Gamma Knife has become a recognised treatment for small-medium-sized tumours and vascular disorders that develop in the brain and skull base, including secondary metastasised tumours and arteriovenous malformations (AVMs), for example. As a form of stereotactic radiosurgery, it presents a treatment option for complex cases where open surgery would carry significant risk. Gamma Knife uses 192 precisely focused beams of radiation, which converge to produce a high dosage of radiation to a highly focused target. This means that it is capable of treating complexly located lesions without damaging the surrounding healthy brain tissue.
10-year anniversary
This year marks the 10-year anniversary of the Gamma Knife installation at the Queen Square Radiosurgery Centre (QSRC) within the University College Hospital (UCH) in central London – a centre of clinical excellence that operates within the National Hospital of Neuroscience and Neurology (NHNN). Providing stereotactic radiosurgery treatment options using Gamma Knife for complex lesions in the brain, this centre has been consistently listed as one of the world’s top three centres for neuroscience and neurology. However, the process of actually installing the Gamma Knife at QSRC was complex – and involved tonnes of equipment and live radioactive sources being fitted into the basement of UCH (where the QSRC is located), entailing the use of cranes, a prefabricated breakout wall, and temporarily moving a steel staircase.
UK hospitals with medical ionising radiation facilities such as X-rays, nuclear scans, and radiotherapy units, must meet the Care Quality Commission (CQC – NHS regulator group) regulations. These aim to minimise the risk of harm to patients and staff, and set out the responsibilities of duty-holders – to ensure that what is needed for sufficient radiation protection and the basic safety standards are met.Unlike linear accelerators (LINACs) such as teletherapy and radiotherapy units, which produce radiation electronically – only active when turned on – Gamma Knife machines house live Cobalt sources.
The sources are sourced from Canada, and are made using Cobalt 59 in a nuclear reactor, which absorbs neutrons from the neutron flux in the reactor to become Cobalt 60. Each Gamma Knife unit contains 192 pellets of Cobalt, each measuring 1 mm x 1 mm, and is transported in a 9-tonne cask. After arriving on site, the sources are loaded into the Gamma Knife using a specifically designed machine – which itself weighs 3 tonnes – that removes the current pellets, as well as transferring the new ones into the Gamma Knife. As the casks and loading machine themselves weigh tonnes, cranes are often needed for loading and unloading the sources, which are returned to Canada after the five-year half-life for disposal/re-use.
Replacing the sources
As the Cobalt 60 decays, it becomes weaker, and as the sources approach their five-year half-life, they have to be replaced. The sources must be delivered in a flask source container, weighing 2-3 tonnes, which then needs to have surface measurements taken on arrival to ensure that it is sealed. Although half as strong, the sources are still extremely strong after five years – up to 222 terabecquerels (1 TBq = 1 000 000 000 000 Bq – one becquerel being one unit of measurement of radioactivity). This means that there are 222 TBq radioactive disintegrations per second. Even so, after five years, treatment times become twice as long. Gamma Knife treatment length can vary, but for complex cases with multiple lesions the procedure can take several hours. So, it is not logistically feasible for treatments to double in length every five years in terms of management. The planning software for the machines automatically takes the sources’ strength into account when planning treatments, so that clinical staff are aware of when the source is approaching half-life.
Applying for a licence
Hospitals looking to introduce a Gamma Knife must apply for a licence to house live radioactive sources on site. This is the same process for brachytherapy equipment, and is in place to ensure that conditions around security and safety are upheld by sites holding active sources. The licence is granted by the Office for Administration of Radioactive Substances Advisory Committee (ARSAC), and includes requirements for tamper-proof doors, and lead-lined shielding for walls etc
As with many hospitals that deliver radiation, staff exposure is stringently monitored, with lead-lined PPE in place, just as it is in place with X-ray and LINACs etc. Radiation monitoring for staff in hospitals is an integral part of good practice. It is the core discipline for various roles, including RSOs, Health and Medical Physicists, Radiologists, and Nuclear Medicine Techs. This is because it is important to be acutely aware of exposure rates for staff. However, as the Gamma Knife is self-shielding, radiation is only able to escape during treatment and at the QSRC treatment room, with vinyl indications on the floor marking out the exposure in the area, in line with ARSAC conditions
Specialised infrastructural requirements
Because Gamma Knife requires housing this live radioactive source, hospitals must also meet specialised infrastructural requirements to provide this treatment option, providing sufficient protection from radiation. The QSRC therefore uses specialised CAT4 doors, in line with the safety and security conditions, which not only insulate the radiation, but are designed to withstand 40 minutes of power tools – protecting the cobalt source from theft.
A multifaceted team comprised of architects, engineers, project managers, radiation safety advisors, radiologists, physicians, and chief technicians (who are going to be using the department), should be involved in the planning of a building with lead protection. Generally, due to the small number of facilities that provide this service (which drives volume of work for centres), it is highly desirable to plan the protection of a lead-lined room for the maximum possible load. This means, roughly speaking, that the radiation protection of a general-purpose leadlined room about 6 x 4 x 3 metres in size calls for a wall thickness in all directions equivalent to 2 mm of lead (from the World Health Organization’s Manual on Radiation Protection in Hospitals And General Practice, 1975).
