Evaluation of apoptosis and micronucleation induced by reactor neutron beams with two different cadmium ratios in total and quiescent cell populations within solid tumors

Purpose: Response of quiescent (Q) and total tumor cells in solid tumors to reactor neutron beam irradiation with two different cadmium (Cd) ratios wa s examined in terms of micronucleus (MN) frequency and apoptosis frequency, using four different tumor cell lines. Methods and Materials: C57BL mice bearing EL4 tumors, C3H/He mice bearing S CC VII or FM3A tumors, and Balb/c mice bearing EMT6/KU tumors received 5-br omo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmot ic pumps to label all proliferating (P) cells. Thirty min after i.p. inject ion of sodium borocaptate-B-10 (BSH), or 3 h after oral administration of p -boronophenylalanine-B-10 (BPA), the tumors were irradiated with neutron be ams. The tumors without B-10-compound administration were irradiated with n eutron beams or gamma -rays. This neutron beam irradiation was performed us ing neutrons with two different Cd ratios. The tumors were then excised, mi nced, and trypsinized. The tumor cell suspensions thus obtained were incuba ted with cytochalasin-B (a cytokinesis blocker), and the MN frequency in ce lls without BrdU labeling (=Q cells) was determined using immunofluorescenc e staining for BrdU. Meanwhile, for apoptosis assay, 6 h after irradiation, tumor cell suspensions obtained in the same manner were fixed, and the apo ptosis frequency in Q cells was also determined with immunofluorescence sta ining for BrdU. The MN and apoptosis frequencies in total (P + Q) tumor cel ls were determined from the tumors that were not pretreated with BrdU. Results: Without B-10-compounds, the sensitivity difference between total a nd Q cells was reduced by neutron beam irradiation. Under our particular ne utron beam irradiation condition, relative biological effectiveness (RBE) o f neutrons was larger in Q cells than in total cells, and the RBE values we re larger for low Cd-ratio than high Cd-ratio neutrons. With B-10-compounds , both frequencies were increased for each cell population, especially for total cells. BPA increased both frequencies for total cells more than BSH d id. Nevertheless, the sensitivity of Q cells treated with BPA was lower tha n that of Q cells treated with BSH. Whether based on the MN frequency or th e apoptosis frequency, similar results concerning the sensitivity differenc e between total and Q cells, the values of RBE, and the enhancement effect by the use of B-10-compound were obtained. Conclusion: Apoptosis frequency, as well as the MN frequency, can be applie d to our method for measuring the Q cell response to reactor neutron beam i rradiation within solid tumor in which the ratio of apoptosis to total cell death is relatively high, as in EL4 tumor. The absolute radiation dose req uired to achieve the same endpoint for Q cells is much higher than that for total cells when combined with B-10-compound, especially with BPA. (C) 200 1 Elsevier Science Inc.