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Dosimetry Study Within a Millimeter From Beam Exiting Air-Tissue Interface Using Film and Monte-Carlo Simulation On a Co-60 Beam

R Tailor

R Tailor*, J Yang, MD Anderson Cancer Center, Houston, TX

SU-E-T-235 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Objective is to assess dose accuracy and uniformity to cells in a well-plate irradiated on Co-60 beam using film (EBT2) and Monte-Carlo simulation.
Situations are encountered where dosimetry near beam-exiting air-interface is needed. Typical example is irradiation of cells in well-plate. Small amount of cell emulsion may amount to as small as 1mm thickness which results in leaving large air space above the cells. Due to contamination concerns, air space above cells can not be avoided. Preferred geometry for irradiation is to place cell-plate on a build-up slab (water-equivalent), and point the beam upward. On Co-60 beam, film EBT2 was employed to measure doses 0.1, 0.4, 0.7, 1.2 mm upstream from air-interface. Film optical-density (OD) was measured with a pocket densitometer, and converted to dose using film-calibration specific to film-batch. To validate film data, Monte-Carlo code DOSXYZNRC was employed for simulation. Doses were calculated employing (i) calculation grid 0.1-1.0 mm along beam direction, and (ii) 15000 million incident particles to achieve < 0.2% precision.
Doses measured with film close to beam-exiting air-interface are ~16% lower than the maximum doses 1.2 mm up-stream and beyond. Monte-Carlo calculations validate the doses measured with film within measurement uncertainty (2%). Dose variation is steep in the first 0.1 mm zone near air-interface, but it is within 10% further up-stream.
In cell irradiations, dose in-homogeneity in the top 1mm zone is significant. Layer close to air-interface receives ~16% lower dose than the layer 1.2mm up-stream. Dose homogeneity with the stated geometry can be improved by having (i) larger amount of cells to provide emulsion thickness > 3mm, and (ii) non-homogeneous emulsion for cells to settle at the bottom. Since dose in-homogeneity is primarily due to air interface, results are expected to be qualitatively similar for other beams (Cs-137 or 6 MV.

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