Efficient independent planar dose calculation for FFF IMRT QA with a bivariate Gaussian source model.

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Flattening filters (FF) in photon beams significantly contribute to head scatter.
• Flattening filter-free (FFF) beams offer potential advantages in radiotherapy but require accurate dose calculation methods for quality assurance (QA).
• Independent dose calculation algorithms are crucial for verifying treatment plans in intensity-modulated radiotherapy (IMRT).

Purpose of the Study:

• To directly compare source models for photon beams with and without flattening filters (FF).
• To develop an efficient, independent algorithm for planar dose calculation in flattening filter-free (FFF) intensity-modulated radiotherapy (IMRT) quality assurance (QA).

Main Methods:

• Developed a source model using a point source and Gaussian functions for primary photons and scatter.
• Calculated IMRT beam fluence using backprojection and integration; modeled FFF off-axis ratios with a polynomial.
• Employed an analytical kernel (sum of Gaussian functions) for dose deposition and convolution for ionization chamber volume averaging.

Main Results:

• Achieved good agreement for in-air output factors (S_c) in both FF (<0.25%) and FFF (<0.10%) beams.
• Demonstrated significant reduction in head-scattered photons with FF removal (34.7% at 6 MV, 49.3% at 10 MV).
• Validated the algorithm with FFF head-and-neck IMRT plans, achieving high passing rates (96.2% for 6 MV, 95.5% for 10 MV) with a 2%/2 mm gamma criterion.

Conclusions:

• The developed source model accurately represents FF and FFF photon beams.
• The efficient, independent planar dose calculation algorithm is suitable for FFF IMRT QA.
• This algorithm facilitates accurate and reliable quality assurance for FFF IMRT.