Program Information
Dosimetric Effects Resulting From Increased MLC Error and Expanded Dose Dynamic Leaf Tolerances in Sliding Window IMRT
T Netherton*, G Ding , Vanderbilt University, Nashville, TN
Presentations
SU-E-T-316 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: To examine the dosimetric effects on the target dose coverage of MLC error (actual vs. planned position) for sliding window IMRT treatment plans.
Methods: Dynalog files from a treated 7 field H&N sliding window IMRT plan were imported into a Dynalog to .mlc converter. The errors of the MLC’s were multiplied by factors of f= 5, 10, 50, and 500 while capping the dose dynamic leaf tolerance at 0.2cm (Eclipse TPS default). Resulting .mlc files were imported into the TPS to study the changes in dose after increasing the magnitude of MLC error. Additionally, the dose dynamic leaf tolerance was expanded to 0.3cm, 0.4cm, and 0.5cm for each case.
Results: For the linac investigated, A and B MLC error was found in the direction opposite to MLC motion, toward the X1 jaw. This systematic shift in the MLC position results in a shift of the dose distribution per field when the inherent error is multiplied by a factor. The complete treatment plan had maximum and minimum dose differences (modified vs. planned) exhibiting a similar trend as individual fields with dose differences reaching 3% of the prescribed dose with f=10 and an expanded tolerance of 0.3cm.
Conclusion: A clinically significant difference of 5% of the prescribed dose is seen when the MLC error is multiplied by a factor f~15 while within the default dose dynamic leaf tolerance of 0.2cm. This suggests that if a large deviation of the MLC’s from the planned positions were to occur for just 1/5 of the active leaf positions that differences in the delivered vs. planned dose per fraction could exceed 5%. Using the default MLC tolerance, no clinically significant difference in the mean target dose was found, indicating that the default MLC tolerance of 0.2cm is adequate for accurate IMRT treatment plan delivery.
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