Abstract
A program has been developed to evaluate the delivered fluence of step-and-shoot segmental and sliding window dynamic multileaf collimator (MLC) fields. To automate these checks, a number of tools have been developed using data available from the dynamic log files that can be created each time a dynamic delivery occurs. Experiments were performed with a Varian 2100EX with a 120 leaf MLC equipped with dynamic capabilities. A dynamic leaf sequence is delivered and measured with film or an amorphous silicon imager. After delivery, the dynamic log file is written by the accelerator control system. The file reports the expected and actual position for each leaf and the dose fraction every 0.055 seconds. Leaf trajectories are calculated from this data and expected and actual fluence images are created from the difference of opposing leaf trajectories. These images can be compared with the expected delivery, measurements, and calculations of fluence. Tools have been developed to investigate other aspects of the delivery, such as specific leaf errors, beam hold-off flags sent by the control system to the MLC, and gap widths. This program is part of a semi-automated quality assurance (QA) system for pretreatment fluence verification and daily treatment verification of dynamic multileaf collimation (DMLC) delivery. PACS number(s): 87.53.–j, 87.52.–g
Highlights
Quality assurance of IMRT delivery techniques such as segmented multileaf collimationSMLC ͑step-and-shootand dynamic multileaf collimationDMLC ͑sliding windowis critical to ensure accurate delivery of optimized treatment plans
More specific evaluation of the control system and MLC function can be done using the information contained in the dynamic log files, or ‘‘DynaLog Files’’ in the case of Varian MLC’s
The purpose of this paper is to describe the analysis program and demonstrate its functionality for commissioning and routine QA of IMRT techniques
Summary
Quality assurance of IMRT delivery techniques such as segmented multileaf collimationSMLC ͑step-and-shootand dynamic multileaf collimationDMLC ͑sliding windowis critical to ensure accurate delivery of optimized treatment plans. The dosimetic effects of leaf position uncertainty may be determined from variations in ionization measurements.6 These techniques provide dosimetric information but do not provide detailed information for diagnosing delivery problems. Other verification techniques that calculate the fluence distribution from the MLC leaf trajectory file have the advantage of not requiring sequence delivery, but only consider anticipated delivery constraints; not necessarily constraints such as the tolerance setting, dose rate, programmed monitor unitsMU, or system delay time.. Other verification techniques that calculate the fluence distribution from the MLC leaf trajectory file have the advantage of not requiring sequence delivery, but only consider anticipated delivery constraints; not necessarily constraints such as the tolerance setting, dose rate, programmed monitor unitsMU, or system delay time.7 These techniques require that the behavior of all clinically relevant parameters and system characteristics be programmed into the verification software. More specific evaluation of the control system and MLC function can be done using the information contained in the dynamic log files, or ‘‘DynaLog Files’’ in the case of Varian MLC’s.8–11 These files contain
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