TA30-Independent Verification of IMRT Delivered With a Multileaf Collimator
Memorial Sloan-Kettering Cancer Center, New York City, New York.
Introduction: Independent verification is an important component in radiation therapy quality assurance. In this presentation, we describe computational and empirical methods for independent verification of intensity modulated radiation therapy (IMRT) delivered with a multileaf collimator (MLC). The methods presented are applicable to both dynamic and segmental delivery.
Methods: The computational algorithms consist of two parts. The first part calculates the delivered intensity distribution in-air, and the second part calculates the absolute dose distribution in a homogeneous phantom. The effects included in the intensity calculation are direct exposure, mid-leaf and interleaf transmissions, rounded leaf-end, tongue-and-groove, extra-focal source distribution, and scatter from the leaves. Dose calculation in phantom is based on measured data, including the output factors, tissue-maximum ratios, and off-center ratios. The effect of intensity modulation is accounted for by pencil beam convolution. Empirical measurements utilize ion chambers and film. Chamber measurement is considered as the standard, but is limited to point measurement only. Two dimensional distributions are measured with film. Special care is needed for film dosimetry to correct for film over response due to the low energy scattered photons.
Results: The accuracy of the computational algorithms has been verified with chamber and film measurements for a variety of IMRT fields ranging from simple patterns to clinical intensity-modulated fields. These fields cover a wide range of field sizes and varying degrees of intensity modulation. The majority of the dose comes from direct exposure. Significant contribution also comes from mid-leaf transmission due to the large amount of time spent by part of the field under the MLC. The effects of interleaf transmission and tongue-and-groove effects can be predicted by the algorithms. For very large fields, scatter from the MLC may contribute up to 5%. For film measurement, the effect of film over response was analysed by Monte Carlo methods and can be included in film dosimetry. The agreement between calculation and measurement in general was within 2-3% in dose, or within 1mm in distance in high dose gradient regions. Typical calculation time per beam is less than 30 seconds on a 266 MHz Alpha station.
Summary: Computational and empirical methods have been developed for independent verification of IMRT treatment. The agreement between calculated and measured results is typically within 2-3% in dose or 1mm in distance. Calculation is now used for routine independent check, while empirical verification is performed when new conditions arise, such as new disease sites, new treatment techniques, or new software release.
TA30－使用多叶准直器实施调强放射治疗的独立验证
Chen-Shou Chui, Thomas LoSasso, ans Asa Palm
Memorial Sloan-Kettering Cancer Center, New York City, New York.
引言：独立验证是放射治疗质量保证中的重要组成部分。在本报告中，我们将讲述用于多叶准直器调强放射治疗独立验证中的计算方法和经验方法。这些方法同时适用于动态调强和静态调强。
方法：计算的算法由两部分组成。第一部分计算空气中的强度分布，第二部分计算均匀模体中的绝对剂量分布。强度计算中考虑了直接辐射、一个叶片中间和叶片间透射、弧形端面、凸凹槽效应、焦点外源分布和来自叶片的散射等因素。模体中的剂量计算是根据输出因子、组织最大剂量比和离轴比等测量数据进行的。通过笔形束卷积考虑调强的效应。经验性测量使用电离室和胶片进行。电离室测量被认为是金标准，但它只限于点剂量测量。剂量的二维分布用胶片测量。对于胶片剂量计，由于低能散射光子的存在，要特别注意校正胶片的过度响应。
结果：从简单强度分布的调强射野到临床的调强射野，通过电离室和胶片测量验证了算法的准确性。这些射野包含了很多的射野大小和不同的强度分布。大部分剂量来自于直接辐射。由于多叶准直器形成射野照射时间较长，有相当一部分剂量来自于叶片中间的透射剂量。算法可以预测叶片间透射和凸凹槽效应。对于很大的射野，多叶准直器散射线的贡献可达5%。对于胶片测量，胶片过度响应的效应通过蒙特卡罗方法分析并包含在胶片剂量计算中。计算和测量的剂量差异一般在2-3%，高梯度区距离差异在1mm以内。使用266 MHz Alpha 工作站，每个射野的计算时间通常在30秒以内。
结论：我们开发了用于调强放射治疗独立验证的计算方法和经验方法。计算和测量的剂量差异一般在2-3%左右，高梯度区距离差异在1mm以内。目前，常规独立验证使用计算的方法，当出现新情况（比如刚开始治疗某一肿瘤时、使用新的治疗技术时或使用新的软件版本时）进行经验验证。