Due to leaf travel range limitations of current DMLC systems, an IMRT field width of
>
14.5cm is split into two or more adjacent sub-fields. Hot or cold spots may occur along the split line because of the possible gaps or overlaps between sub-fields. To alleviate this problem, we developed a technique that feathers the split line of IMRT fields. Feathering of split line was achieved by dividing IMRT fields into several sub-groups with different split line positions. Cylindrical targets with varying widths (>14.5 cm) were created to study split line positions. We investigated a feathering technique of split line positions by introduction of a pseudo target(s) outside of the patient (phantom). The position of the pseudo target was determined by analyzing the divergence of IMRT fields. Introduction of pseudo targets resulted in maximum shifts of ~1cm from the original split line positions, thereby causing a pseudo feathering effect. The dose around split line was measured for a hypothetical case of cylindrical target with an adjacent critical structure positioned in a 30cm
3
phantom. Measured dose distributions demonstrated that dose to the critical structure was 10% higher than planned when the split line crossed through the center of the target. When the split line was feathered, the measured dose distribution overlapped with the calculated distribution. The feathering technique developed in this work does not require any modifications of the radiation fields during the course of treatment because only one treatment plan is used to deliver the entire course of radiation treatments.