Study of the Interplay effect in Radiotherapy using Monte-Carlo methods.

Radiotherapy of bronchial tumours must take into account respiratory movement. While the target (tumour) is being irradiated, it can move several centimetres, which has the effect of potentially leading to target underdosing and organ at risk (OAR) overdosing. For conventional irradiation this movement is usually taken into account by using safety margins. But, when the treatment is intensity modulated (delivery by a dynamic beam that does not irradiate the whole target at once in order to be more consistent with the complex shape of a target) there is an additional risk of an unfortunate combination of collimator blade movement and respiratory movement (interplay effect). This work aims to evaluate this interplay effect for clinical cases using the reference calculation (Monte Carlo). The conclusion is that this effect is negligible for realistic clinical conditions (treatment in 3 fractions over a duration of more than 20 respiratory cycles).

This work allowed to remove all the doubts that the Radiotherapy team had before starting the irradiation in stereotactic condition (i.e. a small number of fractions and a large dose per fraction) of the pulmonary targets. Since this work, the treatments are done in modulated intensity.

In perspective of this work, a prediction system of the effect for each patient could be proposed.

Collaborations and acknowledgements

This work was carried out using the CALMIP super-computer (HPC) under the project 2016-P19001

One picture :

The largest profile corresponds to a classical clinical calculation, based on a static image of the tumour. The other dose profiles are made by simulating the movement and observing the different effects (collapse of the slopes of the profiles or blurring effect and interplay effect on the plateau)

Collaborations and acknowledgements