Tan L T, Warren J, Freestone G, Jones B
Clatterbridge Centre for Oncology, Wirral, Merseyside, UK.
Br J Radiol. 1996 Oct;69(826):953-62. doi: 10.1259/0007-1285-69-826-953.
The estimation of maximum bladder doses from orthogonal radiographs is unreliable when triple source systems are used for intracavitary brachytherapy (BT) for gynaecological cancers. For single line source systems, the estimation of maximum bladder doses from radiographs should be more reliable due to the radial symmetry of the isodose distribution. A pilot study has been carried out to compare the estimated maximum bladder doses from standard radiographs with data obtained from CT for single line source BT treatments. 12 patients undergoing treatment for carcinoma of the cervix were selected for CT assessment of their bladder doses. For each patient, the dose rates at the International Commission of Radiation Units and Measurements (ICRU) bladder reference point B1 and a second reference point B2 2.5 cm cranially were computed from orthogonal radiographs and were compared with the maximum bladder dose rate as determined by CT scanning. Dose rates were computed for two different source loading patterns: (1) a 6 cm line source with uniformly distributed linear activity along its length; (2) a 6 cm line source with increased activity in the central 2 cm segment. The mean ratio of the maximum CT bladder dose rate to the dose rate at the ICRU reference point B1 on orthogonal radiographs for the line source with uniform linear activity was 1.32 (range 0.62-2.43, SD = 0.54). When the dose rates at both reference points B1 and B2 were considered, the mean ratio of the maximum CT dose rate to the maximum dose rate from radiographs was only 1.05 (range 0.72-1.72, SD = 0.24). For the line source with increased activity in the central segment, the mean ratio of the maximum CT bladder dose rate to the dose rate at B1 was 1.38 (range 0.60-2.63, SD = 0.64). When both B1 and B2 were considered, the variation in the ratio of the maximum CT dose rate to the maximum dose rate from radiographs was considerably smaller (mean ratio = 1.07, range 0.69-1.76, SD = 0.26). For single line source systems, single point dose estimation using the ICRU reference point on orthogonal radiographs will underestimate the maximum bladder dose although the discrepancy is less than for triple source systems. If the ICRU reference point is used in conjunction with a second reference point 2.5 cm cranially, then underestimation of the maximum bladder dose is unlikely to occur as at least one of the points is likely to be a reasonable estimate of the maximum bladder dose.
当使用三源系统进行妇科癌症腔内近距离放疗（BT）时，通过正交X线片估算膀胱最大剂量是不可靠的。对于单线源系统，由于等剂量分布的径向对称性，通过X线片估算膀胱最大剂量应该更可靠。已经开展了一项初步研究，以比较标准X线片估算的膀胱最大剂量与单线源BT治疗中CT获得的数据。选择12例接受宫颈癌治疗的患者进行膀胱剂量的CT评估。对于每位患者，根据正交X线片计算国际辐射单位与测量委员会（ICRU）膀胱参考点B1以及头侧2.5 cm处第二个参考点B2的剂量率，并与CT扫描确定的膀胱最大剂量率进行比较。针对两种不同的源加载模式计算剂量率：（1）一个6 cm的线源，其线性活度沿长度均匀分布；（2）一个6 cm的线源，其中心2 cm段活度增加。对于线性活度均匀的线源，CT膀胱最大剂量率与正交X线片上ICRU参考点B1处剂量率的平均比值为1.32（范围0.62 - 2.43，标准差 = 0.54）。当同时考虑参考点B1和B2处的剂量率时，CT最大剂量率与X线片最大剂量率的平均比值仅为1.05（范围0.72 - 1.72，标准差 = 0.24）。对于中心段活度增加的线源，CT膀胱最大剂量率与B1处剂量率的平均比值为1.38（范围0.60 - 2.63，标准差 = 0.64）。当同时考虑B1和B2时，CT最大剂量率与X线片最大剂量率比值的变化明显更小（平均比值 = 1.07，范围0.69 - 1.76，标准差 = 0.26）。对于单线源系统，使用正交X线片上的ICRU参考点进行单点剂量估算会低估膀胱最大剂量，尽管差异比三源系统小。如果将ICRU参考点与头侧2.5 cm处的第二个参考点结合使用，那么不太可能发生膀胱最大剂量的低估，因为至少其中一个点可能是膀胱最大剂量的合理估算。
