Technical note: Absolute dose measurements of a vault-free radiosurgery system

Area of Science:

• Medical Physics
• Radiotherapy
• Dosimetry

Background:

• Accurate absolute dose (AD) calibration is vital for radiotherapy machine function.
• Many systems require modifications to meet TG-51 standards.
• TG-21 calibration offers a viable alternative with appropriate setup and factors.

Purpose of the Study:

• To evaluate modified TG-21 and TG-51 protocols for AD calibrations.
• To assess these protocols on the ZAP-X radiosurgery system.
• To utilize ion chambers, film, and thermoluminescent dosimeters (TLDs) for evaluation.

Main Methods:

• Utilized a custom acrylic phantom based on TG-21 and TRS-398 recommendations.
• Calculated AD using N_x and TG-21 parameters with a PTW 31010 chamber.
• Performed measurements in a PTW MP3-XS water tank using TG-51 factors.
• Verified dose with Gafchromic film and TLDs.

Main Results:

• Measurements from TG-51, modified TG-21, film, and TLDs showed agreement within ±2%.
• This indicates high consistency across different calibration methods.

Conclusions:

• Modified TG-51 in water is preferred but can be impractical due to setup challenges.
• The modified TG-21 protocol with a custom acrylic phantom is an accurate and practical alternative for ZAP-X dose calibration.
• Both methods provide confidence in the system's absolute dose calibration.