Inverse Verification of the Dose Distribution for Intensity Modulated Radiation Therapy Patient-specific Quality Assurance Using Dynamic MLC Log Files

The aim of this study was to investigate a novel method for verification of the dose distribution for intensity modulated radiation therapy (IMRT) patient-specific quality assurance (QA) using dynamic multi-leaf collimator (DMLC) log files (Dynalog files). Dynalog files are recorded every 50 ms by using a MLC controller during the IMRT treatment. Dynalog files contain actual MLC positional information for various delivered dose fractions. As the nonuniform fluence is directly influenced by the MLC positional accuracy, our method for IMRT patient-specific QA can be performed using this information. Three nasopharyngeal cancer patients were selected for the evaluation. We developed an in-house program to convert MLC log files from an MLC controller to delivered MLC (dMLC) field files for the interface between the MLC controller and the treatment planning system. The in-house software, DMLC field file (DFF) converter, was written using programming language (Visual C++ 2005, Microsoft, Redmond, WA, USA). For inverse planning, Eclipse (v. 6.5, Varian, Palo Alto, USA) was used. The MLC log files were converted to dMLC files. The IMRT plans were recalculated and compared with the original plans. Comparisons were done via planar dose distributions using OP-IMRT software (v. 1.4, Wellhofer Dosimetrie, Germany) and dose volume histograms (DVHs) for targets and organs at risk (OARs). Gamma index (dose difference: 3%, distance to agreement: 3 mm) calculations were also performed for a quantitative analysis. There were significant differences (maximum dose difference: 587 cGy, maximum volume difference at 3000 cGy: 17%) in the DVHs of the parotid glands between planned MLC (pMLC)based and delivered MLC (dMLC)-based inverse IMRT QA (IVQA) plans for all three patients. The histograms showed an increased dose-volume in the dMLC-based IVQA deliveries compared to reference (Ref.) IMRT plans. Based on the present study, we can confirm the availability of our new approach to perform IMRT patient-specific QA providing a convenient and clear tool for IMRT dose verification. In the future, this method should be available for inverse on-treatment dose verification and for pre-treatment IMRT QA.