Author: Dong Wook Kim, Hojae Kim, Hojin Kim, Jin Sung Kim, Tae Ho Kim, Sangmin Lee 👨🔬
Affiliation: Department of Radiation Oncology, Yonsei Cancer Center, Heavy Ion Therapy Research Institute, Yonsei University College of Medicine 🌍
Purpose: The CyberKnife system, designed for stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT), considers SRS MapCHECK an effective tool for patient-specific QA (PSQA). This work investigates the performance of SRS MapCHECK in CyberKnife PSQA, identifies potential sources of inaccuracy in PSQA analysis, and recommends accurate and reliable use of this tool for small fields.
Methods: SRS MapCHECK is a promising tool for CyberKnife PSQA due to its compact size and high-resolution 2D detector. It enhances dosimetry accuracy by incorporating correction factors for field size and gantry angle. To validate its effectiveness, PSQAs were performed on CyberKnife plans with various fixed cone sizes for brain SRS cases. Static field dose delivery was conducted by rotating robotic arm gantry angles for different fixed cone sizes under various correction settings to assess the impact of the correction factors on dosimetry accuracy.
Results: For PSQAs, dose differences between measurements and TPS calculations were -8.58%, -16.17%, -4.57%, -3.22%, and 0.22% for the 5, 7.5, 10.0, 12.5, and 20.0 mm fixed cones, respectively. Also, reproducing the 20 mm cone plan with smaller sizes (7.5, 10, 12.5, and 15 mm) showed central axis dose differences increasing from 0.22% (20 mm) to -6.58%, -6.39%, and -9.64% (12.5 mm, 10 mm, and 7.5 mm). Static dose delivery experiments revealed that these discrepancies were primarily linked to the correction factors used in SRS MapCHECK. Enabling field size and angular correction factors helped stabilize dosimetry accuracy for larger cone sizes (³15 mm). However, for smaller cones below 15.0 mm, the corrections had limited or even negative effects on dosimetry accuracy.
Conclusion: This work investigated the validity of SRS MapCHECK suited for PSQA of the CyberKnife system. It demonstrated that SRS MapCHECK does not ensure dosimetry accuracy for plans involving fixed cones smaller than 15 mm.