Chen Zhe, Deng Jun, Roberts Kenneth, Nath Ravinder
Department of Therapeutic Radiology, Yale University School of Medicine, New Haven, Connecticut 06504, USA.
Med Phys. 2006 Apr;33(4):968-75. doi: 10.1118/1.2179170.
Our aim in this work was to study the potential dosimetric effect of prostate edema on the accuracy of conventional pre- and post-implant dosimetry for prostate seed implants using the newly introduced 131Cs seed, whose radioactive decay half-life (approximately 9.7 days) is directly comparable to the average edema resolution half-life (approximately 10 days) observed previously by Waterman et al. for 125I implants [Int. J. Radiat. Oncol. Biol. Phys. 41, 1069-1077 (1998)]. A systematic calculation of the relative dosimetry effect of prostate edema on the 131Cs implant was performed by using an analytic solution obtained previously [Int. J. Radiat. Oncol. Biol. Phys. 47, 1405-1419 (2000)]. It was found that conventional preimplant dosimetry always overestimates the true delivered dose as it ignores the temporary increase of the interseed distance caused by edema. The overestimation for 131Cs implants ranged from 1.2% (for a small edema with a magnitude of 10% and a half-life of 2 days) to approximately 45% (for larger degree edema with a magnitude of 100% and a half-life of 25 days). The magnitude of pre- and post-implant dosimetry error for 131Cs implants was found to be similar to that of 103Pd implants for typical edema characteristics (magnitude < 100%, and half-life <25 days); both of which are worse compared to 125I implants. The preimplant dosimetry error for 131Cs implants cannot be compensated effectively without knowing the edema characteristics before the seed implantation. On the other hand, the error resulted from a conventional post-implant dosimetry can be minimized (to within +/-6%) for 131Cs implants if the post-implant dosimetry is performed at 10+/-2 days post seed implantation. This "optimum" post-implant dosimetry time is shorter than those determined previously for the 103Pd and 125I implants at 16+/-4 days and 6+/-1 weeks, respectively.
我们这项工作的目的是研究前列腺水肿对使用新引入的¹³¹Cs种子进行前列腺籽源植入术的传统植入前和植入后剂量测定准确性的潜在剂量学影响，¹³¹Cs种子的放射性衰变半衰期（约9.7天）与沃特曼等人之前观察到的¹²⁵I植入术的平均水肿消退半衰期（约10天）直接可比[《国际放射肿瘤学、生物学、物理学杂志》41, 1069 - 1077(1998)]。通过使用先前获得的解析解[《国际放射肿瘤学、生物学、物理学杂志》47, 1405 - 1419(2000)]，对前列腺水肿对¹³¹Cs植入的相对剂量学影响进行了系统计算。结果发现，传统的植入前剂量测定总是高估了实际给予的剂量，因为它忽略了水肿导致的籽源间距离的暂时增加。¹³¹Cs植入的高估范围从1.2%（对于程度为10%且半衰期为2天的小水肿）到约45%（对于程度为100%且半衰期为25天的较大程度水肿）。对于典型的水肿特征（程度<100%，半衰期<25天），发现¹³¹Cs植入的植入前和植入后剂量测定误差幅度与¹⁰³Pd植入的相似；与¹²⁵I植入相比，这两种情况都更差。如果在籽源植入前不知道水肿特征，¹³¹Cs植入的植入前剂量测定误差就无法有效补偿。另一方面，如果在籽源植入后10±2天进行植入后剂量测定，对于¹³¹Cs植入，传统植入后剂量测定产生的误差可以最小化（至±6%以内）。这个“最佳”的植入后剂量测定时间比先前确定的¹⁰³Pd和¹²⁵I植入的时间短，¹⁰³Pd和¹²⁵I植入的时间分别为16±4天和6±1周。