Self-shielding devices
While these treatment rooms require reinforced floors, prefabricated break-out walls, and lead lining, the Gamma Knife machines are self-shielding. There is thus only exposure risk when the Gamma Knife unit doors are open, during treatment. This means that – in contrast with other radiation medical equipment – such as LINAC machines that require 2 metre thick walls and specific power supply – these rooms only require walls with a thin lead lining. Such requirements are similar to that of a CT room, making Gamma Knife relatively flexible to implement, and presenting an opportunity to convert old CT treatment rooms to house a Gamma Knife
As the Gamma Knife requires lead shielding in the floors, walls, and ceilings, its installation at the Queen Square Radiosurgery Centre was complex – particularly as it was to be located in the basement of a large London hospital (UCH). The Gamma Knife machine itself weighs approximately 20 tonnes, and the source changer used to load the sources weighs around 15 tonnes. The installation thus required reinforced floors, the closure of wards above, and delivery through a false wall in the treatment room. Generally, the requirements for housing a Gamma Knife include ease of access, but the QSRC was a specific case, being ideally located in proximity to Queen Square ‘Imaging’ and Great Ormond Street Hospital.
Two different crane types
Access to the centre presented as an issue, with two different types of cranes used, and all buildings below evacuated during delivery. With the equipment being so heavy, the biggest crane in Europe was used for the QSRC installation, which also involved a road closure via Westminster City Council. Transporting live radioactive sources is heavily regulated, and in addition, the installation was carried out during the 2012 London Olympics, which caused additional security concerns, further restricting access.
The siting of a Gamma Knife centre is also, of course, influenced by the associated facilities required, such as Imaging. A real advantage to operating a Gamma Knife facility is regular and efficient access to imaging. Arguably the most important part of this treatment is the planning – mapping out the lesions, and determining where to target the Gamma Knife. The QSRC is in fact optimally located, with the Queen Square Imaging Centre on the same premises.
Although the technology is always evolving, the physical technology behind Gamma Knife has been used for decades. The factor that enables this treatment to be as ‘cutting edge’ and precise as it is is the ability to plan where to target the Gamma Knife – a crucial element of the treatment process. One of the key purposes of enhanced diagnostic imaging (for example on a 3T MRI) is to better identify the overall volume and target, in order to treat more accurately and successfully. This is proving an exciting quality development introduced at QSRC this year. As imaging possibilities advance, and developments are made in MRI technology, this directly progresses the potential of Gamma Knife to become even more precise.
This being said, the Gamma Knife machines are designed with very few moving parts. The key cobalt sources are housed within eight moveable sectors in the radiation unit that move forwards and backwards over pre-drilled holes in the tungsten, that allow radiation to reach the patient during treatment. This apart, the patient table is the only other moving part of the Gamma Knife, allowing for 99% uptime. With a service contract and preventative maintenance provided by the machine commissioning body, Elekta, components are replaced on a timecycle basis, and accuracy configuration is checked regularly. This generally takes place once per quarter out of hours when the clinic is empty. The service and maintenance of the machines therefore tends to be straightforward, and at current capacity, they can comfortably treat 1,000 patients per year.
Constant evolution
The Gamma Knife machines undergo a process of ongoing evolution, with new developments regularly introduced to improve ease of use. The most significant recent developments have included the release of the Icon model in 2015, the new Vantage frame in 2018, and new planning software released in 2020. The Icon model introduced the ability to offer both frame treatments and mask-based treatments, with the first addition of cones being used in the machine, as well as allowing for fractionated treatments. The release of the Vantage frame, meanwhile, introduced a frame that was MRI-compatible, allowing for clearer imaging for treatment planning.
The Vantage frame therefore complements the plan optimiser software, Lightning, that was released in 2020 by Elekta. Also released at this time was remote planning software which allows for remote planning (and thus made continuing treatments easier during COVID). Not only does this eliminate the need for clinicians to physically be on site to plan a patient’s treatment, but it also gives them the ability to choose the duration and conformality of treatments. Clinicians can therefore offer more bespoke treatment sessions for their patients, which is particularly useful when treating more frail patients, or those who are in the later stages of their cancer journey
Operational direction
Providing Gamma Knife treatment therefore entails operational direction and co-ordination. Multiple imaging sessions per patient, specialised clinical training, long treatment periods, and exposure monitoring for staff, can present implementation and treatment delivery challenges – especially considering the volume of patients seen with the cancer treatment backlog resulting from COVID. QSRC has witnessed considerable growth in the volume of metastatic cases referred for Gamma Knife treatment. Post-COVID, the Centre is seeing many more patients with multiple metastases, and this is where Gamma Knife can offer particularly effective treatment compared with other platforms.
Ian Paddick
Ian Paddick is lead physicist at Queen Square Gamma Knife Radiosurgery Centre. He has 24 years’ experience in Gamma Knife physics, including the commissioning of 10 B/4CPerfexion/ Icon models, bunker design, and Stereotactic MRI. He has also been involved in the clinical start week of over 100 Gamma Knifes. He won the Fabrikant Award at the 2022 ISRS (International Stereotactic Radiosurgery Congress) held last June in Milan, the first recipient of this award from the UK