Fiorino Claudio, Gianolini Stefano, Nahum Alan E
Medical Physics, Servizio di Fisica Sanitaria, H S Raffaele, Via Olgettina 60, 20132 Milano, Italy.
Phys Med Biol. 2003 Aug 21;48(16):2603-16. doi: 10.1088/0031-9155/48/16/303.
The calculation of the percentage cumulative histogram of the rectal wall (DWH) in prostate cancer radiotherapy may be subject to large uncertainties due to the difficulty of assessing the wall thickness on CT images. For this reason often only the external contour is used to define the rectum and then the percentage cumulative dose-volume histogram (DVH) of the rectum including any filling is calculated as a 'surrogate' for the DWH. More recently, other approaches using only the external contour have been proposed to estimate the DWH such as the percentage normalized dose-surface histograms (NDSH). A similar concept can be used when considering the solid rectum (the percentage normalized DVH, NDVH). The purpose of this investigation was to assess the relationships between rectal DVH, NDVH, DSH, NDSH and DWH in the common case of three- and four-field techniques in prostate cancer irradiation. Analytical relationships between the above parameters have been derived for a cylindrical rectum model in the case of three- and four-field techniques. The model is applied to the case of an empty rectum, a full rectum and to the more realistic mixed full/empty rectum situation for a four-field technique delivering 76 Gy (ICRU dose) with 18 MV x-rays. Different positions of the lateral beam with respect to the rectum axis were simulated. In the case of no lumen variation along the z-axis, the DWH is found to be very close to the DVH and to the DSH for empty and full rectum, respectively. The largest differences (up to 15%) between DVH and DSH were seen in the high-dose region (>70 Gy). In the more realistic case of lumen variation along the z-axis, the DWH always lies between NDVH and NDSH and, excluding the full-rectum situation, the DWH differs from the DVH by less than 7% in the 50-75 Gy range. In the case of significant portions of rectum being completely shielded, the DVH may differ from the NDVH/NDSH/DWH by up to 10-15%. In most clinical situations NDVH is within a few per cent of DWH, whilst NDSH may differ from DWH by up to 15-20%, especially in the high-dose region (V70). In conclusion, for most situations, the DVH is highly correlated with NDVH and DWH. A high degree of consistency between NDVH and DWH was found in most clinical cases whilst largest deviations between NDSH and DWH were evident in the high-dose region (70-75 Gy). In the less common case of a very full rectum a poorer correlation between DVH/NDVH and DWH was found whilst NDSH mimicked the DWH very well. In summary, except for the case of a 'very full' rectum, NDVH may be used as a robust surrogate for DWH. The DVH seems to be sufficiently robust if the rectum is prevalently empty.
在前列腺癌放疗中，由于在CT图像上评估直肠壁厚度存在困难，直肠壁（DWH）百分比累积直方图的计算可能存在很大的不确定性。因此，通常仅使用外部轮廓来定义直肠，然后计算包括任何充盈情况的直肠百分比累积剂量体积直方图（DVH），作为DWH的“替代”。最近，有人提出了其他仅使用外部轮廓的方法来估计DWH，例如百分比归一化剂量-表面直方图（NDSH）。在考虑实体直肠时（百分比归一化DVH，NDVH）也可使用类似概念。本研究的目的是评估在前列腺癌照射中常见的三野和四野技术情况下，直肠DVH、NDVH、DSH、NDSH与DWH之间的关系。对于三野和四野技术的情况，已推导出上述参数之间的解析关系，适用于圆柱形直肠模型。该模型应用于空直肠、满直肠的情况，以及更实际的四野技术下满/空混合直肠情况，使用18 MV X射线给予76 Gy（ICRU剂量）。模拟了侧野相对于直肠轴的不同位置。在沿z轴无管腔变化的情况下，发现空直肠和满直肠的DWH分别非常接近DVH和DSH。在高剂量区域（>70 Gy），DVH和DSH之间的最大差异（高达15%）。在沿z轴有管腔变化的更实际情况下，DWH始终介于NDVH和NDSH之间，并且在50 - 75 Gy范围内，除了满直肠情况外，DWH与DVH的差异小于7%。在直肠的很大一部分被完全屏蔽的情况下，DVH与NDVH/NDSH/DWH的差异可能高达10 - 15%。在大多数临床情况下，NDVH与DWH相差几个百分点，而NDSH与DWH的差异可能高达15 - 20%，特别是在高剂量区域（V70）。总之，在大多数情况下，DVH与NDVH和DWH高度相关。在大多数临床病例中发现NDVH与DWH高度一致，而在高剂量区域（70 - 75 Gy），NDSH与DWH之间的最大偏差明显。在满直肠这种不太常见的情况下，发现DVH/NDVH与DWH之间的相关性较差，而NDSH与DWH非常相似。总之，除了“非常满直肠”的情况外，NDVH可作为DWH的可靠替代指标。如果直肠主要为空，DVH似乎也足够可靠。
