Chikao Sugie, Presenter: Nothing to Disclose

Masato Ito, Abstract Co-Author: Nothing to Disclose

Hiroyuki Ogino MD, Abstract Co-Author: Nothing to Disclose

Akihiko Miyamoto MD, Abstract Co-Author: Nothing to Disclose

Nobuyuki Fukaya, Abstract Co-Author: Nothing to Disclose

Yuta Shibamoto MD, Abstract Co-Author: Nothing to Disclose

In stereotactic irradiation and IMRT, the effect of radiation may be reduced during intermittent exposures due to recovery from sublethal damage (SLDR). The purpose of this study was to investigate whether or not survival of tumor cells irradiated in vivo increased during intermittent irradiation, and to compare the results with those obtained in vitro previously.

EMT6 cells growing exponentially in vitro were transplanted subcutaneously into both hind legs of female 8-week-old Balb/c mice, and experiments were performed 10 days later when tumors grew about 1 cm in diameter. Three fractionation schedules were investigated. First, 2 doses of 10 Gy were given at an interval of 15 to 360 min. Second, 5 fractions of 4 Gy were given with interfraction intervals of 2.5 to 15 min each. Third, 10 fractions of 2 Gy were given with interfraction intervals of 1 to 7 min each. Doses of 15 to 20 Gy were also given without interruption to estimate dose-modifying factors. Tumors were excised 20 h later, minced with scissors, and treated with 0.1% neutral protease solution for 30 min. Tumor cells were then plated onto culture dishes and cell survival was determined by an in vivo-in vitro assay.

In the 2-fraction experiment, the increase in cell survival with elongation of interval was much less than that observed in our previous in vitro study. In the 5-fraction experiment, no significant increase in cell survival was observed due to intermittent exposures with intervals of up to 15 min between fractions. In the 10-fraction experiment, there was a trend for cell survival to decrease when the interruption between fractions was 2 to 7 min. These results were in striking contrast with the results of our previous in vitro study, in which cell survival increased significantly when a few minutes or longer intervals were posed between fractions.

Results of the present in vivo studies were different from those of our in vitro studies, and it was suggested that SLDR in vivo may be counterbalanced by other phenomena such as reoxygenation that sensitizes tumor cells to subsequent irradiation.

Sugie, C, Ito, M, Ogino, H, Miyamoto, A, Fukaya, N, Shibamoto, Y, Changes in the Effect of Radiation due to Intermittent Exposures in Murine Tumors: Comparison with in Vitro Results.  Radiological Society of North America 2004 Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 ,Chicago IL. http://archive.rsna.org/2004/4413966.html