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(PP223) In vitro comparison of CyberKnife® stereotactic radiosurgery and standard external-beam radiotherapy.
Balfour, Eric2, Singh, Anu2, Jolly, Puneet*,1, Velana, Alfredo1, Jung, Mira1, Gagnon, Gregory1,2, 2 Georgetown University Hospital, Washington, DC, United States1 Georgetown University School of Medicine, Washington, DC, United States
ABSTRACT- Purpose/Objective: It has been suggested that any fraction delivery lasting greater than 30 minutes may result in a clinically significant loss of cell-sterilizing effect(Fowler 2004). Stereotactic radiosurgery using CyberKnife® typically transpires over an hour s duration whereas dose rates of ~100-300 cGy/min are usually employed for standard external-beam radiotherapy, allowing for treatment of only a few minutes. Sublethal(SLDR) and potential lethal (PLDR)radiation repair in tumors and normal tissues can occur within an hour of radiation exposure. Therefore, some of the efficacy of the radiation treatment may be lost due to this rapid repair during this time. Another important issue is that while dose rate effects (DRE) can be accurately predicted using standard external-beam radiotherapy, variation in DREs is a major consideration with CyberKnife® stereotactic radiosurgery (SRS). This variation is due to a number of factors including repair differences and radiation sensitivity among different tumor types, blood flow to the tumor and inherent variation in dose-rate delivery due to the use of small (5-60mm) collimators which vary dose rate in highly complicated ways throughout the target volume. In the present study, we have attempted to measure differences in cell survival for several tumor cell lines treated with the same doses of radiation delivered using either the CyberKnife® or delivered using the linear accelerator. Materials/Methods: Various tumor cell lines were plated one day prior to treatment. They were to be treated with no radiation, 8Gy and 16Gy respectively. Each cell line was irradiated within a custom incubator at 37 C and 5% CO2 to insure continual metabolic activity, and irradiated either by the CyberKnife® or by linear accelerator, with treatment times of approximately one hour per 8Gy and 10 minutes per 8 Gy, respectively. Homogeneity of dose was within 10% for both irradiation schemes. After 5 days, cells were washed and stained colonies were counted with > 50 cells/colony being scored as positive. Results: We found no consistent difference in radiation response between the two treatment modalities. Interestingly, there was a mild decrease in cell survival in cell lines with fast doubling times when treated utilizing the CyberKnife® (Figure 1). Conclusions: While the in vitro nature of these experiments limits any broad conclusions, these data suggest that the results using CyberKnife® stereotactic radiosurgical treatment is comparable to external-beam radiotherapy with respect to tumor cell survival. In addition, our results demonstrate no significant loss of biological effect by using the CyberKnife® for treatment.
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