Lara Bonner Millar, MD
The Abramson Cancer Center of the University of Pennsylvania
Last Modified: December 16, 2011
Despite the name, Cyberknife is not a surgical instrument and there is no cutting involved in the treatment. Cyberknife is a treatment using external beam radiation and was FDA approved in 2001. For the first several years, Cyberknife was slow to take off, and was typically reserved for patients with unresectable brain or spinal tumors. The CyberKnife is a frameless miniature robotic linear accelerator that uses image-guided radiotherapy (IGRT) in order to treat tumors in different parts of the body, including the prostate. Because the machine is mounted on a robotic arm, the radiation beam can be delivered from thousands of directions. Another key characteristic of Cyberknife is the use of real-time IGRT. What this means is that the motion of the tumor is tracked and the radiation, because it is delivered via a robotic arm, can be directed based on the motion of the tumor.
You might wonder how the robot knows where the tumor is located. Between 1 and 10 days before treatment with Cyberknife, a patient with prostate cancer will see an urologist to have gold markers, known as "fiducials", placed in the prostate as a way of marking the target. This is done under ultrasound guidance, similar to the procedure for prostate biopsy. The tumor position will be tracked during the Cyberknife treatment using the fiducials. A week or so after fiducial placement, a CT simulation is performed. This is a scan where the patient is positioned in the same way that he will be positioned during the actual treatments. The physician uses this scan to delineate the area to be treated as well as those organs to avoid. This information is used to create the treatment plan. There is some variation in the number of treatments and the dose per fraction used, depending upon the treating physician; four or five fractions are commonly used for a total dose in the mid to high 30 Gy range, delivered over 1-2 weeks. While the patient is being treated, cameras take images of the tumor, which are then compared to the images from the CT simulation. The CyberKnife robot moves around the patient to deliver the treatment. Each treatment session may last from 30 and 60 minutes; larger tumors take longer to treat than smaller ones.
Patient convenience may stand out as a reason to pursue Cyberknife over other treatments for prostate cancer. However, there is more to the story. Prostate cancer is known to be one of the slower growing tumor types. Because the proliferation of prostate cancer cells is low, it is believed that these cells are more sensitive to higher doses per fraction than are other cancers. This implies that delivering radiation in a hypofractionated course (meaning delivery of larger doses (fractions) of radiation in fewer treatment sessions) may improve outcomes. It is already well established from multiple studies of standard fractionation, in which a low dose of radiation is given five days a week for 8-9 weeks, that the best cancer control results are obtained with higher doses. The major question for both standard external beam radiotherapy and for Cyberknife hypofractionated radiotherapy is how to deliver a high dose to the prostate gland with minimal or no adverse side effects in nearby organs, namely the bladder and the rectum.
Cyberknife is used for patients with early stage, localized disease. Patients who are considered high risk for lymph node involvement or have documented lymph node involvement are not good candidates for Cyberknife. Additionally, appropriate patients for Cyberknife are those who have a prostate volume of <80-100 cm3, depending on the policy of the institution or treating physician. Some patients with enlarged prostates may try medical treatments to try to shrink the prostate in order to get Cyberknife treatment.
So far, there is no study that has compared Cyberknife with standard external beam in a randomized fashion. The available data for Cyberknife consists of nonrandomized data from single institutions. Most reports document a drop in the PSA after treatment with Cyberknife, however the follow-up is shorter than the follow-up in studies of conventionally fractioned radiotherapy since this treatment technique is relatively new.
Cyberknife, while precise, still does have the potential for side effects because surrounding tissues can still receive some radiation. The side effects include urinary irritation symptoms such as urgency, frequency, and burning with urination. Rectal irritation may also occur, as well as erectile dysfunction. You may notice that these side effects are similar to those described for conventionally fractionated radiation therapy - that's because they are. In the end, it is still external beam radiation, and even the most precise delivery systems can not be free of side effects. We do not know yet how the frequency or severity of side effects will stack up against conventionally fractionated treatment in the long term, but we should have better answers in the years to come.
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