Dosimetric study using different leaf-width MLCs for treatment planning of dynamic conformal arcs and intensity-modulated radiosurgery

This paper systematically studied the dosimetric difference between a 3 mm micro multileaf collimator (MLC), a 5 mm MLC, and a 10 mm MLC for stereotactic radiosurgery using the Brainscan treatment planning system. Thirty-four cases treated with the dynamic conformal arcs technique and 20 cases treated with the intensity modulated radiosurgery/fractionated radiotherapy (IMRS/ IMRT) technique were retrospectively studied. The conformity index, the percentage target coverage, and the dose-volume histogram (DVH) for organs-at-risk (OARs) were used for dosimetric analysis and comparison for different treatment techniques, target volumes, and treatment sites. For the dynamic conformal arcs technique, there were statistically significant differences in the conformity indices between different leaf-width MLCs. The ratio of the conformity indices between different MLCs depended on the target volume. The average conformity index ratios between the 5 mm MLC and the 3 mm MLC were 1.37±0.09, 1.12±0.04, 1.08±0.02 and 1.04±0.01, respectively, for patients with the target volume (V) in groups: (1) V<1 cm³, (2) 1 cm³ <V< 8 cm³, (3) 8 cm³<V<27 cm³, and (4) V>27 CC. The average conformity index ratios between the 10 and 3 mm MLCs were 2.00±0.33, 1.45±0.09, 1.28±0.09, and 1.18±0.05 for patients in these four volume groups, respectively. No statistically significant difference was found for the target coverage among different MLCs. For the IMRS/IMRT technique, the average conformity index and target coverage ratios were 1.01±0.05 and 1.00±0.02, respectively, between the 5 and 3 mm MLCs, and were 1.04±0.07 and 0.97±0.02, respectively, between the 10 and 3 mm MLCs. The 3 mm MLC showed slightly better overall OAR DVHs than the 5 and 10 mm MLCs, especially for the cranial site with small-volume OARs defined. The results suggest that for the dynamic conformal arcs technique, the narrower leaf-width MLC provides better dose conformity than the wider leaf-width MLCs. This advantage decreases when the target volume increases. For the IMRS/IMRT technique, the narrower leaf-width MLC could have better sparing of small OARs than the wider leaf-width MLC.