Bohan M, Yue N, Nath R
Department of Therapeutic Radiology, Yale-New Haven Hospital, 20 York Street, New Haven, CT, USA.
Cardiovasc Radiat Med. 2000 Jan-Mar;2(1):39-41.
There is a widespread belief in the cardiology and radiation oncology community that high dose rate 192Ir intravascular brachytherapy cannot be implemented without massive additional shielding of the conventional catheterization labs. The purpose of this work is to show that this is a myth, which is not based on sound radiation protection principles.
Exposure rates in air were calculated for a variety of point and line sources of 192Ir. Exposures per treatment at different distances from the source were calculated for a typical intravascular brachytherapy treatment of a 15-Gy dose at a radial distance of 2 mm from the source and for source lengths in the range of 0 to 10 cm. Additionally, exposure rates outside the catheterization lab were calculated for various lead shielding thicknesses typical of conventional X-ray facilities. These rates were used along with the NCRP recommendations on radiation facility design to assess shielding requirements.
For a treatment dose of 15 Gy at 2 mm, the occupational exposure per treatment at 2 m in air without any tissue attenuation or shielding was 7.8 mR for a lesion length of 3.0 cm. This exposure/treatment is independent of the dose rate or the activity of the source. However, it increases as lesion length is increased, increasing from 5.4 to 24.9 mR as lesion length increased from 2 to 10 cm. Exposures in unrestricted areas outside the catheterization lab using the NCRP shielding rationale can be kept below 2 mR per treatment and using appropriate workload, use, and occupancy factors below 2 mR per week.
The feasibility of implementing a high dose rate 192Ir intravascular brachytherapy program in a catheterization laboratory is totally independent of the dose rate or the activity of the source. If it is feasible to implement 192Ir brachytherapy in a conventional catheterization lab using low activity 192Ir seeds, then it is also feasible to do so with a high activity 192Ir afterloader.
心脏病学和放射肿瘤学界普遍认为，若不对传统心导管实验室进行大量额外屏蔽，就无法实施高剂量率192铱血管内近距离放射治疗。本研究的目的是表明这是一个没有基于合理辐射防护原则的谬论。
计算了多种192铱点源和线源在空气中的照射率。对于距源径向距离为2毫米、剂量为15戈瑞的典型血管内近距离放射治疗，计算了在距源不同距离处每次治疗的照射量，源长度范围为0至10厘米。此外，针对传统X射线设施典型的各种铅屏蔽厚度，计算了心导管实验室外的照射率。这些照射率与美国国家辐射防护与测量委员会（NCRP）关于辐射设施设计的建议一起用于评估屏蔽要求。
对于在2毫米处15戈瑞的治疗剂量，在空气中距源2米处，无任何组织衰减或屏蔽时，病变长度为3.0厘米的每次治疗职业照射量为7.8毫伦琴。这种每次治疗的照射量与剂量率或源的活度无关。然而，它会随着病变长度的增加而增加，病变长度从2厘米增加到10厘米时，照射量从5.4毫伦琴增加到24.9毫伦琴。根据NCRP屏蔽原理，在心导管实验室外非限制区域的每次治疗照射量可保持在2毫伦琴以下，使用适当的工作量、使用和占用因子，每周可保持在2毫伦琴以下。
在心导管实验室实施高剂量率192铱血管内近距离放射治疗计划的可行性完全独立于剂量率或源的活度。如果使用低活度192铱籽源在心导管实验室实施192铱近距离放射治疗是可行的，那么使用高活度192铱后装治疗机实施也是可行的。
