Patient QA systems for rotational radiation therapy: a comparative experimental study with intentional errors.

PURPOSE: The purpose of the present study was to investigate the ability of commercial patient quality assurance (QA) systems to detect linear accelerator-related errors. METHODS: Four measuring systems (Delta(4®), OCTAVIUS(®), COMPASS, and Epiqa™) designed for patient specific quality assurance for rotational radiation therapy were compared by measuring four clinical rotational intensity modulated radiation therapy plans as well as plans with introduced intentional errors. The intentional errors included increasing the number of monitor units, widening of the MLC banks, and rotation of the collimator. The measurements were analyzed using the inherent gamma evaluation with 2% and 2 mm criteria and 3% and 3 mm criteria. When applicable, the plans with intentional errors were compared with the original plans both by 3D gamma evaluation and by inspecting the dose volume histograms produced by the systems. RESULTS: There was considerable variation in the type of errors that the various systems detected; the failure rate for the plans with errors varied between 0% and 72%. When using 2% and 2 mm criteria and 95% as a pass rate the Delta(4®) detected 15 of 20 errors, OCTAVIUS(®) detected 8 of 20 errors, COMPASS detected 8 of 20 errors, and Epiqa™ detected 20 of 20 errors. It was also found that the calibration and measuring procedure could benefit from improvements for some of the patient QA systems. CONCLUSIONS: The various systems can detect various errors and the sensitivity to the introduced errors depends on the plan. There was poor correlation between the gamma evaluation pass rates of the QA procedures and the deviations observed in the dose volume histograms.

RapidArc treatment verification in 3D using polymer gel dosimetry and Monte Carlo simulation.

The aim of this study was to verify the advanced inhomogeneous dose distribution produced by a volumetric arc therapy technique (RapidArc) using 3D gel measurements and Monte Carlo (MC) simulations. The TPS (treatment planning system)-calculated dose distribution was compared with gel measurements and MC simulations, thus investigating any discrepancy between the planned dose delivery and the actual delivery. Additionally, the reproducibility of the delivery was investigated using repeated gel measurements. A prostate treatment plan was delivered to a 1.3 liter nPAG gel phantom using one single arc rotation and a target dose of 3.3 Gy. Magnetic resonance imaging of the gel was carried out using a 1.5 T scanner. The MC dose distributions were calculated using the VIMC-Arc code. The relative absorbed dose differences were calculated voxel-by-voxel, within the volume enclosed by the 90% isodose surface (VOI(90)), for the TPS versus gel and TPS versus MC. The differences between the verification methods, MC versus gel, and between two repeated gel measurements were investigated in the same way. For all volume comparisons, the mean value was within 1% and the standard deviation of the differences was within 2.5% (1SD). A 3D gamma analysis between the dose matrices were carried out using gamma criteria 3%/3 mm and 5%/5 mm (% dose difference and mm distance to agreement) within the volume enclosed by the 50% isodose surface (VOI(50)) and the 90% isodose surface (VOI(90)), respectively. All comparisons resulted in very high pass rates. More than 95% of the TPS points were within 3%/3 mm of both the gel measurement and MC simulation, both inside VOI(50) and VOI(90). Additionally, the repeated gel measurements showed excellent consistency, indicating reproducible delivery. Using MC simulations and gel measurements, this verification study successfully demonstrated that the RapidArc plan was both accurately calculated and delivered as planned.