This report demonstrates that SBRT can achieve high rates of durable disease control for patients with low-risk prostate cancer while resulting in low levels of bladder and rectal toxicity. The current results extend prior independently conducted studies by the authors [9
], demonstrating the potential of SBRT monotherapy to provide durable disease control with few serious complications in low-risk prostate cancer patients. Our 5-year progression-free survival rate of 93% compares favorably with that obtained with surgery, LDR or HDR brachytherapy [21
In a recent update of the Stanford experience, which included 67 low-risk patients [27
], King et al. succinctly reviewed the rationale for hypofractionation in the management of prostate cancer. At a median follow-up of 2.7 years, the PSA relapse-free survival was 94%, and toxicity was equal to or lower than observed in dose-escalation studies. Disease control rates above 90% are entirely consistent with predictions based on an α/β ratio for prostate cancer of 1.5 Gy. Using the linear-quadratic radiobiologic model, 36.25 Gy yields an equivalent dose at 2 Gy per fraction, or EQD2, of 91 Gy for this α/β.
In addition, both disease control and toxicity outcomes with SBRT compare favorably to other treatments for low-risk prostate cancer. In a study comparing outcomes for radical prostatectomy and IMRT to a dose of at least 72 Gy [28
], no significant difference in 5-year biochemical disease-free survival (bDFS) rates was detected for low-risk patients (prostatectomy resulted in a bDFS of 92.8% vs. 85.3% for IMRT, p
= 0.20). Similar 5-year bDFS rates, ranging from 76% to 92% for radical prostatectomy, 69% to 89% for external beam radiotherapy at doses of 66 to 72 Gy, and 83% to 88% for seed brachytherapy, have been reported in retrospective comparisons of these various treatments [21
]. A recent report of a multi-institutional retrospective study comparing HDR brachytherapy to seed brachytherapy showed bDFS to be about 90% for both modalities. Somewhat higher 5-year bDFS rates, in the 92-95% range, have been obtained in other studies of surgery, high-dose and hypofractionated EBRT, and seed brachytherapy for low-risk patients [29
]. Thus, the 5-year bDFS of 92.7% obtained in the current study is clearly within the range of disease control expected using modern surgical and high-dose radiation techniques.
In the coming years, the long-term outcomes of several other studies of SBRT for organ-confined prostate cancer will be reported. Katz et al. reported 3-year results on 304 patients with low- and intermediate-risk disease, with favorable outcomes [11
]. An update with 42 months median follow up was presented at ASTRO 2010 [33
], and 5-year data from this study should be available in 2011. An additional 114 low-intermediate risk prostate patients were treated with SBRT in Naples in 2006, so that data will reach 5-year maturity next year. Acute toxicity from a prospective study underway at the University Hospitals Case Medical Center were presented at the 2009 ASCO meeting [34
]. Georgetown has also treated prostate cancer using SBRT; early data were presented at the 2010 ASCO meeting [35
]. Two prospective studies funded by Accuray, examining the effects of delivering either a homogeneous, EBRT-like dose distribution or an HDR-like, heterogeneous distribution [10
] should complete enrollment in the next 6 months, adding another 600 patients to the collective data pool. A phase III study comparing 12-fraction versus 5-fraction SBRT for localized prostate cancer is currently under review by the RTOG, and a proposed, phase III study from the University of Miami will compare extended fractionation (26 fractions) versus accelerated hypofractionation (5 fractions) for low-intermediate risk disease. As data from these various studies mature, we will develop a clearer picture of long-term outcomes following SBRT.