The effect of radio-adaptive doses on HT29 and GM637 cells

Background: The shape of the dose-response curve at low doses
differs from the linear quadratic model. The effect of a
radio-adaptive response is the centre of many studies and well
known inspite that the clinical applications are still rarely
considered. Methods: We studied the effect of a low-dose
pre-irradiation (0.03 Gy-0.1 Gy) alone or followed by a 2.0 Gy
challenging dose 4 h later on the survival of the HT29 cell line
(human colorectal cancer cells) and on the GM637 cell line (human
fibroblasts). Results: 0.03 Gy given alone did not have a
significant effect on both cell lines, the other low doses alone
significantly reduced the cell survival. Applied 4 h before the 2.0
Gy fraction, 0.03 Gy led to a significant induced radioresistance
in GM637 cells, but not in HT29 cells, and 0.05 Gy led to a
significant hyperradiosensitivity in HT29 cells, but not in GM637
cells. Conclusion: A pre-irradiation with 0.03 Gy can protect
normal fibroblasts, but not colorectal cancer cells, from damage
induced by an irradiation of 2.0 Gy and the application of 0.05 Gy
prior to the 2.0 Gy fraction can enhance the cell killing of
colorectal cancer cells while not additionally damaging normal
fibroblasts. If these findings prove to be true in vivo as well
this may optimize the balance between local tumour control and
injury to normal tissue in modern radiotherapy.