Bristol Gamma Knife Centre maximizes frameless radiosurgery with Leksell Gamma Knife Icon
BRISTOL, UK – The introduction of frameless, mask-based Gamma Knife® radiosurgery at Bristol Gamma Knife Centre (University Hospitals Bristol NHS Foundation Trust, U.K.) has opened up new treatment possibilities. With the advanced motion management, frameless mask fixation and cone beam CT (CBCT) tumor targeting capabilities of its Leksell Gamma Knife® Icon™, Centre physicians have been able to easily fractionate Gamma Knife radiosurgery in suitable cases and use mask-based immobilization in high dose single-session radiosurgery. When feasible, clinicians can even give patients the option to choose the mask over the traditional Gamma Knife frame.
To protect sensitive brain anatomy, Icon enables the same fractionation schedule that Centre clinicians had used for years in linear accelerated based intracranial radiosurgery. To date, three patients have “gone the distance” with Icon, each receiving 25 fractions.
Alison Cameron, MD, consultant clinical oncologist at Bristol Gamma Knife Centre
“So far, we’re the world’s only center to have patients who have completely ‘fractionated out,’” says Alison Cameron, MD, consultant clinical oncologist at Bristol Gamma Knife Centre.”
“I don’t think Elekta ever expected anyone to use Gamma Knife with a fully fractionated treatment like you would with a linac, but we’ve proven you can do it.”
One Fraction Typical
Presently at the Centre, single-session, mask-based Icon radiosurgery is the norm.
Of the 14 patients who have had treatment with the mask, 10 had single-session radiosurgery. Among them was a 58-year-male with four metastases who Dr. Cameron suspected might have had progression of his disease (additional mets) since the last MRI and therefore might not even be eligible for Gamma Knife radiosurgery. The original intent had been radiosurgery with the traditional frame and the protocol was to conduct the pre-treatment MRI with the frame attached, something Dr. Cameron was reluctant to do.
“The worst thing you can do is put the frame on a patient, then do the MRI and find out the disease had progressed so much that it’s not really worth doing,” she explains. “He was scanned without the frame and we discovered his disease hadn’t progressed. Moreover, we determined that his mets were not that small and none was anywhere near a significant organ-at-risk, which meant we wouldn’t need the extreme accuracy of the traditional frame. The accuracy of the mask is as good, so we made a mask and delivered the single treatment to the four mets in one hour and 53 minutes.”
She adds that the treatment went so smoothly that the patient fell asleep and didn’t wake until the last 15 minutes. The infrared motion management (IFMM) system of Icon alerted doctors that the patient had awoken by detecting a tiny movement of his nose (0.5 mm), which was not enough to interrupt treatment.
Dr. Cameron has found that mask immobilization has been more secure than she expected, based on this case and subsequent ones. The IFMM, which tracks a marker on the tip of the patient’s nose during the fraction, will trigger the cessation of beam delivery if motion exceeds a preset threshold, which at Bristol is 1.5 mm sustained for two seconds. However, if the patient drifts greater than 0.7 mm, the team manually pauses the treatment and resets the patient with a new CBCT.
“I’ve been exceptionally impressed,” she says. “I thought that patients were going to constantly ‘trip out,’ but that has not been the case, even for patients who have had 25 fractions. In only a minority of cases have we been required to do a second CBCT to recalculate the plan as a result of patient motion over the threshold. The key is getting the patient snugly in the mask and waiting for them to settle comfortably before the first CBCT. The intrafraction monitoring has been hugely reassuring for me to know that the patient’s aren’t moving during beam delivery.”
Protecting Normal Tissue with Fractionation
The three patients who had fractionated mask-based treatments with Icon had growing pituitary adenomas that were too close to optical pathways to justify single-session frame-based radiosurgery.
“If the lesion is less than two millimeters away from the optic apparatus, using single-session frame-based radiosurgery, you would be giving a dose of greater than 8 or 10 Gray to this anatomy, risking optic neuropathy,” she says. “Conceivably, you could drop the dose to the top of the tumor, but then you’re risking relapse. Neither of these options is good. We know that 25 fractions at 1.8 Gy each is very safe for the optical anatomy and achieves a very high control rate.”
Dr. Cameron adds that the advantage of fractionated Gamma Knife radiosurgery with Icon over linac-based radiotherapy for this and other indications is in the amount of brain receiving radiation. With VMAT or IMRT, large areas of the temporal lobe and hippocampus will receive radiation, theoretically increasing the risk of stroke, neurocognitive effects and second malignancy.
“I can’t prove these side effects will happen but you’re definitely treating more brain when using a linac,” she notes. “The three patients we treated with Icon had only minor side effects. They tolerated the treatment really well and the plans were enormously better.”
Dr. Cameron is confident enough in the mask-based head fixation, motion management and CBCT imaging capabilities of Icon that she has had the opportunity to approve patient requests for the mask.
“One of our patients scheduled for Gamma Knife radiosurgery called to say he had seen the BBC TV spot on our Icon and said he wanted the mask,” she recalls. “I looked at his scan and told him ‘Yes, you can have the mask.’ So, here the patient proactively chose the mask, and for another couple of patients we offered them the options and they chose the mask as well. As long as the target is not next door to an organ-at-risk I’m happy to offer patients the mask.”
Frameless makes Timely Debut for Jennifer Rossiter – Spared Third Open Surgery to Treat Second Recurrence of Tumor
Bristol resident Jennifer Rossiter
“In December of that year I had another MRI scan that showed something in the right frontal lobe,” Rossiter says. “Doctors thought perhaps it was scar tissue and adopted a wait-and-see position until May 2015, when they confirmed it was an ectopic metastasis of the original tumor.”
The prospect and associated risks of a third craniotomy were too much for her doctors and Rossiter to consider, so she was referred in July 2015 to Dr. Alison Cameron at the Bristol Gamma Knife Centre.
“I’d had two craniotomies and obviously the risk of anything going wrong increases if you have to have a third one. My forehead has never been the same after the two operations – it feels tight all the time,” she explains. “Dr. Cameron told me that their Gamma Knife system offered a new mask-based radiosurgery option and I would be a good candidate. She said it would remove the risk of anything being put into my skull because it’s non-invasive.”
“Dr. Cameron told me that the treatment would either stop the tumor in its tracks or even shrink it,” she says. “Worst-case scenario, if I did have to have another Gamma Knife treatment, I would definitely opt for the mask-based therapy. It was a good option for me.”
As a bonus, Dr. Cameron informed Rossiter that her tumor was situated at a safe distance from critical structures [e.g., motor, visual pathways], meaning that doctors could deliver a single, very high therapeutic dose of radiation to the lesion without risking damage to these structures. Further increasing the safety and accuracy of the Icon system is its integrated imaging and motion management systems, which enhance tumor targeting and ensure that even the slightest patient motion – which could move the radiation off the target – is detected.
After custom-fitting a thermoplastic mask to Rossiter’s face, physicians performed MRI and CT imaging of her brain with the mask on, and then again just before treatment to make sure in the creation of the treatment plan they had precisely zeroed in on the target. On September 29, she had her Gamma Knife radiosurgery, a 45-minute procedure that a BBC news crew covered (http://www.bbc.com/news/uk-england-bristol-34396991).