A case study on SSD to SAD linear acceleartor calibration transition

Area of Science:

• Medical Physics
• Radiation Oncology
• Quality Assurance

Background:

• Linear accelerator calibration is critical for accurate radiation therapy.
• Transitioning calibration conditions (e.g., source-to-surface distance [SSD] to source-to-axis distance [SAD]) is infrequent and carries significant risk.
• Standardizing calibration protocols across multiple clinical sites enhances consistency and safety.

Purpose of the Study:

• To outline a systematic approach for transitioning a linear accelerator's calibration from SSD to SAD in an active clinical setting.
• To maintain treatment accuracy and avoid interruptions to patient care during the calibration change.
• To improve standardization of calibration practices across different sites within a radiation oncology service.

Main Methods:

• A single Elekta Versa HD linear accelerator was converted from SSD to SAD calibration.
• Pre-conversion evaluation of treatment plans and post-conversion beam recommissioning in the treatment planning system (TPS).
• Output adjustment per AAPM's TG-51 protocol, manual scaling of monitor units (MUs), and rigorous quality assurance (QA) checks were performed. A failure modes and effects analysis (FMEA) and a professional survey were conducted.

Main Results:

• The transition was completed without any interruption to patient treatments.
• Field MU scaling ranged from 2.7% to 6.4% due to the calibration change.
• Patient-specific QA and phantom measurements confirmed machine output accuracy within 2%, with consistent gamma pass rates. FMEA identified staffing as a high-risk factor, and surveys revealed low experience with such transitions.

Conclusions:

• The successful transition from SSD to SAD calibration was achieved while maintaining treatment continuity and safety.
• Multiple verification steps, including QA and peer review, were crucial for ensuring the quality and safety of the procedure.
• This project facilitated improved standardization of calibration conditions across multiple practice sites.