接受预防性颅脑照射的儿童的甲状腺剂量。
Thyroid dose in children undergoing prophylactic cranial irradiation.
作者信息
Stevens G, Downes S, Ralston A
机构信息
Department of Radiation Oncology, Royal Prince Alfred Hospital, Sydney, Australia. gstevensradonc.rpa.cs.nsw.gov.au.
出版信息
Int J Radiat Oncol Biol Phys. 1998 Sep 1;42(2):385-90. doi: 10.1016/s0360-3016(98)00222-3.
PURPOSE
To determine the radiation dose received by the thyroid gland as a result of prophylactic cranial irradiation (PCI) in childhood leukemia and the factors influencing that dose.
METHODS AND MATERIALS
The dose to the thyroid resulting from simulated cranial irradiation with parallel opposed lateral fields of an adult anthropomorphic (ART) phantom with both 6 MV X-rays and Cobalt-60 gamma-rays was measured using thermoluminescent dosimeters (TLDs). The dependence of thyroid dose on the distance of the field from the thyroid and the proportions of thyroid dose from stray radiation (leakage, scatter from jaws, etc.) and tissue scattered radiation were measured. The effects of a shadow tray and shielding blocks were also determined. Calculation of thyroid dose using the Clarkson scatter integration method was performed for 6 MV X-rays to compare with the measured doses. In vivo thyroid dose estimates were made using TLD measurements for three children receiving PCI with 6 MV X-rays.
RESULTS
Using open, unshielded fields, the thyroid region of the phantom received 1.2-1.4% of the prescribed cranial dose for 6 MV X-rays and 1.5-1.7% for Cobalt-60. For both treatment units, stray radiation accounted for approximately two thirds of the thyroid dose and tissue scatter accounted for the remaining one third. The thyroid dose increased as the field moved closer to the thyroid, with an increasing proportion of the dose due to tissue scatter. Placement of a thyroid shielding block on a shadow tray reduced the thyroid dose by only 20% compared with the open, unshielded setup. Thyroid dose from 6 MV using open fields was affected by the orientation of the collimator. When the inferior field edge was defined by the lower jaw, the dose was reduced by 27% compared with the upper jaw. Good correlation of dose to the thyroid region was obtained between phantom measured doses, in vivo measured doses and calculation of dose using the Clarkson method.
CONCLUSION
For PCI doses of 1800 or 2400 cGy in the adult phantom, the dose to the thyroid was 20-40 cGy (1-2%). For small children this could rise to approximately 5% of the prescribed dose, of which half was due to stray radiation. As the thyroid in children is very sensitive to radiation and the dose-response curve for thyroid tumor induction is linear, attempts to shield the thyroid during cranial irradiation are mandatory. Cobalt-60 units should not be used, as the thyroid dose was higher than using 6 MV X-rays. Collimator orientation and the use of shadow trays and shielding were important factors in determining thyroid dose.
目的
确定儿童白血病患者因预防性颅脑照射（PCI）而接受的甲状腺辐射剂量以及影响该剂量的因素。
方法和材料
使用热释光剂量计（TLD）测量成人人体模型（ART）在6兆伏X射线和钴 - 60伽马射线平行相对侧野模拟颅脑照射时甲状腺所接受的剂量。测量甲状腺剂量与照射野到甲状腺距离的关系，以及甲状腺剂量中来自杂散辐射（泄漏、颌部散射等）和组织散射辐射的比例。还确定了铅挡块和屏蔽块的影响。对6兆伏X射线使用克拉克森散射积分法计算甲状腺剂量，以与测量剂量进行比较。对三名接受6兆伏X射线PCI的儿童进行TLD测量，进行体内甲状腺剂量估计。
结果
使用开放、无屏蔽野时，人体模型的甲状腺区域在6兆伏X射线照射下接受规定颅脑剂量的1.2 - 1.4%，在钴 - 60照射下接受1.5 - 1.7%。对于两个治疗单元，杂散辐射约占甲状腺剂量的三分之二，组织散射占其余三分之一。随着照射野靠近甲状腺，甲状腺剂量增加，组织散射导致的剂量比例增加。与开放、无屏蔽设置相比，在铅挡块上放置甲状腺屏蔽块仅使甲状腺剂量降低20%。使用开放野时，6兆伏X射线的甲状腺剂量受准直器方向影响。当下野边缘由下颌确定时，与上野相比剂量降低27%。在人体模型测量剂量、体内测量剂量和使用克拉克森方法计算剂量之间，甲状腺区域的剂量具有良好的相关性。
结论
对于成人人体模型中1800或2400厘戈瑞的PCI剂量，甲状腺剂量为20 - 40厘戈瑞（1 - 2%）。对于幼儿，这可能升至规定剂量的约5%，其中一半归因于杂散辐射。由于儿童甲状腺对辐射非常敏感，且甲状腺肿瘤诱发的剂量 - 反应曲线是线性的，在颅脑照射期间必须尝试屏蔽甲状腺。不应使用钴 - 60治疗单元，因为其甲状腺剂量高于使用6兆伏X射线时的剂量。准直器方向以及铅挡块和屏蔽的使用是确定甲状腺剂量的重要因素。
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