What is stereotactic radiation therapy, which patients are suitable, and what does the future hold?
Stereotactic radiation therapy (SRT) is a highly advanced and precise technique differing from other external beam radiation treatments in that it delivers high-dose, highly focused radiation in one to a few short treatments. When used in brain/head tumours, SRT is known as stereotactic radiosurgery (SRS). For other parts of the body, it is called stereotactic body radiation therapy (SBRT) or stereotactic ablative radiotherapy (SABR).
The technique aims to safely ablate the tumour and achieve permanent local control whilst minimising dose to surrounding healthy tissue and organs. As radiation therapy is non-invasive and avoids the need for an anaesthetic, SRT offers an alternative to surgery when surgery may be impossible, too risky, is contraindicated or refused, or when patients are too frail.
The treatment itself is performed in the outpatient setting, is not painful, and is generally well-tolerated. Patients are usually referred following careful consideration and discussion at a multidisciplinary team meeting. A meticulous planning and quality assurance process is required prior to treatment with the aid of experienced radiation therapists, medical physicists, nurses, and the treating radiation oncologist.
SRS is commonly used to treat brain metastases, benign tumours (e.g. acoustic neuromas, pituitary tumours and meningiomas), and other conditions such as arteriovenous malformations and complex cases of trigeminal neuralgia.
SBRT/SABR can be used in primary as well as metastatic disease. Suitable primary tumours include early-stage non-small cell lung cancer, liver, renal and localised prostate cancer. In the metastatic setting, the technique can be used in patients with isolated or up to three lung metastases, adrenal tumours, disease in the spine, and other metastatic deposits known as oligometastases.
Various machines can be used to deliver SRT including Gamma Knife®, Cyberknife®, TomoTherapy® and linear accelerator (linac)-based systems. The latter may not require the insertion of fiducial markers and offers a more rapid treatment time. While there are subtle differences, all have the same key features enabled by advances in technology since SRT was first conceived in Sweden about 50 years ago.
The latest linac-based technology, Elekta Versa HD™ with the Brainlab ExacTrac® Dynamic X-ray System, is a patient positioning and monitoring system. It was introduced to WA in February 2021 at Hollywood Private Hospital’s new Consulting Centre. A linac with similar capabilities is located at Fiona Stanley Hospital. While the radiobiology of tumours and the dose of radiation required for cell kill is well established, the ongoing technological advances in radiation oncology focus on enhanced imaging and accuracy.
The new linac is equipped with thermal-surface camera technology which works in tandem with real-time X-ray tracking. In addition to standard image-guided radiation therapy (IGRT), the 4D thermal camera with its 300,000 surface points can externally monitor patient position and detect movement even during treatment.
This ‘motion management’ adds another dimension in patient tracking so that treatment is delivered with submillimetre accuracy. Any changes in position can be corrected using the specialised robotic ‘couch’ which can be adjusted along six independent degrees of freedom including pitch, roll and yaw. This removes the need in some patients for the small but permanent tattoo markers used traditionally to help position patients for treatment. Patients with brain tumours may also be able to avoid the full immobilisation mask.
Radiation oncology technology and treatment techniques continue to evolve and to date have resulted in higher cure rates, faster treatment times, reduced number of treatments, enhanced patient comfort and less side effects.
The recent world launch of the MR-linac has heralded the next major advance. By integrating an MRI scanner into a linac, real-time visualisation of the tumour during treatment is possible together with the ability to reshape the dose based on daily changes in shape, size, and position of the tumour. There are currently two MR-linacs in Australia (Sydney and Townsville). We await news for Perth.
Key messages
- Stereotactic radiation therapy (SRT) is a non-invasive, advanced technique that delivers high-dose, highly focused radiation in one or a few treatments
- SRT can be used to treat a variety of primary tumours (e.g., lung, liver, prostate) and metastatic disease (e.g., brain, spine, lung)
- A next generation patient positioning and monitoring system is now in Perth.
– References available on request
Author competing interests – the author consults with GenesisCare which has the machine described