Delivery of Radiation at the Lowest Dose Rate by a Modern Linear Accelerator is Most Effective in Inhibiting Prostate Cancer Growth.

PURPOSE: External beam radiotherapy is one of the treatment options for organ-confined prostate cancer. A total dose of 70 to 81 Gray (Gy) is given daily (1.8-2.5 Gy/d), with a dose rate of 3 to 6 Gy/min over 28 to 45 treatments during 8 to 9 weeks. We applied the latest technological development in linear accelerators for enabling a wide range of dose rates (from 0.2-21 Gy/min) to test the effect of different delivery dose rates on prostate tumor growth in an animal xenograft model. MATERIALS AND METHODS: A prostate cancer xenograft model was established in CD1/nude mice by means of PC-3 and CL-1 cells. The animals were radiated by a TrueBeam linear accelerator that delivered 4 dose rates ranging from 0.6 to 14 Gy/min, and reaching a total dose of 20 Gy. The mice were weighed and monitored for tumor development twice weekly. A 2-way analysis of variance was used to compare statistical differences between the groups. RESULTS: Tumor growth was inhibited by radiation at all 4 dose rates in the 20 study mice compared to no radiation (n = 5, controls). The most significant reduction in tumor volumes was observed when the same dose of radiation was delivered at a rate of 0.6 Gy/min (P < .01). The animals' weights were not affected by any dose rate. CONCLUSIONS: Delivery of radiation with a TrueBeam linear accelerator at the lowest possible rate was most effective in prostate cancer growth inhibition and might be considered a preferential treatment mode for localized prostate cancer.