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

The objective was to evaluate the performance of a high‐definition multileaf collimator (MLC) of 2.5 mm leaf width (MLC 2.5) and compare to standard 5 mm leaf width MLC ( MLC 5 ) 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 withMLC 2.5andMLC 5systems. The gross tumor volume ranged from 0.07 to 40.57   cm 3with an average volume of 5.9   cm 3 . 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 forMLC 2.5andMLC 5 . The prescription isodose surface was selected as the greatest isodose surface covering ≥ 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 utilizingMLC 5and the volume receiving the same dose usingMLC 2.5. The CI and normal tissue sparing for the simulated spherical targets were better with theMLC 2.5as compared toMLC 5 . For the clinical patients, the CI and CDI results indicated that theMLC 2.5provides better treatment conformity thanMLC 5even at large target volumes. The CI's range was 1.15 to 2.44 with a median of 1.59 forMLC 2.5compared to 1.60–2.85 with a median of 1.71 forMLC 5 . Improved normal tissue sparing was also observed forMLC 2.5overMLC 5 , with the NTD always positive, indicating improvement, and ranging from 0.1 to 8.3 for normal tissue receiving 50% ( NTV 50 ), 70% ( NTV 70 ) and 90% ( NTV 90 ) of the prescription dose. TheMLC 2.5has a dosimetric advantage over theMLC 5in Linac‐based radiosurgery using DCA method for intracranial lesions, both in treatment conformity and normal tissue sparing when target shape complexity increases. PACS number: 87.56J‐, 87.56 jk