The evaluation of low dose hyper-radiosensitivity in normal human skin.

BACKGROUND AND PURPOSE

The laboratory phenomenon of low dose hyper-radiosensitivity (LDHRS) describes an excess of cell kill at doses below 1Gy relative to that predicted by the linear quadratic model. These data have stimulated clinical investigation into LDHRS in vivo.

PATIENTS AND METHODS

Skin was used as a model of normal human tissue. Two studies were initiated investigating the response to low doses of radiation. Study 1 compared once daily skin doses of approximately 0.5 and >1.0Gy in 24 patients receiving pelvic radiotherapy. Skin biopsies before and during radiotherapy were analysed histologically to assess the basal cell density (BCD). Study 2 compared two regimens of equal dose/time intensity--an ultrafractionated regimen (0.5Gy TDS x 12 days) with a conventional regimen (1.5Gy OD x 12 days). Skin biopsies taken during treatment assessed BCD and proliferative index. In both studies the changes in BCD were compared using non-linear regression analysis.

RESULTS

Study 1. The results show a significantly greater reduction in BCD in the low dose group when BCD is plotted against dose. This effect is lost when BCD is plotted against time Study 2. The results demonstrate a significantly greater reduction in BCD in the higher dose/fraction arm. The proliferative response was similar in both treatment groups.

CONCLUSIONS

These data suggest that LDHRS does not occur in skin following doses of approximately 0.5Gy/fraction when regimens of equal dose/time intensity are compared. As only small volumes of normal tissue were irradiated it is difficult to predict the biological relevance of this with respect to larger field low dose per fraction irradiation regimens or risk of cancer induction. Equally we cannot extrapolate to effects resulting from exposure to doses <0.5Gy or to the effects of low doses on other endpoints.