Prostate cancer is the most common cancer diagnosed in men, and ranks second in estimated
      deaths. The current standard of care for localized prostate cancer consists of observation,
      surgery or radiation therapy. From a radiation perspective, accepted options include external
      beam radiation therapy, brachytherapy, or a combination of those. Acceptable external beam
      radiation options are further defined as conventionally fractionated (over approximately 8-9
      weeks), or moderately hypo-fractionated (over approximately 4-6 weeks).

      Stereotactic body radiation therapy (SBRT) is a newer technique, allowing treatment to be
      delivered in an extremely hypo-fractionated regimen (i.e. in 5 fractions). Current consensus
      guidelines describe SBRT as "a cautious alternative to conventionally fractionated regimens
      at clinics with appropriate technology, physics, and clinical expertise."

      The goal of any therapy is to maximize the effect of that treatment on cancer cells and
      minimize any effect on normal tissue. In radiation therapy, this is mainly done through dose
      targeting (i.e. limiting the radiation dose to the targets and minimizing dose to normal
      surrounding organs). Advancements in radiation therapy have allowed for better targeting,
      resulting in shrinking margins of treatment around cancer cells.

      The therapeutic window can also be optimized through the use of fractionation (i.e. amount of
      dose delivered per day) in radiation therapy. Since cancerous and normal tissue cells respond
      differently to changes in fractionation, the therapeutic window could theoretically be
      improved by choosing a fractionation pattern to which cancer cells are more sensitive.
      Historical generalizations put cancer cells into the category of early responding tissues,
      which would make them more sensitive to cell death when radiation is delivered in a
      protracted fractionation pattern, such as over 8-9 weeks of radiation therapy.

      However, studies have emerged which suggest that prostate cancer is unlike other cancers and
      reactive more like late responding tissues. With that in mind, prostate cancer cells could be
      more sensitive to high doses per fraction, which could provide a radiobiological advantage
      and a greater therapeutic window with the use of SBRT.

      Very promising early results utilizing SBRT for prostate cancer treatment have been described
      in the literature, and multiple ongoing phase 3 trials are underway.

      Aside from a potential therapeutic advantage, SBRT also offers a more cost effective solution
      to patients with prostate cancer, with less impact on their daily lives (only needing to come
      in for 5 treatments instead of 40-45 treatments).

      Much of the literature on SBRT has been done utilizing the CyberKnife system. But the Elekta
      Versa HD and Agility Systems available at Hoag provides a unique opportunity for faster
      treatment delivery and 4D imaging during treatment. The Elekta Versa HD and Agility systems
      are approved by the FDA for SBRT treatment of cancer.

      This study seeks to investigate biochemical failure rates for patients treated with SBRT
      utilizing Elekta machines, as well as obtain quality of life data and assess the
      intrafraction motion in approximately 15 patients for 5 years following SBRT.