A treatment planning method and system for determining a treatment plan used to irradiate a treatment volume using a radiation treatment system that includes a multi-leaf collimator (MLC), is disclosed. The treatment plan includes settings for leaf patterns of individual leaves of the MLC, each leaf pattern including a geometry of the individual leaves and forming an aperture for use in the treatment plan. Areas of radiation leakage are identified in each of the leaf patterns, the areas of radiation leakage including first unshielded areas corresponding to gaps that are located between adjacent leaves and outside of the aperture. At least one leakage metric is calculated based on the areas of radiation leakage. A setting for at least one of the leaf patterns can be adjusted based on the at least one leakage metric.
