Nath Ravinder, Yue Ning
Department of Therapeutic Radiology, Yale University School of Medicine, P.O. Box 208040, New Haven, CT 06520-8040, USA.
Cardiovasc Radiat Med. 2004 Apr-Jun;5(2):88-96. doi: 10.1016/j.carrad.2004.05.002.
In intravascular brachytherapy, either photon or beta emitters are often used in a linear arrangement so that blood vessels of lengths in the range of several centimeters can be treated. With a line source, the dose uniformity and the range of doses that various components of the blood vessels receive depend not only on the type of radionuclides used in the treatment but also on the geometric position of radioactive source relative to the blood vessel walls. The aim of this study is to investigate the dose uniformity around the blood vessel and the effects on the uniformity due to the changes of the off-centering of different photon and beta emitters within the lumen.
Dose distributions were calculated on a cylindrical blood vessel of various radii. The radioactive sources of (192)Ir, (125)I, (103)Pd, (188)Re, (32)P, and (90)Y/Sr were studied. All the sources were assumed to be in the form of a line and had a length of 2 cm. The dose rate at a point in space produced by a radioactive source was computed by integrating the point dose rate kernel of the corresponding radionuclide over the 2-cm-long radioactive line. The point dose rate kernel was computed with Monte Carlo simulation of radiation transport. Dosimetric calculations were performed for both concentric and nonconcentric radioactive line source locations. Off-centering effects on the dosimetry were characterized with two newly defined quantities LDU and ADU: LDU describes the longitudinal dose uniformity along blood vessels and ADU describes the azimuthal dose uniformity, i.e., the dose deviation from the expected delivery dose around blood vessels.
The longitudinal dose uniformity did not change significantly with the off-center distance. The azimuthal dose uniformity around the blood vessel deteriorated as the off-center distance increased. The ADU was worse for nonconcentric beta emitters than the photon emitters. For example, if the off-center distance was 1 mm and the radial distance was 1.5 mm, the range of dose around the blood vessel on the central transverse plane (normalized to the corresponding dose under the concentric condition) was from 0.55 to 3.3, 0.56 to 3.3, 0.53 to 3.4, 0.43 to 6.0, 0.38 to 4.3, and 0.31 to 4.7 for (192)Ir, (125)I, (103)Pd, (90)Y/Sr, (188)Re, and (32)P sources, respectively. However, it appeared that there existed a lower limit of underdosing (about 40% of desired delivery dose) caused by the off-centering for the photon emitters. It was also found that both ADU and LDU became almost independent of source length when the length was longer than or equal to 20 mm.
A generalized formalism for expressing the dose uniformity along and around blood vessels generated with a linear source was developed and used to study the longitudinal and azimuthal dose uniformity for different types of radionuclides. Although concentric beta emitters provide uniform dose coverage along blood vessels, nonconcentric beta emitters produced larger dose deviations and worse dose uniformity around the blood vessels than photon emitters. The off-centering introduced significantly higher dose on proximal vessel walls for both beta and photon emitters; however, the underdosing at distal points due to off-centering was somewhat limited for the high-energy photon emitters. The magnitude of off-centering effects for the low-energy photon emitters ((103)Pd) was less than that for beta emitters but more than that for higher energy photon emitters ((125)I and (192)Ir).
在血管内近距离放射治疗中，光子或β发射体通常呈线性排列使用，以便能够治疗长度在几厘米范围内的血管。对于线源，血管各部分所接受的剂量均匀性以及剂量范围不仅取决于治疗中使用的放射性核素类型，还取决于放射源相对于血管壁的几何位置。本研究的目的是研究血管周围的剂量均匀性以及由于管腔内不同光子和β发射体偏心度变化对均匀性的影响。
在不同半径的圆柱形血管上计算剂量分布。研究了（192）铱、（125）碘、（103）钯、（188）铼、（32）磷和（90）钇/锶的放射源。所有放射源均假定为线状，长度为2厘米。通过在2厘米长的放射线上对相应放射性核素的点剂量率核进行积分，计算放射源在空间中某一点产生的剂量率。点剂量率核通过辐射传输的蒙特卡罗模拟计算得出。对同心和不同心放射源位置均进行了剂量学计算。用两个新定义的量LDU和ADU来表征偏心对剂量学的影响：LDU描述沿血管的纵向剂量均匀性，ADU描述方位角剂量均匀性，即血管周围剂量与预期输送剂量的偏差。
纵向剂量均匀性随偏心距离变化不显著。随着偏心距离增加，血管周围的方位角剂量均匀性变差。非同心β发射体的ADU比光子发射体更差。例如，如果偏心距离为1毫米，径向距离为1.5毫米，在中心横截面上血管周围剂量范围（相对于同心条件下的相应剂量进行归一化）对于（192）铱、（125）碘、（103）钯、（90）钇/锶、（188）铼和（32）磷放射源分别为0.55至3.3、0.56至3.3、0.53至3.4、0.43至6.0、0.38至4.3和0.31至4.7。然而，似乎光子发射体因偏心导致的剂量不足存在一个下限（约为预期输送剂量的40%）。还发现当放射源长度大于或等于20毫米时，ADU和LDU几乎与放射源长度无关。
开发了一种用于表达线源产生的沿血管及血管周围剂量均匀性的通用形式，并用于研究不同类型放射性核素的纵向和方位角剂量均匀性。尽管同心β发射体沿血管提供均匀的剂量覆盖，但非同心β发射体在血管周围产生的剂量偏差更大，剂量均匀性比光子发射体更差。偏心对于β发射体和光子发射体在近端血管壁上均引入了显著更高的剂量；然而，对于高能光子发射体，由于偏心在远端点的剂量不足在一定程度上受到限制。低能光子发射体（（103）钯）的偏心效应幅度小于β发射体，但大于高能光子发射体（（125）碘和（192）铱）。
