Ⅰ-ⅡB期精原细胞瘤术后放疗中不同厚度低熔点挡铅对睾丸剂量防护的研究

Dose protection of testis with low melting point lead during postoperative radiotherapy for stage Ⅰ-ⅡB testicular seminoma

目的 评估精原细胞瘤术后放疗中低熔点挡铅厚度及与射野边缘的距离对健侧睾丸受照剂量的影响。方法 选取1例I期精原细胞瘤术后患者的放射治疗计划，利用Synergy医用直线加速器6 MV X射线在SZMC-IV型仿真人体模型上模拟患者照射。采用PTW 0.6 cm^3指型电离室，在距射野边缘不同距离处分别测量无挡铅和有3、5、7、10及15 mm厚度低熔点挡铅屏蔽条件下的吸收剂量，评价不同条件下的剂量防护效果。结果 在上述各挡铅厚度条件下，测量结果的归一化剂量与测量点离射野边界之间的距离呈指数形式衰减。无挡铅时的测量结果相对于靶区处方剂量，由距离射野边缘1 cm处的8.41%降到25 cm处的0.61%。有3、5及7 mm挡铅时，分别由距离射野边缘1 cm处的4.55%、3.98%及3.47%降到25 cm处的0.27%、0.21%及0.17%。有10 mm挡铅时，由距离射野边缘1.5 cm处的2.55%降到25 cm处的0.15%。有15 mm挡铅时，由距离射野边缘2 cm处的1.86%降到25 cm处的0.13%。在处方剂量为26 Gy并且距离射野边缘10 cm或4 cm以上，和处方剂量为36 Gy并且距离射野边缘4 cm以上的条件下，分别采用3、7和15 mm厚度的挡铅屏蔽，能将睾丸的散射剂量降低到0.5 Gy以下。结论 在精原细胞瘤术后常规放疗中，根据不同的处方剂量和照射野范围，采用低熔点铅自制适当厚度的睾丸防护装置，能方便、有效地降低睾丸受照剂量，有利于保护患者的生育能力。

Objective To assess the protective effect of low melting point lead and field margin on the opposite testicular in testicular seminoma patients during postoperative radiation. Methods A patient with stage I seminoma was selected and his phantom measurement was carried out. The PTW 0.6 cm^3 type ionization chamber was used to measure the absorbed dose under the conditions of no lead and low melting point lead with thickness of 3, 5, 7, 10 and 15 mm at different distances from the field edge, respectively. Results Under different lead thickness conditions, the measurement result and the distance between the measured points and the boundary of the field were exponentially attenuated. The relative target dose dropped from 8.41% at 1 cm to 0.61% at 25 cm without lead blocking, and dropped from 4.55%, 3.98% and 3.47% at 1 cm to 0.27%, 0.21% and 0.17% at 25 cm with 3, 5, 7 cm lead, respectively. With 10 mm lead, it dropped from 2.55% at 1.5 cm to 0.15% at 25 cm, and 1.86% at 2 cm to 0.13% at 25 cm with 15 mm lead. The lead shield of 3, 7 and 15 mm thickness can be used to reduce the scatter dose of testis to below 0.5 Gy during radiotherapy for seminoma. Conclusions An appropriate thickness of low melting point lead might reduce the dose of testis conveniently and effectively, which would be beneficial to protect the fertility of the patients with testicular seminoma.