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Comparisonof Head Scatter Factor by Different Ion-Chambers in Flattened and Unflattened Beams

T Miwa1,2*, M Oita2 , H Aoyama3 , (1) Department of Radiological Technology, Seitetsu Hirohata Hospital,Japan, (2) Graduate school of Health Sciences, Okayama University, Japan, (3) Department of Radiological Technology,Okayama University Hospital, Japan


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: The goal of this study was to evaluate the head scatter factors using the different combination of ionization chambers and buildup-cap (mini-phantom) in flattened and unflattened photon beams.

Methods: An Elekta Infinityᵀᴹ linear accelerator(Elekta Inc.) was used for this study. Measurements were done using a 0.6 cc Farmer-type, a 0.13 cc cylindrical, 0.04 cc clindrical ionization chambers. For in-water output factors(Scp) measurements, the chamber was positioned at a depth of dose maximum, 5 g・cm�² and 10 g・cm�². The source to chamber distance was maintained at 100 cm for field sizes ranging from 2×2 cm² to 40×40 cm². For in-air output factors(Sc) measurements, a brass build-up cap with the thickness of 2.4 mm and 4.0 mm for 6-MV and 10-MV. And an acrylic mini-phantom with thickness of the 10 g・cm�² were used, respectively. The chambers were positioned at 100 cm source-to-axis distance (SAD) for field sizes ranging from 2 × 2 cm² to 40 × 40 cm².

Results: The Scp values of both FF and FFF beams measured by all ionization chambers at a depth of dose maximum had similar characteristics except for much smaller or larger fields. The differences of the Scp also varied at a deeper depth in such fields. The Sc values of FF beams measured by the Farmer, CC13 and CC04 ionization chambers with mini-phantom were within 0.3 %, 0.2 % for 6-MV, 10-MV, respectively. By the same token, the Sc values of FFF beams were within 0.3 %, 0.3 % for 6-MV, 10-MV, respectively.

Conclusion: In this study, the head scatter factors derived from the different ionization chambers with buildup-cap(mini-phantom) had similar characteristics both in flattened and unflattened photon beams except for much smaller or larger fields.


Head Scatter, Dosimetry, Flattening Filters


TH- External beam- photons: Small/nonstandard field experimental dosimetry

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