High dose rate intracavitary brachytherapy for carcinoma of the cervix: the Madison system: I. Clinical and radiobiological considerations.

The decision to use five high dose rate intracavitary (HDR-ICR) insertions at weekly intervals for invasive carcinoma of the cervix treated at the University of Wisconsin Comprehensive Cancer Center (UWCCC) was made clinically. It was based on practical considerations and on previous clinical experience worldwide which showed that between 2 and 16 insertions have been used with apparently acceptable results. Although radiobiological considerations favor a large number of small doses, such a large number of HDR-ICR insertions is not clinically practical. Our strategy was to keep the biological effects of external beam and intracavitary insertions in the same ratio as used on a large series of patients treated here with low dose rate (LDR) therapy. This means keeping the same external beam treatment scheme and finding high dose rate (HDR) doses that are biologically equivalent to the previous LDR therapy, as far as possible. External beam and HDR intracavitary dose schedules for the Madison System of treating cervical carcinoma are described in detail. Because there is more repairable damage in late-reacting normal tissues, there is a bigger loss of sparing in these tissues than in tumors when changing from LDR to HDR, so total doses should be reduced more for equal late complications than for equal tumor control. The clinical decision was made to aim at equal tumor control. The possible increase in late complications has to be avoided by reducing the doses to critical normal tissues using extremely careful anatomic positioning of the HDR sources. Critical normal tissues must be kept further away from the radiation sources so that their doses are about 20% lower than with LDR geometry. This requires an extra separation of some millimeters depending on the anatomy and geometry of the individual insertion. The strategy is that the unfavourable radiobiological effects of a few large fractions must be counteracted by better physical dose distributions with HDR-ICR than with the previous LDR insertions. These good distributions are obtainable with the short exposures at HDR.