[Dosimetry methods in determining radiation dosage of the rectum in HDR-brachytherapy of cervix carcinoma]

Dosage measurements in brachytherapy of cervix carcinoma are usually obtained with semiconductor dosimeters intrarectally and calculated using approximation methods for additional points, e.g. chosen according to ICRU Report 38. This procedure allows minimizing organ risk dosages and avoiding side effects. This study compares actual dosage measurements with computed approximations.In 75 applications the measured actual rectum dosage was analyzed retrospectively. Using graphic approximation methods in conjunction with the localization radiographs the expected dose values at 5 detector points of the intrarectal semiconductor dosimeter and at the ICRU rectal reference point were determined. Prospectively for 11 additional applications the expected dosage for various points within the rectum were computed during therapy planning and additionally for specific reference points corresponding to Fletcher's lymphatic trapezoid and Chassagne's pelvic wall points.The retrospective evaluation showed that 95% of values determined by graphic approximation methods varied by as much as +/- 30% from measured values. Factors causing errors were incorrect assessment of the applicator's spatial positioning, non-orthogonal radiographs, incorrect calibration of the semiconductor probe, movement of applicator and/or probe in the time between radiograph and application. In the prospective group 95% of deviations between measured and calculated values lay within an interval of +/- 40% (Figure 1). Possible sources of error could be similar to those in using the graphic approximation, although the reconstruction of spatial positioning of the applicator is possibly more exact. Doses determined at the ICRU rectal reference point were 5.6 +/- 2.5 Gy in the retrospective analysis and 6.1 +/- 1.6 Gy in the prospective study (Figure 2). The standard deviation of measured values at the specific reference points was +/- 30%. The mean dosage distribution was nearly symmetrical with regard to the body axis, i.e. to the applicator position. Reasons for the relatively large standard deviation are e.g. difficulties in defining the reference points as well as in identifying them on the radiographs, also differences in applicator positioning.The retrospective analysis led to a larger error than the prospective one. The graphic approximation method should only be utilized when computer-assisted treatment planning is not possible. Conspicuous are the differences between values obtained in computer planning and actual measured values. As these deviations cannot always be explained unequivocally, both computation and measurement should always be conducted in order to obtain an adequate survey of dosage distribution within the rectum. Computer planning offers the additional advantage of determining the dose at various other reference points.