Optimizing the indexing of a resolution-enhancing tertiary collimator for radiotherapy

A method for improving the resolution of multileaf collimator (MLC) defined radiotherapy fields using a tertiary, slotted grid collimator has been investigated and developed further. The original concept was for each slot to be aligned with each leaf pair of opposing MLC leaves. The total treated area is composed of a series of sub-fields, a pattern of irradiated strips, with the width of each strip defined by the width of the slot and the length by the relative separation of the MLC leaf pair. To complete the field, the patient must be indexed relative to the collimator, with the number of sub-fields required determined by the width of the slots and the spacing between them. Two methods were considered by which this indexing could be achieved: movement of the patient while holding the tertiary collimator fixed, or rotating the grid with the point of rotation defined as the radiation source. Consideration of the movement resolution and precision required for the patient support system for non-cardinal gantry, collimator and table angles cast doubt on the practicality of the use of such a strategy. To assess the effect of divergence on the abutting sub-fields, measurements were also performed to assess the uniformity of single fields generated by the tertiary collimator in planes above and below the isocentre using both methods of indexing. As expected, rotation of the collimator resulted in a similar degree of non-uniformity for any plane chosen, whereas significant dose heterogeneities were introduced to treatment planes within 5 cm above and below the isocentre if the patient support system was used. Therefore, the rotation strategy will be implemented with all future versions of the device.