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.