CyberKnife Stereotactic Radiotherapy as Monotherapy for Low- to Intermediate-Stage Prostate Cancer: Early Experience, Feasibility, and Tolerance

Purpose: The CyberKnife (CK), a linear accelerator mounted on a robotic device, enables excellent dose conformation to the target and minimizes dose to surrounding normal tissue. It is a very suitable device for performing hypofractionated stereotactic body radiotherapy as monotherapy for low-to intermediate-risk prostate cancer patients. We report our early experience using this technique. Materials and Methods: Between June 2008 and June 2009, 10 patients underwent CK monotherapy as treatment for their prostate cancer (stage <= T2b, Gleason score (GS) <= 7, initial PSA <= 15 mu g/L). The prescribed dose was 38 Gy in four daily fractions of 9.5 Gy. The International Prostate Symptom Score and Radiation Therapy Oncology Group symptom scale were prospectively administered before and at 0.5, 1, 2, 3, 6, and 12 months. Results: Median age of the patients was 71 years (range, 66-76). Three patients had stage T2a and 7a T1c disease, one patient had GS of 7, and all others had GS of 6. Median follow-up was 5.1 months. Median initial PSA was 8.3 ng/mL (range, 1.3-13.6 ng/mL). Median planning target volume delineated on computed tomography after matching with the magnetic resonance imaging scan was 107 cc (range, 42-158 cc). The median V100 of the prostate was 95.8% (range, 94.8-97.2). The D95 of the prostate was 38.3 Gy (range, 38.1-38.8 Gy). The constraints for the bladder, rectum, and urethra were well met. The International Prostate Symptom Scores after 3 months were stable compared with the pretreatment scores. Urinary and bowel Radiation Therapy Oncology Group symptoms were mild and within the expected levels. Conclusions: This regimen of stereotactic CK monotherapy for low-to intermediate-risk prostate cancer with excellent dose coverage of the prostate was well tolerated. Data collection is ongoing for further assessment of toxicity and PSA response.