Dosimetric Impact of Mlc Backlash on the Csrs and SBRT Treatment Techniques.
Abstract
Purpose
MLC backlash is a linac failure mode that results in MLC positional error that has been demonstrated to not be prevented by the linac’s internal safety systems [1]. This study aims to inform on the clinical significance of MLC positional error due to MLC backlash.
Methods
The study of Barnes et al, 2022 [1] characterized the MLC positional error due to backlash, which is dependent on both gantry angle and direction of MLC motion. Using this characterization, an Eclipse (Varian Medical Systems, Palo Alto, CA, USA) ESAPI script has been written to overwrite clinical treatment plan control points with updated MLC positions simulating the backlash. Modified plans were then recalculated and compared with the original plans to assess backlash-induced dosimetric errors. Two magnitudes of positional error have been investigated and applied across all leaves; 0.3 mm, representing standardly achievable backlash, and 0.5 mm, a realistic worse case -scenario. The simulated effects of MLC backlash on single lesion Cranial Stereotactic-Radiosurgery (SRS) and Spine Stereotactic Body Radiotherapy (SBRT) target coverage have been assessed for the PTV (D100) metric for 16 and 11 clinical plans, respectively. Organ-At-Risk (OAR) doses are not expected to increase as backlash has the effect of reducing the MLC aperture size. Correlation between PTV (D100) target coverage reduction with PTV volume size has also been assessed via Pearson correlation.
Results
PTV (D100) reduced on average by 2.9 % and 5.1 % for 0.3 mm and 0.5 mm backlash, respectively for SRS and 1.3 % and 2.2 %, respectively for spine SBRT. A moderate correlation (p=0.36) was observed between PTV(D100) target coverage and PTV volume for both treatment cohorts.
Conclusion
MLC backlash of realistic magnitudes can reduce target coverage for Cranial SRS and Spine SBRT treatment plans by potentially significant amounts. The dosimetric impact increases as PTV volume decreases.