Stereotactic Body Radiotherapy (SBRT) in the Management of Clinically Localized Prostate Cancer: Where are We Now?

Background: Stereotactic body radiotherapy (SBRT) is a recent addition to the armamentarium for treating clinically localized prostate cancer. SBRT is a special form of image-guided radiotherapy (IGRT) that involves the delivery of a small number of very high-dose fractions (e.g. 35 to 40 Gy, in 4 to 5 fractions). The slow-growing nature of prostate adenocarcinoma is thought to be associated to a low α[alpha]/β[beta] ratio of ~1.5, that is believed to confer a preferential sensitivity to ultra-hypofractionation. Literature documenting excellent long-term outcomes and relatively low toxicity with high dose-rate (HDR) brachytherapy (using similar dose-fractionation) provided a clinical rationale for SBRT. Methods: A review was performed of available literature focusing on the current status of SBRT for clinically localized prostate cancer, including trends, evidence of late toxicity and unresolved issues. Results: The adoption of SBRT for prostate cancer has been rapid, increasing from <1% in 2004 (in all patient groups), to 8.8% by 2012. Cost and convenience have contributed to this trend, despite the risk that the toxicity may be slightly higher. RTOG 0938 demonstrated the clinical tolerance and feasibility in a multi-institutional setting and is expected to form the basis for a definitive phase III trial comparing SBRT to a more conventional fractionation scheme. Whether the use of rectal spacers or modified dose constraints will improve the risk benefit ratio remains to be elucidated. More work is also needed to assess the role of SBRT as a boost after conventional radiation, in the setting of “salvage” therapy and as palliation for urinary tract obstruction due to prostate cancer. Conclusion: There appears to be a substantial and growing interest in SBRT for clinically localized prostate but much more work is needed to define its role relative to other forms of radiation including conventional external beam radiation and brachytherapy.