Harder D, Looe H, Poppe B
Georg August University, Göttingen, Germany.
CArl von Ossietzky University and Pius Hospital, Oldenburg, Germany.
Med Phys. 2012 Jun;39(6Part12):3743. doi: 10.1118/1.4735232.
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 Az 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 Az versus r relationship is currently reviewed.
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.
As shown by the graphical display of Az 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)×[1-exp(-0.0122 r̂4)] with r in mm.
Time has come for a nonlinear updating of the EPOM shift versus radius relationship of cylindrical ionization chambers applied in photon-beam dosimetry.
根据有效测量点（EPOM）的概念，作为用圆柱形电离室测量的水中光子束深度剂量分布变量的深度z值，通常根据公式zeff = zR +△z进行校正，其中zeff是EPOM的深度，zR是电离室参考点（即其对称轴）的深度，△z是EPOM偏移。例如，EPOM偏移的商定值为△z = -0.6r（国际原子能机构TRS 398，2000）和△z = -0.5r（DIN6800 - 2，2008），其中r =充气体积的半径。EPOM概念适用于下降曲线分支和上升曲线分支。目前正在审查△z与r的关系。
对于6/15 MV光子，圆柱形电离室的EPOM偏移测量基于与用Roos电离室（PTW弗莱堡）测量的水中深度相关剂量分布的比较。其EPOM位置（前表面下方1.5 mm）通过与放射性色胶片比较确定（Looe等人，《物理医学与生物学》2011年；第５６卷，第4267 - 4290页）。Huang等人（《物理医学》2010年；第26卷，第126 - 131页）的实验EPOM偏移值以及Johansson等人（国际原子能机构-SM - 222/35，第243 - 270页（1978年））的经典值也可获得。Tessier和Kawrakow（《医学物理》2010年；第37卷，第96 - 107页）提供了精确的蒙特卡罗值。
如△z与r的图形显示所示，该关系是非线性的，形状像曲棍球棒，“针尖”电离室的值约为 -0.2 mm，半径接近3 mm的常用电离室的值接近 -1.4 mm。对于r在1至4 mm范围内，该关系可以用△z = -(2.25 mm)×[1 - exp(-0.0122 r̂4)]近似，其中r以mm为单位。
是时候对用于光子束剂量测定的圆柱形电离室的EPOM偏移与半径关系进行非线性更新了。
