Wegener Sonja, Sauer Otto A
Department of Radiation Oncology, University of Wuerzburg, Wuerzburg, Germany.
J Appl Clin Med Phys. 2018 Nov;19(6):274-281. doi: 10.1002/acm2.12458. Epub 2018 Oct 9.
Relative dose measurements with small ionization chambers in combination with an electrometer placed in the treatment room ("internal electrometer") show a large dependence on the polarity used. While this was observed previously for percent depth dose curves (PDDs), the effect has not been understood or preventable. To investigate the polarity dependence of internal electrometers used in conjunction with a small-volume ionization chamber, we placed an internal electrometer at a distance of 1 m from the isocenter and exposed it to different amounts of scattered radiation by varying the field size. We identified irradiation of the electrometer to cause a current of approximately -1 pA, regardless of the sign of the biasing voltage. For low-sensitivity detectors, such a current noticeably distorts relative dose measurements. To demonstrate how the current systematically changes PDDs, we collected measurements with nine ionization chambers of different volumes. As the chamber volume decreased, signal ratios at 20 and 10 cm depth (M20/M10) became smaller for positive bias voltage and larger for negative bias voltage. At the size of the iba CC04 (40 mm³) the difference of M20/M10 was around 1% and for the smallest studied chamber, the iba CC003 chamber (3 mm³), around 7% for a 10 × 10 cm² field. When the electrometer was moved further from the source or shielded, the additional current decreased. Consequently, PDDs at both polarities were brought into alignment at depth even for the 3 mm³ ionization chamber. The apparent polarity effect on PDDs and lateral beam profiles was reduced considerably by shielding the electrometer. Due to normalization the effect on output values was low. When measurements with a low-sensitivity probe are carried out in conjunction with an internal electrometer, we recommend careful monitoring of the particular setup by testing both polarities, and if deemed necessary, we suggest shielding the electrometer.
在治疗室中使用小型电离室与静电计相结合（“内部静电计”）进行相对剂量测量时，结果显示很大程度上依赖于所使用的极性。虽然之前在百分深度剂量曲线（PDD）中观察到了这种情况，但这种效应尚未得到理解或无法预防。为了研究与小体积电离室配合使用的内部静电计的极性依赖性，我们将一个内部静电计放置在距等中心1米的位置，并通过改变射野大小使其暴露于不同量的散射辐射中。我们发现，无论偏置电压的符号如何，静电计受到照射都会产生约 -1 pA 的电流。对于低灵敏度探测器，这样的电流会明显扭曲相对剂量测量结果。为了证明该电流如何系统性地改变PDD，我们使用九个不同体积的电离室进行了测量。随着电离室体积减小，对于正偏置电压，20厘米和10厘米深度处的信号比（M20/M10）变小；对于负偏置电压，信号比变大。对于iba CC04（40立方毫米）大小的电离室，M20/M10的差异约为1%；对于研究的最小电离室iba CC003室（3立方毫米），在10×10平方厘米射野下差异约为7%。当静电计移离源或进行屏蔽时，额外电流会减小。因此，即使对于3立方毫米的电离室，两种极性下的PDD在深度上也会趋于一致。通过屏蔽静电计，对PDD和侧向射野轮廓的明显极性效应大大降低。由于进行了归一化，对输出值的影响较小。当使用低灵敏度探头与内部静电计一起进行测量时，我们建议通过测试两种极性仔细监测具体设置，如有必要，建议对静电计进行屏蔽。
