How flatbed scanners upset accurate film dosimetry

Area of Science:

• Medical Physics
• Radiological Imaging
• Dosimetry

Background:

• Film dosimetry offers high spatial resolution for dose distribution verification.
• Flatbed scanners are cost-effective for digitizing irradiated film.
• Lateral Scan Effect (LSE) is a known disadvantage of flatbed scanners, altering readout along the scan axis.

Purpose of the Study:

• To investigate the causes of LSE in flatbed scanners digitizing Gafchromic film.
• To differentiate LSE origins from anisotropic light scattering, exploring crosstalk, optical path length, and polarization.
• To quantify the impact of LSE on dose measurements.

Main Methods:

• Investigated LSE on Epson 1680 Expression Pro and 10000XL scanners using Gafchromic EBT, EBT2, and EBT3 films.
• Examined crosstalk using neutral density filters and polarization effects using linear polarizers.
• Measured film optical density (OD) from 0.25 to 1.1 (0.2 to 9 Gy) in transmission mode using RGB channels.

Main Results:

• LSE magnitude depends on scanner and film type, increasing up to 14% at maximum lateral positions for 9 Gy.
• Crosstalk was significant (up to 2%) only in high-contrast regions for small fields.
• Optical path length effects caused up to 3% deviation in extreme lateral positions.
• Light polarization emerged as the main LSE contributor, varying across RGB channels.

Conclusions:

• Gafchromic EBT films digitized with flatbed scanners exhibit optical effects including LSE.
• Accurate dosimetry necessitates correction for LSE, requiring determination per color channel and delivered dose.
• Understanding and correcting for LSE is crucial for reliable film dosimetry verification.