Optimization of proton pencil beam positioning in collimated fields

Area of Science:

• Medical Physics
• Radiation Oncology
• Particle Therapy

Background:

• Pencil beam scanning (PBS) proton therapy allows lateral field shaping using collimator systems.
• Optimizing collimator shape and spot intensity modulation is crucial for precise dose delivery.

Purpose of the Study:

• To investigate the relationship between spot and collimator edge positions.
• To minimize the lateral penumbra of proton fields by optimizing spot placement.

Main Methods:

• Analytical models were developed to describe spot profiles based on position.
• Theoretical findings were validated using Monte Carlo simulations (TOPAS, RayStation) and radiochromic film measurements.
• In silico experiments explored various conditions, including irradiation in water, range shifters, and 2D fields.

Main Results:

• Placing spots beyond the collimator edge ('overscanning') significantly sharpens the lateral dose fall-off.
• Overscanning reduced the lateral penumbra by approximately 20% in air.
• This effect was confirmed in water, with further improvements seen when combined with fluence modulation.

Conclusions:

• Spot overscanning effectively reduces the lateral penumbra in scanned proton fields.
• This technique can improve sparing of nearby organs at risk, particularly at higher energies and shallower depths.
• Combining spot overscanning with fluence modulation or dynamic collimation offers significant potential for optimizing normal tissue sparing.