Zavgorodni S F
Department of Medical Physics, Royal Adelaide Hospital, South Australia, Australia.
Med Phys. 2000 Apr;27(4):685-90. doi: 10.1118/1.598930.
A number of relocatable head fixation systems have become commercially available or developed in-house to perform fractionated stereotactic radiotherapy (SRT) treatment. The uncertainty usually quoted for the target repositioning in SRT is over 2 mm, more than twice that of stereotactic radiosurgery (SRS) systems. This setup uncertainty is usually accounted for at treatment planning by outlining extra target margins to form the planning target volume (PTV). It was, however, shown by Lo et al. [Int. J. Radiat. Oncol., Biol., Phys. 34, 1113-1119 (1996)] that these extra margins partly offset the radiobiological advantages of SRT. The present paper considers dose calculations in SRT and shows that the dose predictions could be made at least as accurate as in SRS with no extra margins required. It is shown that the dose distribution from SRT can be calculated using the same algorithms as in SRS, with the measured off-axis ratios (OARs) replaced by "effective" OARs. These are obtained by convolving the probability density distribution of the isocenter positions (assumed to be normal) and the original OARs. An additional output correction factor has also been introduced accounting for the isocenter dose reduction (2.4% for a 7 mm collimator) due to the OARs "blurring." Another correction factor accommodates for the reduced (by 1% for 6 MV beam) dose rate at the isocenter due to x-ray absorption in the relocatable mask. Mean dose profiles and the standard deviations of the dose (STD) were obtained through simulating SRT treatment by a combination of normally distributed isocenters. These dose distributions were compared with those calculated using the convolution approach. Agreement of the dose distributions was within 1%. Since standard deviation reduces with the number of fractions, N, as STD/square root(N), the planning predictions in fractionated stereotactic radiotherapy can be made more accurate than in SRS by increasing N and using "effective" OARs along with corrected dose output.
许多可重新定位的头部固定系统已在市场上有售或由内部开发，用于进行分次立体定向放射治疗（SRT）。通常所说的SRT中靶区重新定位的不确定性超过2毫米，是立体定向放射外科手术（SRS）系统的两倍多。这种设置不确定性通常在治疗计划时通过勾勒额外的靶区边界来形成计划靶体积（PTV）加以考虑。然而，Lo等人[《国际放射肿瘤学、生物学、物理学杂志》34卷，1113 - 1119页（1996年）]表明，这些额外的边界部分抵消了SRT的放射生物学优势。本文考虑了SRT中的剂量计算，并表明在不需要额外边界的情况下，剂量预测至少可以和SRS一样准确。结果表明，SRT的剂量分布可以使用与SRS相同的算法来计算，将测量的离轴比（OAR）替换为“有效”OAR。这些“有效”OAR是通过将等中心位置的概率密度分布（假定为正态分布）与原始OAR进行卷积得到的。还引入了一个额外的输出校正因子，以考虑由于OAR“模糊”导致的等中心剂量降低（7毫米准直器时为2.4%）。另一个校正因子用于考虑由于可重新定位面罩中的X射线吸收导致的等中心剂量率降低（6兆伏射线时降低1%）。通过结合正态分布的等中心来模拟SRT治疗，获得了平均剂量分布和剂量的标准偏差（STD）。将这些剂量分布与使用卷积方法计算的结果进行了比较。剂量分布的一致性在1%以内。由于标准偏差随分次次数N的增加而降低，即STD/√N，通过增加N并使用“有效”OAR以及校正后的剂量输出，分次立体定向放射治疗中的计划预测可以比SRS更准确。
