Intensity modulated radiation therapy (IMRT) surface dose measurements using a PTW advanced Markus chamber
Intensity modulated radiation therapy (IMRT) surface dose measurements using a PTW advanced Markus chamber
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DOI：
10.1007/s13246-010-0004-x
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发表时间：
2010-03-01
影响因子：
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通讯作者：
Metcalfe, P.
中科院分区：
文献类型：
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作者：
Chen, F. Q.;Gupta, R.;Metcalfe, P.
一种新的PTW先进的Markus电离室已在IMRT领域实施，用于测量ICRU和ICRP参考深度0.07 mm的表面剂量[ICRU报告39，测定来自外部辐射源的剂量当量，1985；ICRP第60号出版物，1990年，国际放射防护委员会的建议，1991年]。这个腔室有一个小的半径与修订的保护环设计，因此不太容易产生表面过度反应的影响。改进Markus腔的过响应修正量为3.3%，明显小于原Markus腔的过响应修正量10.1%。经过过响应校正后，可以通过数据外推或在Markus箱顶部加一层塑料片保护层来精确测量表面剂量。在小范围的表面剂量测量中，如3 × 3 cm，先进Markus腔的极性效应为12%，明显高于原有Markus腔的5%的极性效应。对于12 x 12 cm的视场尺寸，先进Markus室测量的表面剂量（0.07 mm）在开放视场为19.8%，在无调制步进射击IMRT视场为19.2%。强度调制IMRT场引起的表面剂量变化也进行了研究。对于强度调制的阶梯楔形IMRT场，表面剂量从最高强度段的15.7 +/- 1%到最低强度段的26.9 +/- 1%不等。将腔室测量结果与EBT型GAFCHROMIC (R)薄膜的测量结果进行了比较。
A new PTW advanced Markus ionization chamber has been implemented in IMRT fields, to measure surface dose at ICRU and ICRP reference depth of 0.07 mm [ICRU Report 39, Determination of dose equivalents resulting from external radiation sources, 1985; ICRP Publication 60, 1990 recommendations of the International Commission on Radiological Protection, 1991]. This chamber has a small radius with a revised guard ring design, therefore less prone to surface over-response effects. The over response correction for advanced Markus chamber is 3.3%, which is significantly smaller than 10.1% which was the original Markus chamber over response. After over response correction, the surface dose can be accurately measured by either data extrapolation or by adding one layer of plastic sheet protector to the top of Markus chamber. The surface dose measurements for small fields, e. g 3 x 3 cm, the polarity effect of advanced Markus chamber is 12%, which is significantly higher than the 5% polarity effect of the original Markus. For a 12 x 12 cm field size, surface dose ( at 0.07 mm) measured by advanced Markus chamber is 19.8% for open field and 19.2% for an unmodulated step-and-shoot IMRT field. The variation in surface dose due to intensity modulated IMRT fields has also been investigated. For an intensity modulated, step-wedge IMRT field, surface dose varies from 15.7 +/- 1% for the highest intensity segment to 26.9 +/- 1% for the lowest intensity segment. The results of chamber measurements have been compared against EBT type GAFCHROMIC (R) film results.