A Study of the Impact of Multi-Leaf Collimator Rotation

In the treatment of cancer, the goal is to destroy a tumor with minimal impact on normal tissues in the patient. Radiosurgery is a technique used to deliver a lethal dose of radiation to a tumor and spare surrounding critical structures (such as the optic nerve or spinal cord). Multiple beams of radiation are selected such that these beams intersect at the tumor. The treatment planning problem in radiosurgery is the selection of a number of beams and the orientations of those beams, such that at their intersection they deliver a lethal dose to the tumor without exceeding the tolerance of any critical structure in their path. Because tumor geometry is often irregular, it is useful to shape the beams to match the geometry of the tumor. A multi-leaf collimator (MLC) mounted at the beam portal of the radiation source can be used to shape a beam. The MLC is a series of aligned pairs of leaves. The MLC may be rotated around the center of the beam portal. This work investigates the impact of MLC rotation (IMLCR) in treatment planning for radiosurgery. MLC rotation was studied using a scoring function which models the maximum safely deliverable dose. The variation observed in the scoring function indicates that collimator rotation can raise the maximum deliverable dose. When the tumor is large and/or has nearby critical structures (tolerance <=50% of the tumor's prescription), collimator rotation has the greatest impact and is worth considering. Collimator angle selection could play an important role in improving the quality of treatment plans for radiosurgery.