Machine Log File and Calibration Errors-based Patient-specific Quality Assurance (QA) for Volumetric Modulated Arc Therapy (VMAT)
Authors: Ma Yangguang , Mou Xuanqin , Beeraka Narasimha M. , Guo Yuexin , Liu Junqi , Dai Jianrong , Fan Ruitai 
Published: November 2023

Introduction:

Dose reconstructed based on linear accelerator (linac) log-files is one of the widely
used solutions to perform patient-specific quality assurance (QA). However, it has a drawback that the accuracy
of log-file is highly dependent on the linac calibration. The objective of the current study is to represent a
new practical approach for a patient-specific QA during Volumetric modulated arc therapy (VMAT) using
both log-file and calibration errors of linac.

Methods:

A total of six cases, including two head and neck neoplasms, two lung cancers, and two rectal carcinomas,
were selected. The VMAT-based delivery was optimized by the TPS of Pinnacle^3 subsequently,
using Elekta Synergy VMAT linac (Elekta Oncology Systems, Crawley, UK), which was equipped with 80
Multi-leaf collimators (MLCs) and the energy of the ray selected at 6 MV. Clinical mode log-file of this linac
was used in this study. A series of test fields validate the accuracy of log-file. Then, six plans of test cases
were delivered and log-file of each was obtained. The log-file errors were added to the corresponding plans
through the house script and the first reconstructed plan was obtained. Later, a series of tests were performed
to evaluate the major calibration errors of the linac (dose-rate, gantry angle, MLC leaf position) and the errors
were added to the first reconstruction plan to generate the second reconstruction plan. At last, all plans were
imported to Pinnacle and recalculated dose distribution on patient CT and ArcCheck phantom (SUN Nuclear).
For the former, both target and OAR dose differences between them were compared. For the latter, γ was
evaluated by ArcCheck, and subsequently, the surface dose differences between them were performed.

Results:

Accuracy of log-file was validated. If error recordings in the log file were only considered, there
were four arcs whose proportion of control points with gantry angle errors more than ± 1°larger than 35%. Errors
of leaves within ± 0.5 mm were 95% for all arcs. The distinctness of a single control point MU was bigger,
but the distinctness of cumulative MU was smaller. The maximum, minimum, and mean doses for all targets
were distributed between -6.79E-02-0.42%, -0.38-0.4%, 2.69E-02-8.54E-02% respectively, whereas for
all OAR, the maximum and mean dose were distributed between -1.16-2.51%, -1.21-3.12% respectively. For
the second reconstructed dose: the maximum, minimum, and mean dose for all targets was distributed between
0.0995~5.7145%, 0.6892~4.4727%, 0.5829~1.8931% separately. Due to OAR, maximum and mean
dose distribution was observed between -3.1462~6.8920%, -6.9899~1.9316%, respectively.

Conclusion:

Patient-specific QA based on the log-file could reflect the accuracy of the linac execution plan,
which usually has a small influence on dose delivery. When the linac calibration errors were considered, the
reconstructed dose was closer to the actual delivery and the developed method was accurate and practical.