The Princess Margaret - Radiation Therapy Centre
Protons are subatomic particles that can be accelerated into very high energy beams that can be used to treat cancer. Proton beams deliver their energy to a very well defined space, and can be used to produce radiation treatment volumes that are very conformed to the shape of the target. Proton therapy has been used to treat various cancers since the 1980's, and is the very first 3D conformal therapy. Proton therapy facilities are very expensive and the treatment beams are not easy to manipulate or to direct to the target with high precision, especially if it is a mobile target like the prostate gland. More modern technology like image-guided intensity modulated radiotherapy (IG-IMRT) and cyberknife therapy can provide similar treatment volumes with improved targeting and at much less cost, and have largely superseded proton therapy for very high precision external radiotherapy.
Cyberknife is a trade name for robotic radiotherapy. A compact radiation treatment unit is mounted on an industrial robotic arm that can be programmed to approach the intended target from many different directions. Like IMRT, it delivers the radiation in beamlets, but unlike IMRT it can only deliver one beamlet at a time. It can produce very highly conformed treatment plans, but complicated plans can take up 40 minutes or more to deliver each treatment fraction one beamlet at a time. Because of this limitation, cyberknife therapy is being investigated to give very short courses of prostate radiotherapy in a very few large fractions, or by using the robot to target only a part of the prostate to deliver very high doses of radiation to the tumour itself. Cyberknife is rarely used to give conventionally fractionated therapy because it is so slow. These compressed treatment plans best suited to cyberknife show promise, but remain experimental at this time.