IMRT-application with an add-on MMLC

Leakage reduction for the Siemens Moduleaf

The ModuLeaf, an add‐on miniature multileaf collimator (MMLC) for the Siemens ONCOR linear accelerator, provides high resolution field shaping with a maximum interleaf leakage dose of 1.50% at 6 MV. However, beyond the maximum treatment field size, the distribution of leakage and scatter along the y‐axis is different from that of the x‐axis, with maximum leakage values of 1.53% and 0.39%, respectively. Such differences cannot be modeled in the Pinnacle treatment planning system. Also, within the 10 cm×12 cm treatment region, leakage from the crack between closed leaf ends was 3.76%. To resolve these issues, gaps in the ModuLeaf frame were filled with lead sheets, and the Siemens MLC was operated in MLC mode (rather than bank mode). As a result, a rectangle of 10.4 cm×11 cm was formed with the MLC leaves closed behind the Y jaws, whose opening was 10.4 cm. This significantly reduced the difference between the leakage patterns in the x and y directions, with maximum leakage doses of 0.43% outside the treatment region and 1.67% near the crack between abutting ModuLeaf leaves. The modification also reduced the mean square error between Pinnacle profiles and measured profiles in the tail region.

PACS number: 87.56.jk

Dosimetric verification of micro-MLC based intensity modulated radiation therapy

A methodology for the dosimetric verification of micro-multileaf collimator MMLC) based intensity modulated radiation therapy (IMRT) plans intended for stereotactic applications is described. The procedure is similar to that of conventional IMRT patient-specific quality assurance with some notable exceptions. Relative dosimetry measurements are performed with radiographic film, a commercial film-scanning system and a dose-image registration program. Film dosimetry results are within +/- 3.0% of calculated distributions or within 2.0 mm distance to agreement. Absolute dosimetry measurements are performed with a small volume ion-chamber and a commercially available stereotactic phantom. The cumulative dose from all beams is within +/- 2.0 % of the prescribed dose. Large deviations may be observed from individual beams since the smaller IMRT fields tend to have very few high-dose and low-gradient regions. An independent program that examines the treatment MLC file is used to estimate the central axis dose from each beam and provide a dose image that can be assessed alongside the intended fluence distribution prior to treatment. Tolerances for relative and absolute dosimetry of MMLC-based IMRT treatments are tighter than what is typically reported for conventional MLC-based IMRT. Also, the time commitment for the IMRT QA is slightly longer than of conventional MLC-based IMRT due to QA processes which check the mechanical alignment of the MMLC device with the laser and radiation isocentre.