CyberKnife Stereotactic Body Radiotherapy (SBRT)

A new and very promising technology for the treatment of localized prostate cancer

In addition to state-of-the art IMRT/IGRT, we are also pleased to be a national leader in the use of CyberKnife Stereotactic Body Radiotherapy (SBRT) for selected prostate cancer patients. This process is illustrated in the highlighted figures and described below.

Background

CyberKnife SBRT technology utilizes the same basic concepts as IMRT/IGRT, with ceiling-mounted stereotactic X-ray imagers locking on the same implanted gold fiducial markers within the prostate that we described in the IGRT/IMRT section, yet delivers the therapeutic radiation in a more complex, laborious and precise way. The most unique aspect of the CyberKnife device is its capability to continuously track the prostate (or any defined target lesion within the body) by an automated "artificial intelligence" X-ray feedback loop, which continuously updates the location of the prostate fiducial markers throughout the entire treatment. This allows sub-millimeter precision of beam aiming from start to finish. The CyberKnife device treats from a wide variety of beam angles, delivered from a lightweight linear accelerator, which is mounted on a sophisticated and flexible articulating robotic arm. The CyberKnife device and its geometry are illustrated in Figure 1.

In contradistinction to "traditional" prostate radiotherapy approaches such as IGRT/IMRT, which typically take 6-9 weeks to complete, CyberKnife SBRT has some very important differences:

  • Much shorter course of treatment (1-5 treatments total), using much larger daily doses.
  • Longer individual treatments
    • ~ 1 hour per CK treatment versus ~ 10 minutes per IGRT/IMRT treatment
  • Sharper (more "surgical") treatment margins.
  • Relatively more"biologically ablative" to targeted tissues
    • Most appropriately applied to well defined, sharply marginated volumes.
  • Re: prostate cancer:
    • CyberKnife SBRT as monotherapy is most appropriately applied to patients with early stage lesions that have a very low chance of disease beyond the immediate prostate region.
    • CyberKnife SBRT may also be used as a "boost" to the prostate in conjunction with wide field pelvic IMRT in more advanced lesions (versus a longer course of customized IMRT/IGRT without CyberKnife). Advanced cases tend to call for "individualized" solutions.
    • CyberKnife SBRT may also be used as a local "salvage" method for patients with biopsy proven cancer recerrence in their prostate following "conventional" radiotherapy (Potentially a higher-risk application, currently restricted to clinical trial format, which we offer)
    • CyberKnife delivers a more potent form of radiation versus standard IMRT/IGRT. Although this is advantageous from a cancer control point of view, there is also a potentially higher risk of prostate swelling and urinary outflow obstruction in patients with preexisting obstructive prostate problems. These patients may be more safely and effectively treated with "conventional" longer course IMRT/IGRT, which is comparatively biologically "gentler" to a patent with this potential complication. This is an individualized consideration, which is part of the evaluation and treatment recommendation process - much more of a concern for patients with significant potential urinary obstructive problems than for those without this issue.

Process

There are several steps involved in the CyberKnife SBRT process, including the following:

1. Radiation Oncologist Consultation: The original step in the process is a thorough review of the patient's medical records, lab results, imaging studies, and physical examination during a consultation that usually lasts about an hour. If CyberKnife emerges as the preferred alternative the treatment process described below will ensue, starting with step 2 "fiducials."

NOTE: CyberKnife SBRT is a newer technology versus IMRT/IGRT, and although the results appear promising in the 3-5 year follow-up time frame, longer-term efficacy confirmation on a par with IMRT (10 years), brachytherapy (10 years) or radical prostatectomy (10 years) have not been documented. This means that some insurers do not cover CyberKnife SBRT for prostate cancer, deeming the procedure "experimental," though there has been significant improvement in prostate CyberKnife insurance coverage over the last several years. Medicare in California currently covers the prostate CyberKnife procedure as long as the patient is enrolled on a www.clinicaltrials.gov listed clinical trial. This is not a problem in our practice as we are the principal investigator in three such trials.1,2,3 A recent update of our original CyberKnife Clinical Trial is presented in Figure 2.

2.  Fiducials: The first step in the CyberKnife treatment planning process is the implantation of several tiny gold markers, known as fiducials, into the prostate.

NOTE: Unlike with IMRT/IGRT, where fiducials may or may not be required, fiducials are mandatory for ALL CyberKnife prostate patients. This is a minimally invasive procedure done under local anesthesia. The implanted fiducials serve as the three-dimensional targeting reference structure for the CyberKnife SBRT process.

3. Simulation: The next step is the obtaining of CT and MRI images of the prostate - a process known as simulation. These images are then imported into the CyberKnife treatment planning computer for detailed anatomic analysis and contouring by the radiation oncologist, followed by three-dimensional treatment planning by our physics/dosimetry staff.  This process represents a patient-specific "medical engineering" project and forms the basis upon which the daily treatments are designed. Figure 3 represents a completed CyberKnife SBRT plan, constructed from superimposed CT and MRI images.

NOTE: The simulation process for our prostate CyberKnife patients includes a placement of a Foley catheter in the urethra to allow radiological visualization of its course through the prostate, to enable accurate urethral sparing from the high dose regions. This requirement is unique to CyberKnife patients, due to its more potent and sharp dose gradients within the prostate, and not currently used for our IMRT/IGRT patients, whose intraprostatic radiation dose gradients are comparatively less extreme. In other words, it is more important to map the exact urethra location in CyberKnife patients, so that it may be most accurately spared from the high dose regions.

4. CyberKnife Treatment: Following completion of the simulation process, the scheduled course of daily CyberKnife treatments will begin about a week later, typically completed over 4 - 5 days. Each CyberKnife treatment session takes 45-60 minutes and consists of multiple automated steps - including stereotactic prostate fiducial X-ray image acquisition, computerized beam aiming adjustment and beam on, repeated dozens to hundreds of times per session, depending upon the complexity of the CyberKnife treatment plan, until the entire treatment has been delivered.  The process is painless and patients remain fully active and may continue working throughout the entire treatment course.

5. Medical Care and follow-up: During the treatment course, the patient will be checked during their course of treatment by the radiation oncologist, primarily to assess and treat side-effects as well as answer any questions from the patient or their family.  After the course of CyberKnife SBRT has been completed the patient will be given a follow-up appointment and this is a part of every patient's final day sign-out process. Usually the first follow-up radiation oncologist appointment is 1-2 weeks after conclusion of treatment, with multiple follow-up evaluations during year one post-treatment, and typically twice per year follow-up thereafter. Our CyberKnife follow-up schedule is modestly more frequent than out typical IMRT/IGRT follow-up schedule, due to the fact that many more of our CyberKnife patients are enrolled on clinical trials. Over the very long-term (> 5 years) , some patients will be followed by us indefinitely, while others will eventually be discharged back to their referring physician or primary physician, and this decision is individualized to each patient's specific situation and desire. Again, due to the CyberKnife clinical trial aspect, long-term follow-up is more prevalent in our CyberKnife patients to document long-term outcomes for possible publication in peer reviewed medical literature.  Ultimately we aim to make the treatment and follow-up process as efficient for the patient and family as possible, while maintaining the necessary degree of completeness and attention to detail, until the final outcome of treatment is determined.