Holupka E J, Humm J L, Tarbell N J, Svensson G K
Department of Radiation Oncology, Harvard Medical School, Boston, MA 02115.
Int J Radiat Oncol Biol Phys. 1993 Sep 30;27(2):345-52. doi: 10.1016/0360-3016(93)90246-r.
The effect of systematic and stochastic setup error on the dose delivered to the gap region for the three field radiation treatment of medulloblastoma is studied. The consequences of such setup error is discussed.
The treatment of medulloblastoma is typically a 3 field technique, in which two lateral cranial fields are matched with a spine field. The x-ray dose delivered to the region between the matched fields depends upon the gap size. The choice of the gap width between the cranial and spinal fields is controversial. It is currently a compromise between minimizing the risk of dose hot spots to the spine, and the associated clinical complications, as well as the magnitude of cold spots (underdosing) across the gap, with the associated risk of disease recurrence. In this paper, we examine the effect of gap width with a moving junction, referred to as "field feathering", on the dose across the field junction for a 6MV photon beam. In addition, we have studied 129 portal films and 40 simulation films to assess the accuracy and precision of patient setup during treatment with a plan involving feathered fields. Selected landmarks observable on both portal and simulation films were identified and the variation in the distances to the field edges measured. The distribution of patient setup error was convoluted with the beam profiles for a 6MV linac. These convoluted field edges were used obtain dose profiles across the gap region as a function of gap separation. The consequences for therapy are discussed. In addition, analysis of patient setup error on an alternative treatment involving beam modifiers to broaden the beam penumbra is discussed.
The magnitude of the spatial stochastic and systematic setup error was determined to be approximately three and two millimeters respectively. The dosimetric consequences of patient setup error lead to over and under dosing in the spinal gap region for the three field technique. The degree of under or over dose depends on the nature and magnitude of the patient setup error.
The effect of patient setup error can lead to significant dosimetric errors in the dose to the gap region depending on the magnitude of the setup errors. The effective over and under dose can be compensated by the use beams modifiers such as a beam spoiler or vibrating jaws.
研究系统性和随机性摆位误差对髓母细胞瘤三野放射治疗中间隙区域剂量的影响，并讨论此类摆位误差的后果。
髓母细胞瘤的治疗通常采用三野技术，即两个侧颅野与一个脊髓野匹配。传递到匹配野之间区域的X射线剂量取决于间隙大小。颅野和脊髓野之间间隙宽度的选择存在争议。目前是在将脊髓剂量热点风险降至最低、相关临床并发症以及间隙处冷点（剂量不足）大小及其相关疾病复发风险之间进行权衡。在本文中，我们研究了移动交接处（称为“野羽化”）的间隙宽度对6MV光子束野交接处剂量的影响。此外，我们研究了129张射野片和40张模拟片，以评估在涉及羽化野的计划治疗期间患者摆位的准确性和精确性。识别了在射野片和模拟片上均可观察到的选定标记物，并测量了到野边缘距离的变化。患者摆位误差的分布与6MV直线加速器的射束轮廓进行卷积。这些卷积后的野边缘用于获取作为间隙间距函数的间隙区域剂量分布。讨论了对治疗的影响。此外，还讨论了对涉及使用射束修饰器以加宽射束半值层的替代治疗中患者摆位误差的分析。
确定空间随机和系统性摆位误差的大小分别约为3毫米和2毫米。患者摆位误差的剂量学后果导致三野技术在脊髓间隙区域出现剂量过高和过低的情况。剂量过高或过低的程度取决于患者摆位误差的性质和大小。
患者摆位误差的影响可能导致间隙区域剂量出现显著的剂量学误差，具体取决于摆位误差的大小。有效剂量过高和过低可通过使用射束修饰器（如射束扰流器或振动钳口）来补偿。
