Experimental pencil beam kernels derivation for 3D dose calculation in flattening filter free modulated fields

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Flattening Filter Free (FFF) linear accelerators (linacs) offer dosimetric advantages but require accurate characterization.
• Previous methods for calculating pencil-beam kernels were limited to ideal flat fluences, struggling with spatially varying energy fluences typical of FFF beams.
• An exact analysis is needed to handle actual fluence variations encountered in FFF beams for precise dose calculations.

Purpose of the Study:

• To develop and validate a method for obtaining pencil-beam kernels for megavoltage photon beams from FFF linacs.
• To implement an exact deconvolution formalism that accounts for spatially varying fluences.
• To assess the accuracy and robustness of the derived kernels for independent dose calculations in modulated fields.

Main Methods:

• Pencil-beam kernels were derived by deconvolution of experimental measurements at various depths using the Hankel transform.
• Kodak EDR2 films were irradiated with a 10 MV FFF photon beam in a water-equivalent phantom.
• A correction was applied to the low-dose portion of the kernel to accurately reproduce experimental output factors.

Main Results:

• The 3D kernel for a FFF beam was successfully obtained with an estimated uncertainty of 0.2% in the derivation procedure.
• Independent dose calculations for eighteen clinically relevant modulated fields showed a high agreement with measurements (gamma-index passing rate >99% for 3%/3mm criteria).
• The new procedure demonstrated improved reliability and robustness compared to previous methods.

Conclusions:

• The developed deconvolution method accurately characterizes FFF photon beams by handling actual fluence variations.
• This approach enables precise and reliable independent dose calculations for complex modulated fields in radiotherapy.
• The method offers a significant advancement for quality assurance and treatment planning in FFF linac-based radiation therapy.