CyberKnife Therapy for Prostate Cancer
Frequently Asked Questions
» What is CyberKnife, and how can it be used to treat prostate cancer?
» What happens when CyberKnife is used to treat prostate cancer?
» Why is CyberKnife used to treat prostate cancer?
» How effective is CyberKnife for treating prostate cancer?
» Who is treated with CyberKnife?
Q. What is Cyberknife and how can it be used to treat prostate cancer?
CyberKnife is a robotically-controlled radiation delivery machine. It was developed about in the 1990s primarily as a device to deliver Sterotactic Radiosurgery (SRS) for tumors of the brain. SRS has been used for more than three decades to treat brain tumors. In the last several years, this technology has been augmented so it can be applied to other parts of the body, including the prostate for men with prostate cancer.
It is important to understand that CyberKnife therapy for prostate cancer does not involve a new form of radiation. Rather, a new device delivers the same high energy X-rays (radiation) to the prostate. The X-rays coming out of the CyberKnife machine are the same type that are used in standard external beam radiation treatments (IMRT/IGRT).
With CyberKnife, 200 separate beams of radiation are focused on a tumor. The beams are very narrow, so that a very large amount of radiation is delivered to a very small area. The aim is to use a high dose of radiation to treat the prostate cancer while delivering a very small dose to the surrounding normal tissues; in this case, the rectum and bladder. The procedure requires great precision.
Q. What happens when CyberKnife is used to treat prostate cancer?
There are two unique aspects to CyberKnife treatment.
The first is the radiation machine. The CyberKnife is basically a large robotic arm. This robot is designed to carry a heavy load and move with sub-millimeter precision. Attached to the robot arm is a mini-radiation machine called a linear accelerator, or LINAC. Unlike standard radiation machines, which are very large and weigh several tons, this mini-LINAC is relatively small and weighs only a couple hundred pounds. In addition, the radiation beam is very narrow and can be adjusted from the diameter of a pea to a quarter. Both types of machines deliver the same type of high energy X-rays, which have been used for more than 50 years to treat cancer.
The second feature of the CyberKnife is the real-time tracking of the target during the actual radiation treatments. No other device has this capability. The CyberKnife adjusts, in real time, to the position of the prostate.
In the first step in CyberKnife prostate cancer treatment, a urologist places into the prostate four very small slivers of gold, called fiducials. This is done in a procedure very much like a prostate biopsy. The fiducials serve as reference points for the location of the prostate during treatment. This is needed because the prostate is not attached to bone and continually moves. About a week or so later, the patient undergoes a CT scan (and, for most, an MRI too). These scans are used to develop a plan to deliver the radiation with the CyberKnife.
The patient returns the following week for his first CyberKnife treatment. Each treatment involves lying on a table while the robotic arm moves the mini-LINAC to about 200 positions. At each position, a narrow beam of radiation is delivered to the prostate. X-rays are taken during the treatment and a computer identifies the four gold markers in the prostate. If the prostate moves at all during the treatment, the robot adjusts for this motion.
Each treatment takes about 45 minutes. Depending on the situation, the patient receives two to five treatments. In contrast, standard external beam radiation patients receive up to 44 treatments over eight to nine weeks.
Q. Why is CyberKnife used to treat prostate cancer?
There are two main reasons why CyberKnife is a very attractive approach to treat prostate cancer. One is theoretical; the other is based on long-term data.
The CyberKnife approach is to use only a few high dose treatments to treat prostate cancer, as opposed to many low dose treatments (as is done with standard external beam radiation), or with continuous low irradiation (as in "seeds" or low dose rate brachytherapy). The technical term for this is accelerated hypofractionation. There is evidence (in the way prostate cancer cells respond to radiation) that accelerated hypofractionated radiation is better at killing prostate cancer.
Dosing for CyberKnife prostate cancer treatment is developed using this model, which has been used for more than 25 years to develop various radiation dosing schedules.
The second line of evidence that accelerated hypofractionated radiation for prostate cancer is safe and very effective comes from high dose rate brachytherapy (HDR). With this form of radiation, the patient is brought to the operating room and hollow needles are implanted into the prostate. After the patient leaves the operating room with the needles in place, one (or several) HDR treatment(s) is given. With this procedure, high doses of radiation are delivered in only a few treatments (accelerated hypofractionation).
Several studies with long-term follow-up have shown that when this technique is used alone or in combination with external beam radiation, it is safe and very effective.
Q. How effective is CyberKnife for treating prostate cancer?
The CyberKnife was developed by a neurosurgeon at Stanford University, which became the first academic institution to study the use of CyberKnife for prostate cancer. In their first study of fewer than 50 patients with early prostate cancer, Stanford researchers reported that treatment was safe, with very few side effects and no serious side effects. They also reported excellent cancer control with no recurrences.
Recent studies have reported results of more than 1,000 patients treated with CyberKnife for prostate cancer, with follow-up up to eight years, which has shown that CyberKnife is very effective at treating localized prostate cancer.
Twenty-one CyberKnife centers in the United States have conducted a trial in which patients with early prostate cancer received five treatments of accelerated hypofractionated treatment the same way the Stanford researchers treated their initial patients. To date, more than 320 patients have taken part in this study. (Forty-two patients are from BIDMC's Keith C. Field CyberKnife Center.) Results concerning the first 250 patients have been reported; the rate of minor side effects was low, acceptable and similar to what is observed with external beam radiation. Cancer control rates also have been excellent.
Based on these results, most (but not all) insurance compaines cover CyberKnife treatment for prostate cancer. Before you proceed with treatment, ask BIDMC for help determining whether your insurer is one of them.
Q. Who is treated with CybeKnife?
Several types of radiation can be used to treat most stages of prostate cancer. These include external beam radiation therapy (EBRT), brachytherapy and now CyberKnife.
Many patients with early-stage prostate cancer are candidates for CyberKnife therapy. Patients with more advanced prostate cancer may be eligible for combined treatments of five weeks of external beam irradiation and two CyberKnife treatments.