The effect of minimum segment width of collimator on VMAT plan quality for rectal cancer using Monaco TPS

Abstract Purpose: To compare the effect of the minimum segment width of the multi-leaf collimator (MLC) on the quality of rectal cancer planning in the Monaco treatment planning system. Methods: A retrospective analysis of 30 rectal cancer patients was conducted using the Monaco treatment planning system with varying minimum segment widths under the same optimization parameters. The dose of the target area and organs at risk (OARs), conformability index (CI), homogeneity index (HI), time of treatment and monitor units (MUs) were compared across patients under different minimum segment widths. Results: All of the patients had γ-passing rates greater than 95% and they were all statistically significant. From TPS calculations and 3DVH recalculations, deviations above 5% occur at MLC=0.5 and 2.0, particularly larger at 0.5 and with a larger variance for target areas. Of the 9 dosimetric parameters, only D98 and Dmax were statistically significant. As the minimum segment width increases, the mean number of MUs decreased with 724, 525, 469, and 451 respectively, and the mean time of treatment also decreased from 154, 141 to 140s. The differences in target area dose, conformability index, homogeneity index and organs at risk dose with different segment widths were not statistically significant(P>0.05). Conclusion: When designing treatment plans for rectal cancer using Monaco, dose distributions that meet the requirements can be obtained using all 4 segment width optimization patterns. In short, the radiation treatment time can be shortened and the clinical efficiency can be enhanced by increasing the minimum segment width without compromising the treatment outcome.