Stereotactical fields applied in proton spot scanning mode with range shifter and collimating aperture

Area of Science:

• Medical Physics
• Radiation Oncology
• Particle Therapy

Background:

• Achieving sharp lateral dose gradients is crucial for specific clinical indications in proton beam therapy.
• Low-range proton fields delivered via spot scanning often exhibit large lateral gradients due to beamline profiles or range shifter blocks.

Purpose of the Study:

• To investigate a novel method for creating sharp lateral dose gradients in proton beam therapy.
• To evaluate the effectiveness of shifting range shifting blocks upstream combined with a collimating aperture.

Main Methods:

• Experiments utilized a commercial proton therapy treatment head in spot-scanning mode.
• A range shifter was placed upstream, downstream of scanning magnets, in a slot typically used for scattering modes.
• Lateral dose planes were measured using scintillation screens and radiochromic films.
• Dose distributions were calculated using the Monte Carlo dose engine in the RayStation treatment planning system.

Main Results:

• Proton fields with 80%-20% lateral dose fall-off between 1.4 mm and 4.0 mm were achieved for depths of 0-10 cm water equivalent.
• Simulated lateral dose profiles closely matched experimental measurements.
• Field edge sharpening was accompanied by proton spot broadening towards the field center.

Conclusions:

• The upstream shifting of range shifting blocks effectively sharpens proton beam field edges.
• This technique is primarily suitable for small fields where distal and proximal conformality are less critical.
• Further research may explore optimizing this method for broader clinical applications.