Output variation from an intensity modulating dynamic collimator.

Intensity modulated radiation therapy (IMRT) offers a method of delivering radiation dose conforming to the shape of targets while minimizing the dose to the surrounding tissue and nearby critical organs. One popular device is the NOMOS MIMiC Collimator coupled to the CORVUS treatment planning system. The MIMiC collimator, mounted on a linac head, opens and closes one or more of its 40 small leaves as determined by the planner while the linac delivers radiation and the gantry rotates. This dynamic IMRT allows the intensity to be modulated yielding a highly conformal dose distribution. However, the dose output becomes a function of the detailed manner in which the leaves open and close, since the opening and closing are not instantaneous. We investigate the effect of switch rates and delay in the open/close events on the output profiles. The output is enhanced as the switch rate increases. The enhancement factor at any point of measurement is dependent on its distance from the central plane. We interpret these variations in terms of a simple model, which includes the effect of leaf travel time during the process of opening and closing. We also include the time delay in establishing the specified pressure in the pneumonic device, which controls the opening and closing of the leaves. The information presented here offers a means for incorporating these output changes into the planning system. This may avoid the current situation where many patient plans need to be renormalized based on the actual measurement taken during the delivery of the specified intensity pattern to a phantom.