Inverse X-irradiation split-dose effect in a murine lymphoma cell line

The effects of single and split-dose X-irradiation of the radiosensitive (S/F) and radioresistant (AIV) sublines of murine L51798Y lymphoma cells were studied. The two-dose fractionated survival curve for AIV cells indicated the presence of sublethal damage repair. However when the S/F cells were given split-dose irradiation, not only was there an absence of repair, but the survival level resulting from the challenging dose was lower than the single-dose survival. Concomitant with this effect was a temporary block in cell multiplication and a dramatic increase of the cell volume. The subsequent increase in split-dose survival coincided with the resumption of cell division and an accompanying decrease in modal cell volume. When cells were held at 230C between the doses, thus preventing cells from progressing through the cell cycle, the decrease in survival with split-dose irradiation was abolished. This unique 'inverse split-dose effect' is attributed to the lack of sublethal damage repair in S/F cells, coupled with cell-cycle progression of the initial survivors to more radiosensitive phases of the cycle after the conditioning dose. The potential importance of the present findings for radiotherapy of tumour cells is discussed. (U.K.)