Verification by treatment plan and physical measurement of the monitor unit (MU) objective function for stereotactic ablative body radiotherapy (SABR) lung planning

Treatment plan quality was investigated to determine whether the total monitor units (MUs) could be reduced by using the MU objective function within the EclipseTM planning system (Varian Medical Systems) for stereotactic ablative body radiotherapy (SABR) lung plans. The plans were produced using RapidArc® (Varian's volumetric modulated arc therapy) technique. This study used physical measurements to assess the deliverability and quality of the resultant plans on the treatment machine. Ten patients (dose prescriptions ranging from 6.25 to 18 Gy per fraction) were re-planned retrospectively using objective limit MUs to approximately 50% of the clinical plan (CP) MUs. A number of parameters including dose volume histogram, conformity index, gradient measure, total MUs, treatment time, mean leaf gap, small aperture score (SAS) and gamma (2%/2 mm) were compared. To assess deliverability, plans were measured on a treatment machine using the MatriXX Array and the CompassTM system (IBA Dosimetry). Whilst the quality of plans with and without MU objective function was not statistically different, the number of MUs and overall treatment time was reduced by 23% and 1.71 min, respectively. Mean leaf gap increased by 22.3%, SAS reduced by 49% and normalised MU (MU/cGy) reduced by 22.0%. Importantly, verification and improvements in plan deliverability were measured. The MU objective method showed an overall improvement in plan quality and deliverability; no plans were statistically worse. By reducing the number of MUs for treatment the delivery time is reduced thereby decreasing the time the patient is on the radiotherapy couch. Our institution now uses this technique for clinical lung SABR patients routinely.