Analysis of Motion-dependent Clinical Outcome of Tumor Tracking Stereotactic Body Radiotherapy for Prostate Cancer

BACKGROUND: To analyze clinical outcome of CyberKnife (CK) tumor-tracking stereotactic body radiotherapy (SBRT) for prostate cancer (Pca) according to the magnitude of intra-fractional prostate motion. METHODS: Medical records and daily treatment logs for 71 patients who received CK tumor-tracking SBRT were retrospectively analyzed. Statistical relationships between prostate motion and various outcome results, including local recurrence (LR), biochemical failure (BF), and treatment-related toxicity, were investigated in order to evaluate motion-dependent efficacy of tumor-tracking SBRT for Pca. RESULTS: In a total 71 patients, 3 (4.2%) patients with LR, 12 (16.9%) patients with BF, and 22 (31%) patients with grade-II or worse toxicities to rectal or bladder (22 to rectal, 22 to bladder and 8 patients to both) were observed in a median follow-up of 47 months. Magnitudes of intra-fractional tumor motion along superior-inferior, right-left, and anterior-posterior (AP) axes were 0.15 ± 0.31, 0.12 ± 0.19, and 0.73 ± 0.32 mm, respectively. Radial magnitude was estimated to be 1.0 ± 0.35 mm. Intra-fractional movement was not significantly correlated with tumor control. However, it was significant correlated with the incidence of grade-II or worse toxicity to rectum or bladder particularly when tumor motion was in the AP axis. CONCLUSION: Our quantitative results revealed that toxicity related to SBRT treatment was highly sensitive to intra-fractional prostate movements, although local-tumor control was not affected by such movements. Our results demonstrate that precise motion correction is essential in prostate SBRT, even if it seems to be small.