Responses of Total and Quiescent Cell Populations in Solid Tumors to Boron and Gadolinium Neutron Capture Reaction Using Neutrons with Two Different Energy Spectra

In neutron capture therapy, whose effectiveness depends on the tumor distribution of neutron capture compound and the neutron energy distribution, controlling quiescent tumor cells with clonogenic potential is critical for therapeutic gain, as is the case in conventional radio‐ and chemotherapy. Tumor‐bearing mice were continuously given 5‐bromo‐2′‐deoxyuridine (BrdU) to label all proliferating cells. After administration of sodium borocaptate‐(10)B (BSH), dl‐p‐boronophenylalanine‐(10)B (BPA) or gadodiamide hydrate (Omniscan), the tumors were irradiated with neutrons of different cadmium (Cd) ratio, and then isolated and incubated with cytochalasin‐B (a cytokinesis blocker). The micronucleus (MN) frequency in cells without BrdU labeling (quiescent cells) was determined using immunofluorescence staining for BrdU, and that for total cells was obtained from tumors not pretreated with BrdU. Without drugs, quiescent cells showed lower MN frequencies than total cells, but neutron irradiation reduced gamma‐ray sensitivity difference between the two. Relative biological effectiveness (RBE) of neutrons compared with gamma‐rays was greater in quiescent cells than in total cells, and low Cd ratio neutrons tended to exhibit large RBE values. With neutron capture compounds, MN frequency for each cell population was increased, especially when high Cd ratio neutrons were used. BPA increased the MN frequency for total cells to a greater extent than BSH. However, the sensitivity of quiescent cells treated with BPA was lower than that in BSH‐treated quiescent cells. This tendency was clearly observed in high Cd ratio neutrons. Omniscan only slightly increased the MN frequency in both cell populations, compared with irradiation alone, without drugs. From the viewpoint of increasing the quiescent cell sensitivity, tumors should be irradiated with high Cd ratio neutrons after BSH administration.