Dose uncertainty from calculation grid resolution and its alignment with MLC.

The aims are (1) to present a clinical brain case where the dose distribution calculated at 2.5-mm grid resolution was significantly different from that of 1.0 mm with Varian HD120 multileaf collimator (MLC; 2.5-mm central leaf width), and (2) to present our analysis results to better understand the influence of dose calculation grid resolution and its spatial alignment with MLC. A simulation using a static MLC pattern with 10 open strips was conducted to demonstrate the possibility that the characteristics of MLC dose profiles can be completely hidden. The effects of the spatial MLC alignment with the dose grid were investigated for grid resolutions from 1.0 mm to 5.0 mm. In the end, 20 recent stereotactic radiotherapy plans (5 brain, 5 lung, 5 spine, and 5 other) were retrospectively selected, and their dose distributions were recalculated at 1.0 mm, 1.5 mm, and 2.5 mm resolutions for comparisons. Film dosimetry results were used as references. At 2.5-mm grid resolution, the MLC open-strip patterns were not visible in the dose distribution when the MLC leaf interspaces were at exact alignment with the dose grid. The effects were smaller at 1.0-mm grid resolution. The dose error at the centers of the open strips varied significantly (7% to 39%) depending on the alignments. Overall, the planning target volume mean dose of 1.5-mm plans was higher than 1.0-mm plans by 1.3 ± 0.3% (mean ± 1SD) with a maximum of 2.6%, while the mean dose of 2.5-mm plans was lower by -0.5 ± 0.5% with a maximum mean dose of -3.7%. In conclusion, the choice of dose calculation grid resolution can affect the plan dose distribution significantly. The dose grid resolution must be smaller than the width of MLC leaf at a minimum. A finder resolution is expected to produce less uncertainty in the calculated dose and influenced less by the MLC-grid spatial alignment.