Novalis Tx®, CyberKnife®, TomoTherapy®
Linac Radiosurgery and Radiation Therapy
Linac is short for the term linear accelerator. Linear accelerator machines produce radiation that is referred to as high energy X-ray. A linear accelerator machine is designed to be a general purpose radiation delivery machine and in general requires modifications to enable it to be used for radiosurgery or IMRT (intensity modulated radiation therapy). Often, the modification is the addition of another piece of machinery.
Linac machines may be dedicated or non-dedicated. Dedicated linac machines have the additional equipment to perform higher level treatments permanently attached to the radiation couch. This is the preferred method. Non-dedicated linac machines may be used for conventional radiation therapy in the morning and after adding the attachment, are used for higher level treatments in the afternoon. Non-dedicated linac machines are unable to acquire the same degree of precision and accuracy that dedicated machines may have.
Radiosurgery can be performed with linear accelerator machines. By definition, radiosurgery is a one session surgical procedure directed by a neurosurgeon and a radiation oncologist. The entire procedure occurs in one day, including immobilization, scanning, planning and the procedure itself. With radiosurgery, the radiation dose given in one session is usually less than the total dose that would be given with radiation therapy. However, the tumor receives a very high one time dose of radiation with radiosurgery, and smaller doses over time with radiation therapy. This is important since more radiation is delivered to surrounding healthy areas when a person undergoes a few (2–5) treatment sessions instead of one; this can result in more side effects, some of which may be permanent. More importantly, a reduced amount of radiation delivered to the tumor with each radiation therapy treatment, rather than a very high one time dose, can result in less tumor control and poorer outcomes than with radiosurgery.
Currently, there is marketing from hospitals, physicians and manufacturers of newer linear accelerators to entice patients to use their machines, which would result in more profits. Much of the marketing is not supported by evidence-based outcome studies which have been peer reviewed and published. In one case, most of the published research has been conducted by the developers of the machine, which is privately owned by a group of investors.
An article published in the October 2004 edition of the Journal of Neurosurgery (J Neurosurg 101: 585–589, 2004) states the following, in part:
“Regardless of whether radiation is delivered by LINACs or proton generators, stereotactic radiation therapy [more than one session] is performed in an attempt to reduce the risks associated with radiation falloff [with these types of machines] in surrounding normal tissues adjacent to the targeted tissue.
Radiotherapy is an effective treatment for a wide variety of clinical problems. It is performed differently [than radiosurgery], with different expectations… The use of a robotic device for movement of the radiation emitter or stereotactic frame does not affect the radiobiological effect of the treatment.”
Linac technology is most often used in multi-session treatments in order to not do damage to healthy surrounding tissue. The total dose of radiation is higher than with one session radiosurgery, but it is given in several smaller amounts, which may allow the tumor to continue growing.
The best use of linac technology may be its ability to target larger brain and body cancers that cannot be treated with one session radiosurgery. More precise techniques using one session Gamma Knife® machines or one-session linac technology are best utilized within the brain. There is no benefit to fractionated radiation treatments when one session radiosurgery can be performed. Multiple radiation treatments may mean less tumor control and more permanent side effects.
What Is Important When Using Linear Accelerator Equipment?
The ownership of the technology. Groups of physicians or investors may own a treatment machine that is located within a hospital or institution. To ensure radiosurgery or radiotherapy is the correct procedure for the patient, the patient should ask whether the technology is owned by the hospital or by the physician recommending treatment. If the machine is privately owned the patient may wish to seek a second opinion.
Strength of the technology: MeV. The greater the strength of the machine, the further into tissue it can penetrate. The strength of the machine may be a function of the method it uses to deliver the radiation and not necessarily equal to the MeV rating.
Imaging technology utilized. MRI technology is considered the gold standard when imaging to target a brain tumor. MRIs provide an unparalleled view inside the body. MRI scans are able to see normal tissue versus brain tumor tissue. CT scanning uses X-ray technology to scan a tumor. Tumor borders cannot be seen on a CT scan. Bone nearby a tumor will obscure clear CT imaging of the tumor. The precise details of soft tissue are less visible on CT scans than on MRI scans. Some linear accelerator technology can only use CT scans. There are experts that believe this may contribute to permanent side effects and inferior results for tumor growth control with linear accelerator technology, that are not as good as with Gamma Knife® technology. Other linac technology can fuse MRI and CT images.
The backing of a publicly held company. Technology is approved by the FDA with little clinical testing. The manufacturer simply states that his equipment is similar to something that already exists on the market. Testing is not required by the FDA for safety or clinical applications for medical equipment. Publicly held companies must meet the U.S. Securities and Exchange Commission standards for finances and ethics and have statements and annual reports that are available to consumers.