Abstract
The purpose of this study is to investigate the dosimetric impact of multi‐leaf collimator (MLC) positioning errors on a Varian Halcyon for both random and systematic errors, and to evaluate the effectiveness of portal dosimetry quality assurance in catching clinically significant changes caused by these errors. Both random and systematic errors were purposely added to 11 physician‐approved head and neck volumetric modulated arc therapy (VMAT) treatment plans, yielding a total of 99 unique plans. Plans were then delivered on a preclinical Varian Halcyon linear accelerator and the fluence was captured by an opposed portal dosimeter. When comparing dose–volume histogram (DVH) values of plans with introduced MLC errors to known good plans, clinically significant changes to target structures quickly emerged for plans with systematic errors, while random errors caused less change. For both error types, the magnitude of clinically significant changes increased as error size increased. Portal dosimetry was able to detect all systematic errors, while random errors of ±5 mm or less were unlikely to be detected. Best detection of clinically significant errors, while minimizing false positives, was achieved by following the recommendations of AAPM TG‐218. Furthermore, high‐ to moderate correlation was found between dose DVH metrics for normal tissues surrounding the target and portal dosimetry pass rates. Therefore, it may be concluded that portal dosimetry on the Halcyon is robust enough to detect errors in MLC positioning before they introduce clinically significant changes to VMAT treatment plans.
Highlights
Modern radiation therapy uses the multi‐leaf collimator (MLC) heavily to shape and control the beam delivered to the patient during treatments using approaches such as intensity‐modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT)
While MLC positioning errors in any treatment plan delivered on the Halcyon are undesirable and should be minimized when possible, the type and magnitude of errors can greatly impact the clinical significance of these errors
Systematic errors consistently introduced more clinically significant changes to dose delivered to normal tissues and to target volume coverage than did random errors of the same magnitude
Summary
Modern radiation therapy uses the multi‐leaf collimator (MLC) heavily to shape and control the beam delivered to the patient during treatments using approaches such as intensity‐modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). Some studies have compared dosimetric differences in plans with random MLC leaf errors (where leaves are shifted some random amount within provided parameters) with differences in plans with systematic MLC leaf errors (where leaves are shifted identical amounts). Both error classes have been shown to unfavorably impact treatment plans, systematic errors are reported to be more significant than random errors of the same magnitude.[1,2]
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