The relationship between dose heterogeneity ("hot" spots) and complications following high-dose rate brachytherapy

It is generally believed that "hot" spots should be avoided in radiotherapy because they lead to complications. Dose homogeneity within the target volume is much more difficult to achieve during brachytherapy than during external beam irradiation, and implants are rarely geometrically perfect. To not underdose some parts of the target volume, therefore, it may be necessary to accept hot spots in other parts of the target volumes, but it is not at all clear from the literature how much dose heterogeneity should be considered excessive. We undertook this study in an effort to determine just how high a dose to a hot spot is associated with clinically significant complications.

We studied 40 patients treated by high-dose rate brachytherapy with or without external irradiation. For each patient, we calculated the minimum dose to the "hottest" 1 cubic centimeter (cc) volume (Dmax1) and, for 18 patients, the minimum dose to the hottest 10 cc volume (Dmax10) as well.

Considerable dose heterogeneity existed within the target volume. The Dmax1 ranged from 150-2000% (median 320%) of the minimum target dose (MTD). The median MTD/fraction was 2.50 Gy (range 1.50-25.00), and the median Dmax1/fraction was 10.00 Gy (range 3.75-150.00). The median Dmax1 from the entire course of brachytherapy was 75.00 Gy (range 25.00-550.00). Adding the doses from planned external irradiation, plus any prior irradiation to the same area, the median total Dmax1 was 112.50 Gy (range 30.00-580.00), yet the incidence of complications, even among those in the highest quartile of this dose range, was not greater than the lowest quartile. The total median Dmax10 was 85.00 Gy (range 32.00-130.00), but the incidence of complications was, again, similar whether the dose was in the lower or the upper half of this range (32.00-85.00 Gy, or 86.00-130.00 Gy, respectively).

We had expected to find that the patients with the highest Dmax1 and/or Dmax10 would be the ones most likely to suffer complications, but the results did not support this hypothesis. Thus, dose heterogeneity, within the scope of our study, turned out to be rather unimportant with regard to complications. This finding contradicts the conventional wisdom and suggests that concerns about hot spots need not preclude optimization to ensure adequate dosage to all parts of the target volume.