Measurement of radiotherapy CBCT dose in a phantom using different methods

Area of Science:

• Medical Physics
• Radiation Oncology
• Diagnostic Imaging

Background:

• Cone beam computed tomography (CBCT) is integral to precise patient positioning in modern radiation therapy, particularly for hypofractionated treatments like IMRT and SRT.
• Reported CBCT doses raise concerns about potential radiation tolerance limits for critical organs.
• A lack of standardized CBCT dosimetry protocols necessitates independent verification of manufacturer-stated doses.

Purpose of the Study:

• To investigate and verify dosimetry for CBCT on a Varian Trilogy linear accelerator using established AAPM and IAEA protocols.
• To compare measured CBCT doses with manufacturer-provided values for standard head and body protocols.
• To assess the agreement between multiple dosimetry detection methods.

Main Methods:

• Applied IAEA and AAPM dosimetry approaches to a Varian Trilogy linear accelerator with on-board imager v1.5.
• Utilized three detection methods: 100mm and 300mm pencil ionization chambers, a 0.6 cm³ ionization chamber, and Gafchromic film.
• Performed measurements using custom 45cm and standard 15cm PMMA phantoms simulating head and body exposures.

Main Results:

• Excellent agreement (within 3%) was observed among the different detector systems.
• Measured CBCT doses significantly deviated from Varian's stated doses.
• For the standard head protocol, measured doses were approximately 40% higher than the manufacturer's stated dose.

Conclusions:

• Independent dosimetry verification of CBCT protocols is essential due to potential discrepancies with manufacturer-stated doses.
• The study highlights the critical need for end-users to validate radiation doses delivered by CBCT systems.
• Accurate CBCT dosimetry is vital for ensuring patient safety and optimizing radiation therapy efficacy.