Stereotactic fields shaped with a micro-multileaf collimator: systematic characterization of peripheral dose.

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Stereotactic radiotherapy delivers precise, localized doses.
• However, small doses can reach out-of-field regions, potentially causing patient harm.

Purpose of the Study:

• To systematically measure out-of-field radiation doses from stereotactic radiotherapy.
• To compare these measurements with predictions from commercial treatment planning software (TPS).
• To identify factors influencing out-of-field dose and explore methods for reduction.

Main Methods:

• Dose measurements were performed up to 45 cm from the isocenter for stereotactic fields generated by a BrainLAB mini-multileaf collimator (MMLC) on a Varian 600C linear accelerator.
• Various treatment parameters were varied to assess their impact on dose.
• Measurements were compared against TPS calculations.

Main Results:

• TPS significantly miscalculates out-of-field doses, predicting constant dose beyond 10-15 cm while measurements show a consistent decrease.
• Measured out-of-field doses were 5-10% at 5-10 cm off-axis distance (OAD), 1% at 15 cm OAD, and 0.1% at 45 cm OAD.
• Larger MMLC field sizes and shallower depths increased out-of-field dose; collimator rotation offered significant reduction potential.

Conclusions:

• Commercial TPS is inaccurate for predicting out-of-field doses in stereotactic radiotherapy.
• Peripheral doses, though small, are substantial in radiation protection terms, especially for patients with long life expectancies.
• Optimizing treatment techniques, such as collimator rotation, is crucial for minimizing out-of-field dose and improving patient outcomes.