Changes in prescribed doses for the Seattle neutron therapy system.

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Neutron therapy utilizes specialized treatment planning systems for dose distribution calculations.
• The PRISM system at the University of Washington Medical Center has historically calculated neutron dose distributions.
• Accuracy in absorbed dose calculations is critical for effective radiation therapy.

Purpose of the Study:

• To enhance the accuracy of neutron absorbed dose calculations.
• To investigate the impact of algorithmic and data improvements in the PRISM treatment planning system.
• To re-evaluate previously reported neutron therapy prescribed doses based on improved calculations.

Main Methods:

• Modified the PRISM algorithm to include an analytical expression for central axis wedge factor dependence on field size and depth.
• Incorporated a newly commissioned set of measured data, enabling the use of wedge profiles at various depths.
• Compared absorbed dose calculations between the original and improved PRISM algorithms.

Main Results:

• The improved PRISM algorithm demonstrated discrepancies compared to older versions, primarily due to accounting for central axis wedge factor dependence and depth-specific wedge profiles.
• Older versions used a constant central axis wedge factor and a single wedge profile, leading to inaccuracies.
• The new algorithm's ability to use depth-dependent wedge profiles significantly impacted dose calculations.

Conclusions:

• The enhancements to the PRISM system have improved the accuracy of neutron absorbed dose calculations.
• Discrepancies identified highlight the limitations of previous calculation methods.
• Previously reported prescribed doses for neutron therapy require revision due to these findings.