Bladder dose estimation during intracavitary brachytherapy for carcinoma of the cervix using a single line source system

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 B-1 and a second reference point B-2 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 al the ICRU reference point B, 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 al both reference points B-1 and B-2 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 rare at B-1 was 1.38 (range 0.60-2.63, SD=0.64). When both B-1 and B-2 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.