全身电子束治疗中电子束的阻止本领比。

Stopping-power ratios for electron beams used in total skin electron therapy.

机构信息

Department of Radiation Oncology, Vanderbilt University School of Medicine, Nashville, Tennessee, USA.

出版信息

Med Phys. 2021 Sep;48(9):5472-5478. doi: 10.1002/mp.15121. Epub 2021 Aug 4.

DOI:10.1002/mp.15121

PMID:34287969

Abstract

PURPOSE

The electron beams for total skin electron therapy (TSET) are often degraded by a scatter plate in addition to extended distances. For electron dosimetry, both the AAPM TG-51 and IAEA TRS-398 recommend the use of two formulas developed by Burns et al [Med. Phys. 23, 489-501 (1996)] to estimate the water-to-air stopping-power ratios (SPRs). Both formulas are based on a fit to SPRs calculated for standard electron beams. This study aims to find: (1) if the formulas are applicable to beams used in TSET and (2) the impact of the ICRU report 90 recommendations on the SPRs for these beams.

METHODS

The EGSnrc Monte Carlo code system is used to generate 6 MeV high dose rate total skin electron (HDTSe) beams used in TSET. The simulated beams are used to calculate dose distributions and SPRs as a function of depth in a water phantom. The fitted SPRs using the empirical formulas are compared with MC-calculated SPRs.

RESULTS

The electron beam quality specifier, the depth in water at which the absorbed dose falls to 50% of its maximum value, R , decreases approximately 1 mm for each additional 100-cm extended distance ranging from 2.24 cm at SSD = 100 to 1.72 cm at SSD = 700 cm. For beams passing through a scatter plate, R is 1.76 cm (1.14) at SSD = 300 and 1.48 cm (0.85 cm) at SSD = 600 cm with an Acrylic plate thickness of 3 mm (9 mm), respectively. The discrepancy between fitted and MC-calculated SPRs at d as a function of R is <0.8%, and in many cases <0.4%. The difference between fitted and MC-calculated SPRs as a function of depth and R is within 1% at depths <0.8R for beams with R ≥ 1.14 cm. The ICRU-90 recommendations decrease SPRs by 0.3%-0.4% compared to the use of data recommended in ICRU-37.

CONCLUSION

The formulas used by the major protocols are accurate enough for clinical beams used in TSET and the error caused using the formulas is <1% to estimate SPRs as a function of depth and R for depths <0.8R for beams used in TSET with R ≥ 1.14 cm. The impact of the ICRU-90 recommendations shows a decrease of SPRs by a fraction of a percent for beams used in TSET.

摘要

目的

总皮肤电子治疗（TSET）的电子束除了延长距离外，通常还会被散射板衰减。对于电子剂量学，AAPM TG-51 和 IAEA TRS-398 都建议使用 Burns 等人开发的两个公式[Med. Phys. 23, 489-501 (1996)]来估计水-空气阻止本领比（SPRs）。这两个公式都是基于对标准电子束计算出的 SPRs 的拟合。本研究旨在确定：（1）这些公式是否适用于 TSET 中使用的光束，以及（2）ICRU 报告 90 号建议对这些光束的 SPRs 的影响。

方法

使用 EGSnrc 蒙特卡罗代码系统生成用于 TSET 的 6 MeV 高剂量率总皮肤电子（HDTSe）束。模拟的光束用于计算水模中深度处的剂量分布和 SPRs。使用经验公式拟合的 SPRs 与 MC 计算的 SPRs 进行比较。

结果

电子束质量指定器，即吸收剂量下降到其最大值的 50%的水深处，R ，从 SSD=100 处的 2.24cm 左右每增加 100cm 延长距离大约减少 1mm，到 SSD=700cm 处的 1.72cm。对于穿过散射板的光束，当 SSD=300 时，R 为 1.76cm（1.14cm），当 SSD=600cm 时，R 为 1.48cm（0.85cm），分别使用厚度为 3mm（9mm）的亚克力板。d 处拟合值和 MC 计算值之间的 SPRs 差异<0.8%，在许多情况下<0.4%。对于 R≥1.14cm 的光束，在深度<0.8R 处，拟合值和 MC 计算值之间的 SPRs 差异在 1%以内。与使用 ICRU-37 中推荐的数据相比，ICRU-90 建议将 SPRs 降低 0.3%-0.4%。