A hematopoietic stromal lesion associated with fractionated radiotherapy (FxRT): time- and dose-effects.

Earlier, we reported that the local exposure of femoral bone marrow to a clinically-relevant course of fractionated radiotherapy [FxRT; 2.0 Gy (q24h x 5) --> 74 Gy] resulted in the delayed appearance of a stromal lesion which temporally appeared after exposures to a threshold dose of 20 Gy FxRT. To further define this threshold dose, the temporal recovery of blood-forming elements ("cobblestone area" forming colonies; CAFC(7-28d)) and the microenvironmental stroma (long-term bone marrow cultures; LTBMC) of the marrow was examined over a 17-week period following 10 and 30 Gy FxRT. After a subthreshold dose of 10 Gy, each of the 4 CAFC subpopulations were significantly dampened, with recovery initiated within a 6-week interval of 10 Gy of FxRT. Above the threshold dose (30 Gy FxRT), the CAFC subpopulations were again reduced to a level similar to that observed with 10 Gy FxRT. However, after exposures to 30 Gy FxRT, CAFC recovery was significantly well beyond the 6-week interval observed with a 10 Gy Dose of FxRT. Similarly, cell production in LTBMC prepared from marrow exposed to either 10 or 30 Gy FxRT was significantly dampened for at least 3 weeks following the FxRT. Moreover, while cell production in LTBMC derived from marrow exposed to 10 Gy was eventually restored to normal, the dampened cell production observed in LTBMC prepared after 30 Gy FxRT persisted for a period in excess of 17 weeks. Collectively, these observations provide additional support to our earlier observation suggesting that FxRT generates two forms of dose-dependent damage in the marrow: the first an early lesion arising in the blood-forming CAFC subpopulations; the second form, a delayed lesion that involves the persistent expression of a dysfunctional microenvironmental phenotype, appearing to disrupt the normal regulation of hematopoietic stem cell (HSC) proliferation and differentiation of the HSC during the recovery process.