This very interesting question gets to the heart of how radiation oncologists treat cancer. The goal of radiotherapy is to deliver a very high radiation dose to the tumour, while protecting the nearby normal tissues. This is usually achieved with a combination of “high-tech” methods that permit accurate identification of the target and delivery of the radiation, and daily fractionation of the radiotherapy. Fractionation is the process of dividing a course of external beam treatment into small daily increments over a period of weeks, to minimize the risk of injury to any normal tissue that it’s not possible to avoid.
As technology for radiation targeting and delivery improved, normal tissues became better protected, allowing for the possibility of compressing lengthy treatment into shorter courses. This is called hypofractionation, and is used both as routine treatment and in experimental protocols. Taking the concept of hypofractionation to the extreme involves treating a target with a single massive dose of radiotherapy, which essentially coagulates or “ablates” it just as a surgeon might. This is called radiosurgery (versus radiotherapy), and it is what the CyberKnife was developed to do.
The CyberKnife is an external beam radiotherapy unit with two main differences compared to a standard unit. It is mounted on an industrial robotic arm, so it can move in more directions than the arc of a conventional unit, and it has the ability to reduce the size of its radiation beam (to “collimate” it) much more. The main advantage of the Cyberknife is to get very small beams of radiation into very difficult areas; it was originally developed to help neurosurgeons treat benign and malignant brain tumours they couldn’t get at otherwise. Its application has since been expanded to other sites, including prostate cancer.
The CyberKnife may be used in conventional fashion with fractionated radiotherapy to the whole prostate, but the advantage is relatively small when compared to other high-precision techniques such as image-guided intensity modulated radiation therapy (IMRT) and tomotherapy.
A very interesting potential use of the CyberKnife for prostate cancer is as a radiosurgery tool, to give extra radiation to the tumour or to treat only part of the prostate, but this is still experimental. One major challenge to overcome is to identify which part of the prostate most needs treating. Until we can do this with confidence, we will continue to treat the whole gland.
The Hôpital Notre-Dame du Centre hospitalier de l’Université de Montréal (CHUM) has recently opened the first CyberKnife facility in Canada.
