossible to create homogeneous dose plateaus. This means that PTV margins cannot be directly applied in BT. Application of PTV margins in lateral and anterior-posterior directions can even lead to a significant and overall dose escalation (∼8% per mm margin applied) for the individual patient and for the entire patient population. In the specific direction along the intrauterine tandem, safety margins can partly account for uncertainties, though. In conclusion, safety margins can only be partially applied in intracavitary BT, and it is not recommended to perform PTV delineation. The PTV seems not to be useful for dose reporting, and dose normalisation to PTV is strongly discouraged since it can lead to dose escalation.