Technical Note: Penumbral width trimming in solid lung dose profiles for 0.9 and 1.5 T MRI-Linac prototypes

Area of Science:

• Medical Physics
• Radiation Oncology
• Biomedical Engineering

Background:

• Longitudinal magnetic fields can influence radiation beam characteristics.
• Understanding these effects is crucial for advanced radiotherapy techniques, especially in heterogeneous tissues like lung.

Purpose of the Study:

• To investigate the impact of longitudinal magnetic fields (0.9 T and 1.5 T) on beam penumbra in solid water and solid lung phantoms.
• To compare the penumbral width differences between magnetic field (0 T) and non-magnetic conditions.

Main Methods:

• Utilized Gafchromic® EBT3 film for measurements within solid water and solid lung phantoms.
• Employed two magnetic field strengths (0.9 T and 1.5 T) using an MRI-linac prototype.
• Measured penumbra at various depths and field sizes (3x3 cm² and 10x10 cm²) to determine the 80%-20% penumbral width.

Main Results:

• A maximum penumbra narrowing of 4.4 ± 0.1 mm was observed at 1.5 T, compared to 2.5 ± 0.1 mm at 0.9 T.
• Exit profiles showed greater penumbra reduction (up to 2.6 ± 0.2 mm) than entrance profiles.
• The 1.5 T field significantly reduced the difference in penumbral width between solid water and lung phantoms.

Conclusions:

• Longitudinal magnetic fields effectively trim radiation beam penumbra in both solid water and lung tissue.
• The degree of penumbra reduction is field strength-dependent, with 1.5 T yielding a more pronounced effect.
• These findings support the potential of MRI-guided radiotherapy for improved dose conformity in lung treatments.