Dynamic Log Files Research Articles

SU-GG-T-84: Dose Verification of 2D Synchronized DMLC IMRT Delivery to a Moving Target

Purpose: Anatomic change caused by respiratory motion is a major challenge of accurate dosedelivery in Intensity Modulated Radiation Therapy(IMRT). Dynamic MLC (DMLC) tracking delivery has been studied by a few groups to mitigate the motion effect. In this work, some clinically practical issues related to the 2D synchronized DMLC IMRTdelivery were investigated. Method and Materials: An IMRTtreatment plan was generated for one specific phase of 4D CT data. Two‐dimensional superimposing method was applied to modify the original leaf sequence by adding the average tumor trajectory (ATT) onto the leaf sequences in beam's eye view plane. The modified leaf sequences were delivered using a Varian 2100CD linac with Millennium 120‐leaf DMLC. Dynamic log files were analyzed to check the machine beam hold‐off and simulate the fluence map in the coordinate system of the moving targets. Comparison between the delivered fluence map and the expected fluence map was done. Results: Without the motion correction, the delivered fluence map is largely different from the planed fluence map. With the motion corrected leaf sequences, such difference was significantly reduced: the maximum point‐to‐point dose difference decreased by 40%; and the number of dose mismatching points decreased by 5 times. When the leaf speed exceeds DMLC specifications and machine tolerance is tight, there will be beam hold‐offs, which will cause errors in the delivereddose distributions. Conclusion: The dose error in delivery of 2D synchronized DMLC IMRT was studied. The dose map comparison shows that the dose error delivered with a motion corrected leaf sequence can be significantly reduced. Since the actual delivereddose is affected by DMLC mechanic limits, leaf velocities of the modified leaf sequences should be less than MLC maximum speed to guarantee the synchronization during delivery.

SU‐GG‐T‐149: 4D IMRT Quality Assurance (QA) by Dynamic Log File Analysis

Purpose: 4D IMRT was a technique developed to account for dosimetry errors from intra‐fraction motion, especially from anatomy transformation caused by respiration. To perform 4D IMRT dose validation some dynamic phantoms are used. Measurements with dynamic phantom involve the complex setting procedure. In order to simplify the QA procedure and quickly check the IMRT plan, a 4D IMRT verification tool was developed. The delivered fluence map to a moving target can be simulated from dynamic log (Dynalog) files using this tool. Method and Materials: A 4D CT was acquired for the patient. The breathing cycle was divided into 10 phases. An IMRT plan was generated for the exhale phase, and its leaf sequence was exported. The original leaf sequence was modified by superimposing the tumor trajectory onto the leaf trajectory in a two dimensional beam's eye view plane. The modified leaf sequence was delivered by Varian 2100 CD with Millenium 120 leaf MLC. After dose delivery Dynalog files were analyzed by an in‐house software to simulate the fluence map of a moving target. The fluence map measured by the dynamic phantom was compared with the simulation of delivered fluence map. Results: The comparison between the measured fluence map using the dynamic phantom and the simulated fluence map by Dynalog files shows that the difference of isodose lines was within 3mm and the difference of line profiles was within 3%. Conclusion: A 4D IMRT QA tool was developed to simulate the delivered fluence map to a moving target. The simulated fluence map shows a good agreement with the measured fluence map based on dynamic phantom measurements. When compared to phantom measurement, this tool can assist to quickly check the 4D IMRT plan and handle irregular tumor trajectories.

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Intensity-modulated radiation therapy (IMRT) represents one of the most significant technical advances in radiation therapy. In the dynamic multileaf collimator (MLC) method of IMRT delivery, because of the relatively small gaps between opposed leaves and because most regions are shielded by leaves most of the time, the delivered dose is very sensitive to MLC leaf positional accuracy. A variation of +/-0.2 mm in the gap width can result in a dose variation of +/-3% for each clinical dynamic MLC field. Most often the effects of leaf motion are inferred from dose deviations on film or from variations in ionization measurements. These techniques provide dosimetric information but do not provide detailed information for diagnosing delivery problems. Therefore, a dynamic log file (Dynalog file) was used to verify dynamic MLC leaf positional accuracy. Measuring for narrow gaps using the thickness gauge could detect a log file accuracy of approximately 0.1 mm. The accuracy of dynamic MLC delivery depends on the accuracy with which the velocity of each leaf is controlled. We studied the relationship between leaf positional accuracy and leaf velocity. Leaf velocity of 0.7 cm/sec caused approximately 0.2 mm leaf positional variation. We then analyzed leaf positional accuracy for the clinical dynamic MLC field using Dynalog File Viewer (Varian Medical Systems, Inc., Palo Alto, CA), and developed a new program that can analyze more detailed leaf motions. Using this program, we can obtain more detailed information, and therefore can determine the source of dose uncertainties for the dynamic MLC field.

SU‐FF‐T‐210: Performance Evaluation and Quality Assurance of Enhanced Dynamic Wedges

Purpose: Enhanced dynamic wedges have been used in clinical practice for many years. Obvious superiority of dynamic over physical wedges is accompanied by the increased overhead involved in verifying the accuracy and reliability of their use. Contrary to very limited quality assurance (QA) required to ensure proper functioning of the physical wedges, dynamic wedges, like any other dynamic treatment, require a robust quality assurance program. This work expands upon previous suggestions and describes a comprehensive QA program for Varian Enhanced Dynamic Wedges (EDW) and presents the results of a sixteen‐month evaluation of these wedges. Method and Materials: Daily, monthly, and annual QA procedures were devised for enhanced dynamic wedges on a dual energy Varian 21ex linear accelerator with seven wedge angles. These include daily constancy checks of the wedge performance as part of the morning QA procedure, monthly evaluation of wedge factors and profiles, and an analysis of dynamic log files generated by linac. Yearly performance evaluation includes measurement of a larger sample of wedge angles and profiles. In addition, individual treatment verification or “per patient” QA consists of visual inspection of the wedge direction at the conclusion of each treatment. Results: The daily constancy checks show a stable delivery of the wedged field with a variation of less than 1%. The monthly wedge factors measured have been within 1.5 % of the commissioning values. A comparison of beam profiles over the same time period shows a stable delivery of the wedged beams and good agreement with the expected segmented treatment tables (STT). Conclusion: An expended comprehensive QA of enhanced dynamic wedges is presented. Results of a sixteen‐month evaluation demonstrate stable and accurate delivery of these fields. Due to the dynamic nature of enhanced dynamic wedge deliveries, a comprehensive QA program is necessary to verify proper delivery of these fields.

Verification of dynamic and segmental IMRT delivery by dynamic log file analysis

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