What is Stereotactic Radiation Therapy?

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Stereotactic radiation therapy is an advanced external beam radiation therapy technique. It differs to most other external beam radiation therapy in that it delivers a focused higher dose to a small tumour target up to 3 cm in size.

Joshua Hiatt BSc MSc (Medical Physics)

Medical Physics Registrar (Radiation Oncology)

The course of radiation treatment is also over a smaller number of treatment sessions of between one to five treatment days, depending upon the radiation oncologist’s prescription which can vary based on the patient’s medical details

Application of Stereotactic Radiation Therapy

The stereotactic radiation therapy technique is not suitable for treating tumours larger than about 3 cm. The stereotactic technique involves directing many very narrow X-ray beams distributed around 360 degrees and beams greater than 3 cm would irradiate too much healthy tissue.

Stereotactic radiation therapy can be used to treat small primary brain tumours, small cancer metastases in the brain and also non-cancerous conditions such as arteriovenous malformation, trigeminal neuralgia and other functional disorders of the brain.

The Stereotactic Treatment Strategy

The equipment and techniques applied in stereotactic radiation therapy are very similar to what may be encountered in conventional radiation therapy. But the required precision and complexity is much more critical.

Stereotactic radiation therapy relies on the same cure principles as conventional radiation therapy. Radiation damages the DNA of tumour cells. However, conventional radiation therapy aims at damaging tumours so that no cancer cells remain viable for the tumour to re-grow.

The prescribed dose and the frequency of treatment is tailored to preferentially kill the more rapidly growing cancer cells. The treatment protocol for stereotactic radiosurgery is different to conventional radiation therapy.

The radiation oncologist prescribes a higher radiation dose given in a smaller number of treatments compared to conventional radiation therapy. The aim is to ablate the tumour to completely disrupt the biological functions of the cancer cell. This can improve the response rate of the treatment.

By precisely targeting a small tumour and exposing it to a much higher dose over a shorter time than is normally prescribed, success of totally eradicating the tumour, will improve.

But there are major limitations that need to be overcome.

SRS is only suitable when the radiation can be accurately given and the patient or the tumour itself does not move during the radiation exposure.

Your radiation oncologist will advise you on the best course of treatment for your specific situation.

The Stereotactic Method

In the case where there is a small tumour to treat, it is not advisable to irradiate all the brain or have the treatment beam passing through a large amount of normal brain tissue.

Instead, the stereotactic technique is achieved by using many very small sized high energy X-ray beams that are carefully designed to limit most of the radiation dose on the small tumour site and as little as possible elsewhere.

Figure 1. The Gamma Knife treatment machine (courtesy of Elekta)

What’s special about the Stereotactic Method?

To deliver such a large radiation dose, using many very small ‘pencil beams’ to treat the tumour, there are special design and technique methods required.

Stereotactic Treatment Machines – Gamma Knife and Linear Accelerators

Gamma Knife

The Swedish neurosurgeon, Professor Lars Leksell, first introduced the concept of stereotactic surgery in 1951. Leksell was later instrumental in the development of the Cobalt-60 Gamma Knife in 1967.

The Gamma Knife (Figure 1.) has Cobalt-60 sources mounted in the shielding head of the machine. Gamma rays from the radioactive source pass through narrow slots in the shielding to the patient (Figure 2.). By carefully positioning the patient’s head into the treatment head cavity, the tumour is accurately irradiated to within 0.5 mm.

The Gamma Knife is still an acceptable treatment machine used for stereotactic radiosurgery.

Linear Accelerators

To match the accuracy of the Gamma Knife, the linear accelerator rotating bearing system and the X-ray collimators, were more tightly engineered (Figure 4, Elekta and 5, Varian).

Figure 4 The linear accelerator treatment machine, designed for high accuracy stereotactic radiosurgery (SRS, left) and radiation therapist setting up the patient for treatment (right) (Courtesy of Elekta).

Figure 5 Varian linear accelerators designed and built for high precision stereotactic radiosurgery (courtesy of Varian Medical Systems).

Treatment machines equipped for stereotactic treatments will typically have what is called a 6-Degree of Freedom treatment couch that can make rotational adjustments to the roll, pitch and yaw of the patient in addition to the vertical, longitudinal and lateral directions.

It’s possible to target the X-ray beam with sufficient accuracy for a treatment size as small as 5 mm. The ‘multileaf collimator system (Figure 6) controls the size and intensity of X-ray beam targeting the tumour.

