Speaking at the recent ICTR-PHE in Geneva, Riccardo Faccini from INFN and Sapienza University presented an alternative: radioguided surgery based on β decay. Electrons travel just a few millimetres in tissue (about 100 times less than gamma rays), eliminating background signal, even in cases with large tracer uptake in nearby healthy organs. This limited penetration also enables use of a lower radio-pharmaceutical activity and reduces the dose to medical staff (Sci. Rep. 4 4401).

"We don't use β for medical imaging, because the signal doesn't get out of the patient, but in the operating environment we can use it," Faccini told the delegates. "The idea is to extend radioguided surgery to more clinical cases."

Probe prototype

Faccini described the team's prototype intraoperative β detection probe. The 1 cm-diameter probe has a core of p-terphenyl (a scintillator with low gamma sensitivity) and a 2 x 2 x 2 mm sensitive area. In tests on phantoms, the probe exhibited a spatial sensitivity of about 2 mm (J. Phys.: Conf. Ser. 620 012009).

One potentially significant use of β guidance is within neurosurgery, where complete removal of a brain tumour is vital and other radioguided surgery techniques are limited by high tracer uptake in brain tissue. To investigate this possibility, Faccini and colleagues examined the somatostatin analogue DOTATOC, which is known to exhibit substantial uptake in meningioma, labelled with the β-emitting radionuclide 90Y – a combination that is already used clinically for radionuclide therapy.

To test the feasibility of this approach, the team first studied the uptake of 68Ga-labelled DOTATOC, which can be visualized by PET, in patients with meningioma and glioma. In meningioma, the tumour-to-non-tumour ratio (TNR) was higher than 20 in many cases. For glioma patients, the TNR was always at least 4. The researchers also performed simulations of 90Y-DOTATOC and observed large uptake in meningioma. For glioma, the TNR was acceptable providing that the probe could take about 6 s to discriminate cancerous tissue.

Ex vivo validation

Faccini presented some results from the first validation of this approach, using the β detection probe to evaluate ex vivo samples from a patient with meningioma. In an initial step, the researchers administered 68Ga-DOTATOC and performed PET imaging to estimate tracer uptake. A low, but sufficient, tumour SUV of about 2g/ml was observed, with a TNR of about 14.

A few weeks later, the patient was injected with 8 mCi 90Y-DOTATOC the day before surgery. During surgery, tumour samples were extracted for evaluation. Results showed that the probe could detect residuals as small as 0.2 ml. The signal from larger tumour samples (100 counts per second (cps)) agreed well with simulations, which predicted 115 cps. The smaller 0.2 ml samples exhibited signals of approximately 40 cps, while signals from healthy tissues were below 5 cps. The researchers also confirmed that there was a low level of radioactivity in the surgical environment.

Next, Faccini and colleagues plan to extend the clinical studies to glioma and neuroendocrine tumours, and are open to other applications with different beta-emitting radiotracers. Other tasks include developing specific radiotracers, and improving the probe, for example by enabling endoscopic use. He emphasized that this is a multidisciplinary project, involving physics, chemistry, nuclear medicine, oncology and engineering. "We still have a long way to go," he said.

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