Responses of total and quiescent cell populations in solid tumors to carbon ion beam irradiation (290 MeV/u) in vivo.

Abstract

Purpose: The aim of this study was to clarify the radiosensitivity of intratumor total cells and quiescent (Q) cells in vivo to accelerated carbon ion beams compared with gamma-ray irradiation.

Materials and methods: Squamous cell carcinoma (SCC) VII tumor-bearing mice received continuous administration of 5-bromo-2'-deoxyuridine (BrdU) to label all intratumor proliferating (P) cells. They then were exposed to carbon ions (290 MeV/u) or gamma-rays. Immediately after and 12 h after irradiation, immunofluorescence staining for BrdU was used to assess the response of Q cells in terms of micronucleus frequency. The response of the total (P + Q) tumor cells was determined from the tumors not treated with BrdU.

Results: The apparent difference in radiosensitivity between total and Q cell populations under gamma-ray irradiation was markedly reduced with carbon ion beams, especially with a higher linear energy transfer (LET) value. Clearer recovery in Q cells than in total cells through delayed assay under gamma-ray irradiation was efficiently inhibited by carbon ion beams, especially those with a higher LET.

Conclusion: In terms of the tumor cell-killing effect as a whole, including intratumor Q cells, carbon ion beams, especially with higher LET values, were extremely useful for suppressing the dependence on the heterogeneity within solid tumors as well as depositing the radiation dose precisely.