PET成像应用于高能光子剂量验证的定量研究

目的 通过PET扫描对高能光子照射不同模体产生的正电子进行定量研究,探讨PET成像在高能光子放疗中生物剂量验证的可行性.方法 利用25和50 MV高能光子照射不同模体(水凝胶、聚乙烯和不均匀模体),照射剂量为2、4、6和8 Gy,照后1 min内立即对模体进行PET扫描,记录正电子计数率随时间的变化,扫描完成后对其进行数据拟合,推算产生核素的半衰期.将模体横截面和矢状面的活度分布图与TPS计算的剂量分布图进行对比,观察正电子活度与剂量分布的相关性.结果 根据各时刻正电子计数率进行拟合,得到水凝胶和聚乙烯中产生的正电子半衰期分别为2.23和19.47 min,与11C和15O的半衰期2.08和20.2 min相差不大.50 MV光子在水凝胶和聚乙烯中产生的正电子数量分别是25 MV光子产生的3.88和3.86倍,正电子产额随模体吸收剂量增加而成比例增加.除了在剂量建成区和模体空腔外,模体深度-活度和离轴-活度分布均与剂量分布相似.结论 高能光子与模体反应生成的正电子数量和分布与剂量关系密切,利用PET成像进行高能光子剂量验证理论上是可行的.

The quantitative study on high energy photon dose verification by PET imaging

Objective To study positron yield quantitatively in different phantoms by means of PET imaging after high energy photon irradiation, and to investigate the feasibility of bio-dose verification by PET in high energy photon radiotherapy. Methods The phantoms (hydrogel, HDPE, inhomogeneous phantom) were scanned with PET 1 min later after exposure to 2, 4, 6, 8 Gy dose with/using 25 and 50 MV photon irradiation. The positron account rates were recorded every minute in PET scanning process and then fitted to get the half-lives of yielded nuclides. Positron distribution in each phantom was compared with dose distribution to investigate the relationship between positron activity and dose delivered. Results The half-lives of nuclides yielded in hydrogel and HDPE were supposed to be 2.23 and 19.47 min,respectively by fitting results, which had negligible difference with half-lives of ¹¹C (20.2 min) and ¹⁵O (2.08 min). The positron amounts induced by 50 MV photon in hydrogel and HDPE were 3.88 and 3.86 times that of 25 MV photon, and increased in proportion to dose delivered. Except for build-up region and cavity, activity distributions in depth and off-axis were similar to dose distribution. Conclusions The amount and distribution of positron induced by high energy photon are related to dose delivered, and it is feasible to do dose verification with PET imaging in high energy photon radiotherapy.