THE BIOLOGICAL EFFECTIVENESS OF INTERMITTENT IRRADIATION AS A FUNCTION OF OVERALL TREATMENT TIME - DEVELOPMENT OF CORRECTION FACTORS FOR LINAC-BASED STEREOTAXIC RADIOTHERAPY

Purpose: Continuous irradiation of relatively short duration as admini stered in gamma-ray stereotactic radiosurgery (SRS) is biologically no t equivalent to the more protracted intermittent exposures during acce lerator-based radiosurgery with multiple arcs, Accelerator-based SRS a nd fractionated stereotactic radiotherapy (SRT) is currently performed with a high degree of variability in equipment and techniques resulti ng in highly variable treatment delivery times, The present work is de signed to quantify the effects of radiation delivery times on biologic al effectiveness, For this, the intermittent radiation delivery scheme s, typical for linac-based SRS/SRT, have been simulated in vitro to de rive biological correction factors, Methods and Materials: The experim ents were carried out using U-87MG human glioma cells in suspension at 37 degrees C irradiated with 6 MV X-rays to clinically relevant doses ranging from 6 to 18 Gy, delivered over total irradiation times from 16 min to 3 h, The resulting cell survival data was used to calculate dose correction factors to compensate for aide variations in dose deli very times, Results: At each total dose level, cell survival increased with increasing total irradiation time, The increase in survival was more pronounced at higher dose levels, At a total dose of 12 Gy, cell survival increased by a factor of 4.7 when irradiation time was increa sed from 16 to 112 min, Dose correction factors were calculated to all ow biologically equivalent irradiations over the range of exposure tim es, Cells irradiated with corrected total doses of 11.5 Gy delivered i ncrementally in 16 min up to 13.3 Gy in 112 min were found to exhibit the same survival within the experimental limits of accuracy, Conclusi ons: For a given total dose, variations in dose delivery time typical of SRS/SRT techniques will result in significant changes in cell survi val, In the dose range studied, an isoeffect dose correction factor of 2 to 3 cGy/min was shown to compensate for the change in delivery tim e for U-87 MG human gloma cells in vitro, (C) 1997 Elsevier Science In c.