Ayyangar K M, Jiang S B
Department of Radiation Therapy, Medical College of Ohio, Toledo 43614, USA.
Med Dosim. 1998 Fall;23(3):161-8. doi: 10.1016/s0958-3947(98)00012-0.
The accuracy of conventional empirical and semi-empirical dose calculation algorithms for radiation therapy treatment planning is limited. The main problem is that these algorithms fail to adequately consider the lateral transport of radiation. Most conventional algorithms use measured dose distribution data as input. These data induce an added inaccuracy to stereotactic radiosurgery dose calculations due to the difficulty of acquiring accurate dosimetric data for very small beams; however, since multiple arcs of large solid angles are usually used in stereotactic radiosurgery, the errors introduced by conventional dose algorithms are quite likely to be diluted. The use of Monte Carlo treatment planning for stereotactic radiosurgery has been investigated and described in the present paper. The OMEGA Monte Carlo code system is used as the dose engine in an in-house developed radiosurgery treatment planning system. The Monte Carlo treatment plans are done for two typical clinical cases. In one case, the collimator of 20 mm diameter is used and the lesion is located in the peripheral part of the brain. In the other case, the collimator diameter is 30 mm and the lesion is in the central part of the brain. The resultant dose distributions are compared with those calculated with a conventional dose algorithm which is based on the standard Tissue Maximum Ratio (TMR)/Off Axis Ratio (OAR) formalism. Without the inhomogeneity correction, the conventional algorithm yields accurate relative dose distributions for both cases compared with the Monte Carlo calculations. The absolute dose at the isocenter may be overestimated by the conventional algorithm by 1.5% for the first case and 2.6% for the second case; however, using the method of ratio of TMRs for inhomogeneity correction, the overestimation can be greatly reduced for both cases. The inclusion of the inhomogeneity correction into the conventional dose algorithm does not alter the relative dose distributions. Based on the clinical cases studied, it may be concluded that the conventional dose algorithm is sufficient for radiosurgery treatment planning and the Monte Carlo based radiosurgery treatment planning is unwarranted.
用于放射治疗治疗计划的传统经验和半经验剂量计算算法的准确性有限。主要问题在于这些算法未能充分考虑辐射的侧向传输。大多数传统算法使用测量的剂量分布数据作为输入。由于获取非常小射束的准确剂量数据存在困难，这些数据会给立体定向放射外科剂量计算带来额外的不准确性；然而，由于立体定向放射外科通常使用多个大立体角的弧，传统剂量算法引入的误差很可能会被稀释。本文对立体定向放射外科使用蒙特卡罗治疗计划进行了研究和描述。OMEGA蒙特卡罗代码系统被用作内部开发的放射外科治疗计划系统中的剂量引擎。针对两个典型临床病例制定了蒙特卡罗治疗计划。在一个病例中，使用直径20毫米的准直器，病变位于脑外周部分。在另一个病例中，准直器直径为30毫米，病变位于脑中央部分。将所得剂量分布与基于标准组织最大比（TMR）/离轴比（OAR）形式的传统剂量算法计算的结果进行比较。在不进行不均匀性校正的情况下，与蒙特卡罗计算相比，传统算法在两种情况下都能产生准确的相对剂量分布。对于第一个病例，传统算法可能会使等中心处的绝对剂量高估1.5%，对于第二个病例高估2.6%；然而，使用TMR比值法进行不均匀性校正，两种情况下的高估都可大大降低。将不均匀性校正纳入传统剂量算法不会改变相对剂量分布。基于所研究的临床病例，可以得出结论，传统剂量算法足以用于放射外科治疗计划，基于蒙特卡罗的放射外科治疗计划没有必要。
