Frazier A, Du M, Wong J, Vicini F, Taylor R, Yu C, Matter R, Martinez A, Yan D
Department of Radiation Oncology, William Beaumont Hospital, Royal Oak, MI 48073, USA.
Int J Radiat Oncol Biol Phys. 1995 Dec 1;33(5):1229-38. doi: 10.1016/0360-3016(95)00203-0.
The goal of this study was to evaluate the dosimetric characteristics of geometric MLC prescription strategies and compare them to those of conventional shielding block.
Circular fields, square fields, and 12 irregular fields for patients with cancer of the head and neck, lung, and pelvis were included in this study. All fields were shaped using the MLC and conventional blocks. A geometric criterion was defined as the amount of area discrepancy between the MLC and the prescription outline. The "least area discrepancy" (LAD) of the MLC conformation was searched by selecting the collimator angle, meanwhile keeping a preselected position along the width of the leaf into the prescribed field. Five LAD conventions were studied. These included the LAD-0, LAD-1/3, LAD-1/2, and LAD-2/3 that inserted the leaves at the 0, 1/3, 1/2, and 2/3 of the leaf end into the prescription field, respectively. In addition, the LAD optimization was applied to the transecting (TRN) approach for leaf conformation that prescribed an equal area of overblocking and underblocking under each leaf. Film dosimetry was performed in a 20 cm polystyrene phantom at 10 cm depth 100 cm from source to axis distance (SAD) for both 6 and 18 MV photons with each of the above MLC conformations and conventional blocks. The field penumbra width, defined as the mean of the separation between the 20% and 80% isodose lines along the normal of the prescription field edge, was calculated using both the MLC and conventional block film dosimetry and compared. In a similar way, the d20 is defined as the mean separation between the 20% isodose line and the prescription field edge, and the d80 is defined as the mean separation between the 80% isodose line and the prescription field edge.
The field penumbra width for all MLC conventions was approximately 2 mm larger than that of the conventional block. However, there was a larger variation of the separation distribution in the penumbra region of the irregular fields for the MLC, which had a standard deviation of 1 mm (a factor of 5 larger than the conventional block). The dosimetry for the circular fields showed that the LAD-TRN, LAD-1/2, and LAD-1/3 approximated the conventional blocking well in terms of d20 and d80; however, no single convention produced the best conformation for both measures. The dosimetric result of the patient treatment fields was similar for all sites. The LAD-1/3, LAD-1/2, and LAD-TRN strategies conformed to within 1 to 1.5 mm of the d80 of the conventional block for both 6 MV and 18 MV, respectively. The LAD-1/2 and LAD-TRN conformations were virtually identical, although it is proven analytically that the LAD-1/2 convention has the least overall area discrepancy of all conventions.
The five MLC conformation conventions resulted in similar dosimetric penumbrae for all field shapes studied. The LAD-1/3, LAD-TRN, and LAD-1/2 produced the more favorable approximation to conventional block. The field penumbra width, although useful for evaluating irregular field shapes, could not describe the large local variations in the penumbra along the field edge for the MLC. These local variations could be of clinical concern when they appear near vital organs. However, the variation in a local region can potentially be reduced by minimizing the jaggedness of the leaf steps in that local region. The dosimetric results were useful as guidelines for the clinicians in the evaluation and adjustment of MLC leaf positions.
本研究的目的是评估几何多叶准直器（MLC）处方策略的剂量学特征，并将其与传统屏蔽挡块的剂量学特征进行比较。
本研究纳入了圆形野、方形野以及针对头颈部、肺部和骨盆部癌症患者的12个不规则野。所有野均采用MLC和传统挡块进行塑形。将几何标准定义为MLC与处方轮廓之间的面积差异量。通过选择准直器角度，同时保持叶片沿宽度方向在预定野内的预选位置，来搜索MLC构象的“最小面积差异”（LAD）。研究了五种LAD惯例。其中包括LAD-0、LAD-1/3、LAD-1/2和LAD-2/3，它们分别将叶片末端的0、1/3、1/2和2/3处的叶片插入处方野。此外，LAD优化应用于叶片构象的横切（TRN）方法，该方法规定在每片叶片下的过挡和欠挡面积相等。在20 cm聚苯乙烯模体中，在距源轴距（SAD）100 cm、深度10 cm处，对上述每种MLC构象和传统挡块的6和18 MV光子进行了胶片剂量测定。通过MLC和传统挡块胶片剂量测定计算并比较了野半值层宽度，野半值层宽度定义为沿处方野边缘法线方向20%和80%等剂量线之间的平均间距。以类似方式，d20定义为20%等剂量线与处方野边缘之间的平均间距，d80定义为80%等剂量线与处方野边缘之间的平均间距。
所有MLC惯例的野半值层宽度比传统挡块大约2 mm。然而，MLC的不规则野半值层区域的间距分布变化更大，其标准差为1 mm（比传统挡块大5倍）。圆形野的剂量测定表明，LAD-TRN、LAD-1/2和LAD-1/3在d20和d80方面与传统挡块拟合良好；然而，没有一种惯例能在这两种测量中产生最佳构象。所有部位患者治疗野的剂量测定结果相似。LAD-1/3、LAD-1/2和LAD-TRN策略在6 MV和18 MV时分别与传统挡块的d80拟合在1至1.5 mm范围内。LAD-1/2和LAD-TRN构象实际上是相同的，尽管经分析证明LAD-1/2惯例在所有惯例中总体面积差异最小。
对于所研究的所有野形状，五种MLC构象惯例产生了相似的剂量学半值层。LAD-1/3、LAD-TRN和LAD-1/2对传统挡块的近似效果更优。野半值层宽度虽然有助于评估不规则野形状，但无法描述MLC沿野边缘半值层的大的局部变化。当这些局部变化出现在重要器官附近时，可能会引起临床关注。然而，通过最小化该局部区域叶片台阶的锯齿状程度，有可能减少局部区域的变化。剂量测定结果可作为临床医生评估和调整MLC叶片位置的指导。
