Dosimetric performance of the new high-definition multileaf collimator for intracranial stereotactic radiosurgery

Abstract

The objective was to evaluate the performance of a high-definition multileaf collimator (MLC) of 2.5 mm leaf width (MLC2.5) and compare to standard 5 mm leaf width MLC (MLC5) for the treatment of intracranial lesions using dynamic conformal arcs (DCA) technique with a dedicated radiosurgery linear accelerator. Simulated cases of spherical targets were created to study solely the effect of target volume size on the performance of the two MLC systems independent of target shape complexity. In addition, 43 patients previously treated for intracranial lesions in our institution were retrospectively planned using DCA technique with MLC2.5 and MLC5 systems. The gross tumor volume ranged from 0.07 to 40.57 cm3 with an average volume of 5.9 cm3. All treatment parameters were kept the same for both MLC-based plans. The plan evaluation was performed using figures of merits (FOM) for a rapid and objective assessment on the quality of the two treatment plans for MLC2.5 and MLC5. The prescription isodose surface was selected as the greatest isodose surface covering >or= 95% of the target volume and delivering 95% of the prescription dose to 99% of target volume. A Conformity Index (CI) and conformity distance index (CDI) were used to quantifying the dose conformity to a target volume. To assess normal tissue sparing, a normal tissue difference (NTD) was defined as the difference between the volume of normal tissue receiving a certain dose utilizing MLC5 and the volume receiving the same dose using MLC2.5. The CI and normal tissue sparing for the simulated spherical targets were better with the MLC2.5 as compared to MLC5. For the clinical patients, the CI and CDI results indicated that the MLC2.5 provides better treatment conformity than MLC5 even at large target volumes. The CI's range was 1.15 to 2.44 with a median of 1.59 for MLC2.5 compared to 1.60-2.85 with a median of 1.71 for MLC5. Improved normal tissue sparing was also observed for MLC2.5 over MLC5, with the NTD always positive, indicating improvement, and ranging from 0.1 to 8.3 for normal tissue receiving 50% (NTV50), 70% (NTV70) and 90% (NTV90) of the prescription dose. The MLC2.5 has a dosimetric advantage over the MLC5 in Linac-based radiosurgery using DCA method for intracranial lesions, both in treatment conformity and normal tissue sparing when target shape complexity increases.

Figure 1

Graphical display of BEV for 100° arc for ${\text{MLC}}_{2.5}$ (a) and ${\text{MLC}}_5$ (b). These two views show MLC's leaf position for every ten degrees surrounding the target volume (in blue). The leaf width between the two views for the 2.5 mm and 5 mm leaf width is shown; the yellow outline demonstrates the difference in conformality of the two MLC systems.

Figure 2

Schematic illustrating the definition of normal tissue difference NTD.

Figure 3

Dependence of Conformity Index on target volume for a simulated spherical target using ${\text{MLC}}_{2.5}$ and ${\text{MLC}}_5$.

Figure 4

Comparison of normal tissue sparing capabilities of ${\text{MLC}}_{2.5}$ and ${\text{MLC}}_5$ for a simulated spherical target.

Figure 5

The percentage difference of the conformity indices between ${\text{MLC}}_5$ and ${\text{MLC}}_{2.5}$ versus target volume.

Figure 6

Dose volume histograms of the normal tissue shell surrounding target for three patients.

Figure 7

Normal tissue difference NTD plotted as a function of target volume for tissue irradiated to 50%, 70% and 90% when ${\text{MLC}}_{2.5}$ is used instead of ${\text{MLC}}_5$.