Abstract:
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Purpose: In modern high-tech radiotherapy such as intensity modulated radiation therapy (IMRT) and stereotactic irradiation, radiation doses are usually administered intermittently. In linear accelerator radiosurgery, 30 minutes or longer time is often required in one treatment session, and in some IMRT procedures, up to 30 minutes are needed to deliver 2 Gy. The present study was conducted to determine if the dose delivered with such intermissions is equivalent to the dose of continuous irradiation, and if not, to estimate the dose-modifying factors in these situations.
Methods and Materials: Murine SCCVII and EMT6 cells in culture were used. Radiation was delivered using a 210 kVp X-ray machine at a dose rate of 2.1 Gy/min. In the first two experiments, two doses of 4 Gy were given with an interruption of 15 min to 6 h or 1 to 10 min, or without interruption. Next, 5 fractions of 1.6 Gy was given with interfraction intervals of 1 to 5 min each or without interruption. Total doses of 6 to 7.5 Gy were also given without interruption to estimate dose-modifying factors. Cell survival was determined by a colony assay. In the fourth experiment, a total dose of 2 Gy was given in 5 fractions with interfraction intervals of 1 to 5 min each or without interruption, and the response to radiation was evaluated by a cytokinesis-block micronucleus assay.
Results: In the single-split dose experiments, a significant increase in the cell survival due to recovery from sublethal damage (SLDR) was observed when the interruption time was 3 min or longer in SCCVII cells and 2 min or longer in EMT6 cells. Compared to cell surviving fractions with no radiation interruption, the cell survival increased by 5% with a 2-min interval and 18% with a 5-min interval in SCCVII. In EMT6, the survival increase was 8% and 13%, respectively. In the 5-fraction experiments, SLDR was evident when the interfraction interval was 2 min or longer. The dose-modifying factor appeared to be approximately 1.03-1.06 when the interfraction interval was 2 min (total interruption time, 8 min) and 1.07-1.12 with the intervals of 5 min (total 20 min interruption).
Conclusion: The results of this study strongly suggest that the effects of IMRT and stereotactic radiosurgery that take more than 15 min from the start to the end of radiation are less than the effect of the same dose administered continuously. In such situations, dose modification is necessary based on the biologically determined dose-modifying factors.
Shibamoto MD, PhD, Y,
Is the Dose of IMRT Equivalent to That of Continuous Irradiation? Recovery from Sublethal Damage during Several Minutes of Radiation Interruption in Cultured Tumor Cells. Radiological Society of North America 2003 Scientific Assembly and Annual Meeting, November 30 - December 5, 2003 ,Chicago IL.
http://archive.rsna.org/2003/3100829.html
