Recovery from potentially lethal damage (PLD) after split-dose (fractionated) irradiation was studied in Vero cells in vitro.
Unfed plateau phase cells were irradiated 0 and 3 Gy as priming dose (D,) and then were incubated for 24 hours. After test dose irradiation (DZ) of 1,3,6 and 9 Gy, cells were left in situ for 0, 1, 2, 3, 4, 5, 6 and 24 hours, and then were typsinized and explanted into culture dishes containing fresh media with feeder cells to make the total number of cells constant.
Cell survival was estimated by colony forming ability and the increase of cell survival fraction with incubation time before explantation was compared between split and single, dose irradiation.
As the incubation time after test dose irradiation increased, the PLD repair progressed rapidly and saturated in 2 to 6 hours. The saturated level of PLD repair in-creased with test radiation dose. These patterns of recovery of PLD after split dose irradiation was quite similar to those after single dose irradiation.
When the dose-survival curves were fitted to the linear-quadratic model, the linear inactivation coefficient (a) decreased progressively to zero but quadratic inactivation coefficient (R) increased only by 1.6 to 1.7 times as the PLD is repaired with progression of time after split dose irradiation as well as after single dose irradiation.
In conclusion, the pattern of recovery of PLD after split dose irradiation was not different from that after single dose irradiation.
And from these data, the effect of recovery of PLD on final cell survival fraction after multiple fractionated irradiation is expected to be great.
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