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.