High efficacy of particle beam therapies against tumors under hypoxia and prediction of the early stage treatment effect using 3'-deoxy-3' [18F]fluorothymidine positron emission tomography

Objective Compared with radiation therapy using photon beams, particle therapies, especially those using carbons, show a high relative biological effectiveness and low oxygen enhancement ratio. Using cells cultured under normoxic conditions, our group reported a greater suppressive effect on cell growth by carbon beams than X-rays, and the subsequent therapeutic effect can be predicted by the cell uptake amount of 3'-deoxy-3'-[F-18]fluorothymidine (F-18-FLT) the day after treatment. On the other hand, a hypoxic environment forms locally around solid tumors, influencing the therapeutic effect of radiotherapy. In this study, the influence of tumor hypoxia on particle therapies and the ability to predict the therapeutic effect using F-18-FLT were evaluated.Methods Using a murine colon carcinoma cell line (colon 26) cultured under hypoxic conditions (1.0% O-2 and 5.0% CO2), the suppressive effect on cell growth by X-ray, proton, and carbon irradiation was evaluated. In addition, the correlation between decreased F-18-FLT uptake after irradiation and subsequent suppression of cell proliferation was investigated.Results Tumor cell growth was suppressed most efficiently by carbon-beam irradiation. F-18-FLT uptake temporarily increased the day after irradiation, especially in the low-dose irradiation groups, but then decreased from 50 h after irradiation, which is well correlated with the subsequent suppression on tumor cell growth.Conclusions Carbon beam treatment shows a strong therapeutic effect against cells under hypoxia. Unlike normoxic tumors, it is desirable to perform F-18-FLT positron emission tomography 2-3 days after irradiation for early prediction of the treatment effect.