Beam flatness modulation for a flattening filter free photon beam utilizing a novel direct leaf trajectory...

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Flattening filter free (FFF) linear accelerators offer dosimetric advantages like increased dose rate and reduced leakage.
• FFF beams have a forward-peaked fluence distribution, necessitating methods for conventional flat beam delivery.
• Existing literature suggests dedicated FFF units, but this study explores beam modulation instead of physical filters.

Purpose of the Study:

• To develop an effective method for delivering modulated flat beam treatments using FFF technology.
• To extend the direct leaf trajectory optimization (DLTO) model for FFF beams, incorporating machine and multileaf collimator (MLC) constraints.
• To assess beam flatness, machine deliverability, and treatment time efficiency of modulated FFF beams.

Main Methods:

• Developed a novel optimization model extending DLTO for modulated FFF beams.
• Incorporated machine and MLC dynamic delivery constraints using linear constraints and a convex objective function.
• Included the tongue and groove (T&G) effect without introducing nonlinearity or nonconvexity.

Main Results:

• Analyzed regular square fields (10x10 cm² to 40x40 cm²), clinical fields, and concave contours.
• Modulated FFF fields achieved quantitative flatness comparable to or better than open flattening filter (FF) counterparts (mostly <3.0%).
• Modulated FFF beams demonstrated acceptable delivery times, comparable to open FF beams, due to efficiency constraints.

Conclusions:

• Optimized modulated FFF beams using the DLTO model can match the dose uniformity and flatness of conventional FF beams.
• This approach leverages FFF beam characteristics for potential further benefits in radiotherapy.
• The developed model provides an effective way to deliver modulated flat beam treatments without a physical flattening filter.