SU-E-T-235: Dosimetry Study within a Millimeter from Beam Exiting Air-Tissue Interface Using Film and Monte-Carlo...

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Accurate dose delivery is critical for cell irradiation experiments.
• Dosimetry near air interfaces presents unique challenges due to density changes.
• Well-plate irradiations often involve thin cell layers with air gaps.

Purpose of the Study:

• To assess dose accuracy and uniformity in well-plate cell cultures irradiated with a Cobalt-60 (Co-60) beam.
• To compare dosimetry results obtained from radiochromic film (EBT2) and Monte Carlo simulations.
• To investigate dose variations near the air-interface in a typical cell irradiation setup.

Main Methods:

• Radiochromic film (EBT2) was used to measure doses at various depths (0.1-1.2 mm) upstream from the air interface.
• Film optical density was converted to dose using batch-specific calibration curves.
• Monte Carlo (MC) simulations using DOSXYZNRC code were performed to validate film dosimetry, with high particle counts for precision.

Main Results:

• Film measurements showed doses approximately 16% lower near the air interface compared to depths 1.2 mm and beyond.
• Monte Carlo simulations validated the film dosimetry results within a 2% measurement uncertainty.
• Dose variation was steepest within the first 0.1 mm from the air interface, becoming less pronounced further upstream.

Conclusions:

• Significant dose inhomogeneity exists in the top 1 mm of cell cultures irradiated near an air interface.
• The air interface causes a substantial dose reduction (approx. 16%) in the uppermost cell layers.
• Improved dose homogeneity can be achieved by increasing cell emulsion thickness (>3 mm) or utilizing non-homogeneous cell settling.