Dynamic collimation for missing tissue compensation

In external radiotherapy, the use of missing tissue compensation has been proposed for the improvement of dose distributions resulting from radiation beams entering through oblique or irregular surfaces of the patient. The well-known inconveniences related to conventional compensators justify the investigation of alternative techniques such as the use of dynamic multileaf collimators. The combination of a number of software and hardware modules was necessary for the implementation of dynamic collimation for missing tissue compensation. First, a software module that calculates, with a ray-tracing technique, the missing-tissue deficit from computed tomography scans and generates a desired 2D intensity modulation pattern has been developed. Then a second software module that converts the 2D intensity map to a leaf motion plan for practical delivery with dynamic collimators has been implemented. Finally, the existing primary-scatter separation dose model of the authors' TPS had to be extended to account for the intensity modulation generated by the dynamic MLC. As an example of the 'virtual' compensator technique, the MLC sequence that would compensate an arbitrary patient irregular surface has been calculated. Then, film was used to measure the dose on a plane perpendicular to the beam axis at the specified depth of compensation. A comparison between measured and calculated dose distributions shows that an acceptable accuracy in dose calculations can be achieved. Finally a comparison between uncompensated and dynamically compensated fields demonstrates that the 'virtual' compensator was able to flatten the off-axis dose across a plane perpendicular to the beam axis at the depth of compensation.