The technique allowed researchers to remove brain tumours from mice with unprecedented accuracy. In future, it could offer a lifeline to patients with hard-to-treat and inoperable brain cancers
Scientists compare the approach to special forces laser-tagging targets for precision bombing missions. The gold nanoparticles are coated in special chemicals that can be imaged in three different ways.
Measuring less than five millionths of an inch across – about a 60th of the size of a red blood cell – the particles cling to tumours and highlight their boundaries.
One of the major difficulties in treating brain tumours is that they are impossible for even highly skilled surgeons to remove without damaging healthy tissue. As a result, the prognosis for patients with glioblastomas, the most aggressive form of brain tumour, is usually bleak. Typical survival time without treatment is three months, and surgical removal of the tumour prolongs survival by less than a year.
“With brain tumours, surgeons don’t have the luxury of removing large amounts of surrounding normal brain tissue to be sure no cancer cells are left,” said lead researcher Professor Sam Gambhir, chair of radiology at Stanford University Medical Centre in California. “You clearly have to leave as much of the healthy brain intact as you possibly can.”
The problem is made worse because glioblastomas have tiny finger-like projections that follow blood vessels and nerve tracts to infiltrate healthy tissue. Microscopic tumours, or “micrometastases”, invisible to the naked eye may also spread out from the primary tumour and invade surrounding areas of the brain. These can then grow into dangerous new cancer sites.
The research is published online in the journal Nature Medicine. Prof Gambhir’s team first showed that the nanoparticles preferentially bound themselves to tumours. They then tested the particles on mice with several different types of human glioblastoma implanted deep into their brains.
After injecting the particles into the animals’ tail veins, they were able to visualise the tumours using three different types of imaging.
Conventional magnetic resonance imaging (MRI) scans provided good pre-operative images of the general shape and location of the tumours.
Then during surgery to remove the tumours, the scientists used “photoacoustic imaging” to help them accurately follow the tumours’ edges in real time.
Photo-acoustic imaging employs pulses of light which are absorbed by material such as the nanoparticles’ gold cores. This causes the particles to heat up and generate detectable ultrasound signals from which 3D images can be obtained.
Even more accuracy is achieved by the third technique, called “Raman imaging”. This uses a special microscope to capture weak light signals emitted by certain materials, including one of the layers coating the gold particles, in several distinct wavelengths.
In the animal tests, the highly sensitive Raman imaging flagged up micrometastases and tiny tumour projections that had earlier been missed.