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

Area of Science:

• Radiation oncology
• Cellular radiobiology
• Normal tissue response

Background:

• Low dose hyper-radiosensitivity (LDHRS) is a phenomenon where cell kill exceeds predictions below 1Gy.
• Clinical investigation into LDHRS in vivo is ongoing.
• Skin serves as a model for normal human tissue response to radiation.

Purpose of the Study:

• To investigate the occurrence of LDHRS in human skin.
• To compare the effects of different low radiation doses and fractionation schedules on skin.
• To assess basal cell density (BCD) and proliferative index in response to radiation.

Main Methods:

• Two studies were conducted using skin biopsies from patients receiving pelvic radiotherapy.
• Study 1 compared 0.5Gy and >1.0Gy daily fractions, assessing BCD.
• Study 2 compared ultrafractionated (0.5Gy TDS) and conventional (1.5Gy OD) regimens, assessing BCD and proliferative index.

Main Results:

• Study 1 showed a greater BCD reduction with lower doses when plotted against dose, but this effect was lost when plotted against time.
• Study 2 demonstrated a greater BCD reduction in the higher dose/fraction arm.
• The proliferative response was similar between the two regimens in Study 2.

Conclusions:

• LDHRS was not observed in skin at approximately 0.5Gy/fraction when comparing equal dose/time intensity regimens.
• The biological relevance of these findings for larger irradiated volumes and different dose ranges remains uncertain.
• Extrapolation to doses below 0.5Gy or other endpoints is not possible from this study.