Why Does Stereotactic Radiosurgery Primarily Treat The Brain?

If someone is prescribed a treatment plan that consists of stereotactic radiosurgery, they can enter the radiotherapy centre confident that they are being treated by one of the most effective, accurate and oldest radiosurgical treatments available.

The principle of stereotactic radiosurgery, as pioneered by Lars Leksell’s Gamma Knife in the late 1940s, is that multiple small beams of radiation that converge on a particular point can have a much greater effect without damaging the surrounding tissue and minimising overall exposure.

This treatment, whilst adjusted and refined over the past half-century, has been consistently used and effective even before there were effective ways to map treatments using MRI and CT scans. 

This naturally leads to a big question; if the Gamma Knife is so effective, why has the stereotactic principle not been widely applied to other forms of radiotherapy treatment outside of the brain and spine?

The answer is complex and one that medical researchers are doing their utmost to try and change.

The Sea Of Life

The almost tautological answer for why the Gamma Knife specifically is not used to treat other parts of the body is that it was only designed for the brain and relies on a very specific set of medical apparatus that has only been used in the head and cannot be used anywhere else.

Part of this is that the frame system that keeps the head in place for a Gamma Knife treatment is based on the Horsley-Clarke frame initially used to create an atlas of various animal brains before being applied to the human head over the course of four decades.

This tool, albeit heavily modified by Mr Leksell and others, is as fundamental and vital to the success of Gamma Knife as the source of radiation itself. This is why the treatment was tested and used before the widespread availability of three-dimensional medical imaging.

It meant that thanks to some rather complex mapping, a series of skull X-rays could be triangulated to position the stereotactic beams in order to treat patients effectively. Once CT scans and MRIs came in, this only made the process even more accurate.

A lot of this can be credited to Mr Leksell’s work himself, who famously said there was no tool too accurate for use on the brain, but whilst an accurate approach is mandatory for obvious reasons, it is also, relatively speaking, easier to achieve.

The skull helps keep everything in place and there is less variation, relatively speaking, with tumour and lesion positions, allowing for consistency and greater accuracy. 

Pair this with the stereotactic frame keeping the head from moving during treatments and it is perhaps understandable how the Gamma Knife started as a highly effective targeted treatment and has only gotten more accurate with time.

By contrast, the rest of the body is far less consistent and mobile. Organs are constantly moving in the body, not just in terms of pulsing, contracting and relaxing, but also shifting positions based on the position of the body.

Organs change shape depending on their use, muscles such as the diaphragm affect their shape, and even posture and the way people lie down can alter the shape, size and position of organs, particularly those in the abdomen.

This is a major problem when planning radiotherapy because the act of breathing creates a moving target that affects treatments, often requiring wider beams and more collateral damage to healthy tissue to ensure that all cancerous cells are destroyed.

There are ways to mitigate some of these issues, such as through the use of frames and casts to at least keep a body part securely in place during treatment, but there are a lot more variations than there are for the brain, requiring not only different treatments but a different treatment philosophy.

There have been attempts to change this over the years, primarily through increasing the speed at which medical images are produced and interpreted, and this could lead to Gamma Knife-like treatments becoming possible with that level of accuracy in the future.

In order for this to be accomplished, however, there needs to be a close to real-time form of three-dimensional imaging of body parts that can be used to plan treatments the same day they are executed, as unlike the brain there is the potential for much greater organ movement.

This concept, known broadly as real-time adaptive radiotherapy, is one that is at least conceptually possible, but it requires a widely-used real-time imaging system that, as of 2024 at least, has not been widely used.