Quantification of static magnetic field effects on radiotherapy ionization chambers

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Radiation Dosimetry

Background:

• Integrated magnetic resonance (MR) imaging and radiotherapy (RT) systems are emerging, posing challenges for accurate dose measurements.
• Ionization chambers, crucial for dose calibration and traceability, may experience increased uncertainty in MR-linac magnetic fields due to air gaps.

Purpose of the Study:

• To investigate and quantify the impact of air gaps on ionization chamber measurements within a magnetic field.
• To assess the effect of magnetic fields alone on ionization chamber response.

Main Methods:

• Measurements were conducted using cylindrical ionization chambers (PTW 31006, Exradin A1SL, PTW 30013) in a Co-60 beam.
• Custom Perspex phantoms were used to introduce variable air gaps around chambers within a 1.5 T electromagnet.
• Experiments were performed with asymmetric air gaps, defined air gaps, and a water-filled phantom to eliminate air gaps.

Main Results:

• Asymmetric air gaps caused variations up to 8.5±0.2% for the PTW 31006 chamber at 1.5 T.
• The minimum peak-to-peak variation observed with an asymmetric air gap was 1.1±0.1% for the Exradin A1SL.
• A defined air gap resulted in a 3.8±0.2% variation for the PTW 30013 chamber.
• Eliminating air gaps with water reduced the variation for the PTW 30013 chamber to a minimal 0.2±0.01%.

Conclusions:

• Air gaps significantly influence ionization chamber measurements in MR-linac magnetic fields, introducing substantial uncertainty.
• The presence and configuration of air gaps are critical factors affecting dose measurement accuracy.
• Water-filled phantoms effectively mitigate air gap effects, crucial for reliable dose calibration in MR-guided radiotherapy.