High-precision radiation therapy techniques such as IMRT or sterotactic radiosurgery, delivers more complex treatment fields than conventional techniques. The increased complexity causes longer dose delivery times for each fraction. The purpose of this work is to explore the radiobiologic effect of prolonged fraction delivery time on tumor response and survival in vivo.

1-cm-diameter Lewis lung cancer tumors growing in the legs of C57BL mice were used. To evaluate effect of dose delivery prolongation, 18Gy was divided into different subfractions. 48 mice were randomized into 6 groups: the normal control group, the single fraction with 18 Gy group, the two subfractions with 30min interval group, the seven subfractions with 5min interval group, the two subfractions with 60min interval group and the seven subfractions with 10min interval group. The tumor growth tendency, the tumor growth delay and the mice survival time were analyzed.

The tumor growth delay of groups with prolonged delivery time was shorter than the group with single fraction of 18 Gy (P<0.05). The tumor grow delay of groups with prolonged delivery time 30min was longer than that of groups with prolonged delivery time 60min P<0.05). There was no significant difference between groups with same delivery time (P>0.05). Compared to the group with single fraction of 18 Gy, the groups with prolonged delivery time shorten the mice survival time while there was no significant difference between the groups with prolonged delivery time 30min and the groups with prolonged delivery time 60min.

The prolonged delivery time with same radiation dose shorten the tumor growth delay and survival time in the mice implanted with Lewis lung cancer. The anti-tumor effect decreased with elongation of the total interfractional time.

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