Analysis of patient-specific quality assurance for Elekta Unity adaptive plans using statistical process control...

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Elekta Unity MR-linac enables daily MRI for online adaptive radiotherapy.
• Adaptive planning options include adapt to position (ATP) and adapt to shape (ATS).
• Adapted plans are generated during treatment, precluding pre-delivery measurement.

Purpose of the Study:

• To assess the quality assurance (QA) of adaptive intensity modulated radiation therapy (IMRT) plans generated on the Elekta Unity MR-linac.
• To evaluate the performance of ATP and ATS adaptive strategies using statistical process control.
• To investigate the impact of fractionation (SBRT vs. conventional) and plan complexity on IMRT QA pass rates.

Main Methods:

• Utilized statistical process control methodology on 512 patient-specific IMRT QA measurements.
• Employed MR-compatible SunNuclear ArcCheck with a 3%/2 mm gamma criterion, global normalization, and 10% low dose threshold.
• Determined lower control limit (LCL) from 68 reference plans and assessed pass rates for 432 ATP and 80 ATS plans using a one-sided process capability ratio ($$).

Main Results:

• The LCL for reference plans was a gamma pass rate of 0.958.
• The $$ for ATP plans was 1.403 and for ATS plans was 0.940.
• SBRT and conventional fractionation $$ were 0.902 and 1.383, respectively. No correlation between modulation degree and gamma pass rate was found, but beam-averaged aperture area showed a significant correlation.

Conclusions:

• All adaptive plans met TG-218 guidelines, demonstrating the feasibility of online adaptive IMRT on the Elekta Unity.
• ATS and SBRT plans exhibited a trend towards smaller beam-averaged aperture areas and slightly lower gamma pass rates.
• Beam-averaged aperture area is a significant factor influencing IMRT QA pass rates in adaptive radiotherapy.