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Synopsis of the Experimental and Monte Carlo Calculated Values of the Radial EPOM Shift of Cylindrical Ionization Chambers for Photon-Beam Dosimetry

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D Harder

D Harder1*, H Looe2, B Poppe3, (1)Georg August University, Göttingen, Germany, (2)CArl von Ossietzky University and Pius Hospital, Oldenburg,Germany

SU-E-T-174 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose:
According to the concept of the effective point of measurement (EPOM), the values of depth z serving as the variable of photon-beam depth dose distributions in water measured with cylindrical ionization chambers are commonly corrected according to the formula zeff=zR+?z, were zeff is the depth of the EPOM, zR the depth of the reference point of the chamber, i.e. of its symmetry axis, and ?z the EPOM shift. Agreed-upon values of the EPOM shift are for instance ?z=-0.6r (IAEA TRS 398, 2000) and ?z=-0.5r (DIN6800-2,2008) with r=radius of the air-filled volume. The EPOM concept holds in the falling as well as in the rising curve branch. The ?z versus r relationship is currently reviewed.

Methods:
Measurements of the EPOM shift of cylindrical ionization chambers for 6/15 MV photons had been based upon a comparison with depth dependent dose distributions in water measured with the Roos chamber (PTW Freiburg). Its EPOM position (1.5 mm below the front face) had been determined by comparison with radiochrome films. (Looe et al, Phys.Med.Biol. 2011;56:4267-4290). Available are also the experimental EPOM shift values by Huang et al, Physica Medica 2010;26:126-131 and the classical value of Johansson et al, IAEA -SM-222/35,243-270(1978). Accurate Monte Carlo values had been supplied by Tessier and Kawrakow, Med.Phys. 2010;37:96-107.

Results:
As shown by the graphical display of ?z versus r, the relationship is nonlinear, shaped as a hockey stick, with values around -0.2 mm for the "pinpoint" chambers and with values near -1.4 mm for the often-used chambers with radii near 3 mm. For r ranging from 1 to 4 mm, the relationship can be approximated by ?z = -(2.25 mm)x[1-exp(-0.0122 r^4)] with r in mm.

Conclusions:
Time has come for a nonlinear updating of the EPOM shift versus radius relationship of cylindrical ionization chambers applied in photon-beam dosimetry.


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