CyberKnife Dose Escalation Prostate Cancer Trial

The optimal radiation schedule for the curative treatment of prostate cancer remains unknown. Prostate cancer patients receiving radiation therapy are typically treated 5 days per week for 8-9 weeks. Recent data suggest that large radiation fraction sizes are radio-biologically favorable over lower fraction sizes in prostate cancer radiotherapy. The sensitivity of a tumor or normal tissue to fraction size of radiation can be approximated by the alpha-beta ratio. It has been suggested that the alpha/beta ratio for prostate cancer is actually as low as 1.5 implying that the current radiation therapy paradigm for prostate cancer treatment might be fundamentally flawed, as high fraction sizes would be expected to damage tumor more readily.

Typical prostate SBRT doses do not appear to have similar efficacy in higher risk prostate cancer suggesting even higher doses are required. Many techniques including dose escalated external beam radiation therapy (EBRT), proton therapy (PT) and brachytherapy have been employed to increase dose to the prostate. Data from the ASCEND-RT trial utilizing low dose rate brachytherapy boost showed a dramatic 21% improvement in biochemical control at 9 years favoring brachytherapy boost compared to conventional dose escalated EBRT radiation therapy. However, no corresponding benefit was identified in overall survival, incidence of bone metastases or prostate cancer specific mortality while a 3 fold increase in late urinary toxicity was noted.

SBRT is well tolerated with minimal acute and late side effects. In this protocol, CK-SBRT will be used to target the microscopic and gross disease in the prostate, seminal vesicles. An escalated dose of 40 Gy in 5 fractions will be delivered to the entire target volume while any nodules visible within the prostate gland on endorectal MRI will receive 50 Gy in 5 fractions.