Figure 6 Shows the thin collimator leaves that are independently driven to shape and control the X-ray intensity to a very small ‘pencil size’ beam (courtesy of Varian Medical Systems)

Immobilisation of the Patient

To precisely deliver such a high radiation dose to such a small target, requires a combination of specialised radiation treatment equipment, high resolution imaging techniques and good methods of immobilisation.

Targeting the tumour must be maximised and damaging normal healthy tissue minimised. Immobilisation of the patient’s head during the X-ray exposure is very important.

Early stereotactic radiosurgery used a special headframe surgically pinned to the patient’s skull. The frame had the dual role of immobilising the head and using metal markers imaged on an X-ray, to target the tumour in the patient’s head.

The Gamma Knife treatment still requires a modified version of this head frame technique.

Thankfully, with improvements in imaging and X-ray beam targeting, the modern linear accelerator approach is significantly less invasive. Normally, a special mask made of a thermal mesh is now used for head immobilisation.

The patient’s mask is prepared during the CT scan session for treatment planning. The plastic mask is heated in warm water and becomes flexible enough to be gently moulded onto the patient and held until it cools and stiffens. The mask is then used for every treatment to maintain the same patient position each time (Figure 7).

Figure 7. Illustration of patient with plastic mask fitted (taken from Mayo clinic website).

Patients have recently raised their concern in having to wear the mask, particularly if they have difficulties with claustrophobia. There are a number of new innovations that have been implemented by some medical companies. Some alternatives are:

The use of head clamps and a bite block;
plastic masks with part of it cut out; and
the use of motion sensors interlinked with the linear accelerator.

Completely mask free systems using technologies such as motion tracking and beam gating are areas of active research. The design combines the use of an open mask (parts cut away) and surface image guidance (SIGRT offered by C-Rad) for an alternative comfortable, accurate SRS treatment method.

Planning the Treatment

Figure 8. Cranial SRS offers treatment planning for a range of indications (copyright: courtesy of Brainlab AG)

The treatment computer planning system is also specially designed to:

use the patient’s CT scan images for outlining the tumour and all the critical structures that should not be treated beyond a maximum prescribed amount; and
have a special dose calculation method for the pencil beams.

Figure 8. shows the computer display screen for the Brainlab treatment planning system. It’s designed for a range of brain disorders requiring X-ray therapy.

Figure 9. Arc trajectory optimisation for cranial radiosurgery (copyright: courtesy of Brainlab AG).

The narrow X-ray beams move in an arc to optimise treatment of the target but miss other surrounding critical organ structures. Figure 9. is an example of how the target is treated by multiple X-ray beam arcs that avoid the patient’s radiation sensitive optic chiasma, each of the two optic nerves and the lens.

Figure 10. An example of a treatment plan radiation dose distribution overlaid onto a CT scan. The plan is an SRS small brain tumour treatment. All the X-ray beams are focused on the tumour centre. Note that there is some unavoidable very low splay of dose shown as pink around the tumour.

Figure 10. is another SRS example planned on the CT scan image for the treatment of a small tumour located in the parietal region of the brain.

Procedures during each SRS treatment

As in conventional radiation therapy, advanced pre-treatment imaging plays a role in the accurate delivery of stereotactic radiation therapy. Immediately before every treatment another series of images of the patient set up on the treatment bed, are taken. These pre-treatment images (with the help of computer software imaging) are compared to the original planning CT scan images. The radiation therapist overlays the images to measure and correct any small shifts so that it re-aligns to the target. The therapist is able to automatically make any treatment bed corrections remotely from the treatment machine control room.

Other Stereotactic Techniques

Confusingly, stereotactic radiation therapy may be called several different names: Stereotactic Radiosurgery (SRS), Stereotactic Body Radiotherapy (SBR), Stereotactic Ablative Body Radiotherapy (SABR).

Why?

There are some distinctions between them. For example, SBR is stereotactic radiotherapy applied to (as you might have guessed) an area of the body other than the brain. Then there are some other technical nuances behind the different names that we won’t go into here.

But they are all based on a similar premise, delivering a focused high dose of radiation to a small volume in a limited number of treatment sessions.

For SABR/SBR, depending upon the site of the tumour, additional immobilisation devices might be used, such as abdominal compression paddles to restrict motion of the lower abdomen.

Keep in touch with the Better Healthcare Technology for a more detailed description on body stereotactic radiotherapy techniques.

Joshua Hiatt, 31 January 2020