ACPSEM position paper: dosimetry for magnetic resonance imaging linear accelerators

Area of Science:

• Medical Physics
• Radiation Oncology
• Imaging Technology

Background:

• Accurate dosimetry is critical for patient outcomes and inter-center dose comparisons.
• Magnetic Resonance Imaging Linac (MRI-linac) systems are emerging in Australasian clinics, necessitating updated dosimetry guidelines.
• Existing dosimetry protocols (e.g., IAEA TRS-398) require adaptation for MRI-linac systems due to unique physical characteristics.

Framework:

• Recommendations focus on ion chamber dosimetry for reference measurements in MRI-linac units.
• MR-safe or MR-conditional water phantoms are required, considering practical setup limitations.
• Solid phantoms are not recommended for reference dosimetry in this context.

Implementation:

• Two MRI-linac configurations (perpendicular and parallel magnetic and radiation fields) impact dose deposition and require specific considerations.
• Ion chamber orientation relative to the magnetic field and radiation beam significantly affects chamber response.
• Beam quality specification using TPR_20,10 is recommended.

Implications:

• Specific corrections for beam quality and magnetic field effects on dosimeter response are proposed.
• Manufacturers' guidance on reference conditions should be followed.
• These recommendations are for a developing field, emphasizing the need for ongoing literature review.