Indexing artefacts using a tertiary collimator and a method to avoid them.

Abstract

A tertiary, slotted grid collimator has been proposed as a method by which the spatial resolution of radiation fields shaped using a multileaf collimator (MLC) may be improved. The prototype device previously reported in the literature allowed each slot in the grid to be aligned with each pair of opposing leaves in the MLC. Irradiation through the collimator resulted in a regular pattern of high dose 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. This indexing is achieved by rotation of the collimator along a radius centred on the radiation source, thus avoiding the effects of beam divergence on the uniformity of delivered composite beams. However, rotation of the tertiary collimator has been shown to result in artefacts in field definition. The source of such artefacts is the proximity of the long edge of a slot to the junction between two MLC leaves, if one leaf is withdrawn with respect to the next. A simple cause for such artefacts was identified as a small misalignment between the slot edge and the leaf junction. Another source of such effects is the finite size of the radiation source, which allows for partial shielding effects beyond the leaf edge. Finally, the effect of the leaf edge penumbra was identified as the cause of potential areas of underdose within the field boundary. Measurements were performed using a mini-diode of the potential magnitude of the last two effects for slot widths of 2.5 and 5.0 mm beneath a single open MLC leaf. For both slot widths, the unwanted overdose due to partial shielding by neighbouring leaves did not exceed 8% of the dose with the slot at the centre of the single open leaf. The potential for underdose within the field was a much more significant effect, especially for the narrower slot, with the peak intensity through the 2.5 mm slot positioned just inside the open leaf only reaching 41% of that measured with it positioned centrally. Such an arrangement of slots and leaves is unavoidable if the separation between neighbouring slots is identical to that of the MLC leaves (1 cm at isocentre). An alternative arrangement is proposed, and has been demonstrated to avoid the generation of the artefacts detailed above.