The Bubbly Photo Neutron Source

"You can park there, beside the ambulance." The small blue van had no air conditioning, and the long drive from Trieste had been exhausting. We began unloading the padded box containing the expensive bottles of heavy water first, followed by the heavy lead and tungsten bricks, and finally the heavy graphite blocks used to reflect neutrons into the therapeutic cavity. If nuclear physics applied to medicine could resemble Lego, this was the clearest example: PhoNeS, the Do-It-Yourself Photo Neutron Source.

The entire radiation therapy department was present: the radiation oncologists in their white coats despite the warm midday sun, the medical physicists (more curious than excited, at least initially) wearing jeans and polo shirts, and the field service engineers overseeing the medical linac that we were about to transform — brick by brick — into a neutron factory for in-hospital Boron Neutron Capture Therapy.

Neutrons, and where to find them

Medical linear accelerators work by accelerating electrons toward a tungsten target. As the electrons are slowed down, photons are emitted in exchange. These photons are then shaped into a precise geometry by a system of collimators and directed with precision to the target volume—the tumor—in the patient.

Boron Neutron Capture Therapy (BNCT), however, works differently. For BNCT, neutrons must be directed toward a target volume that has been previously infused with Boron-10. Neutrons preferentially interact with the boron accumulated in individual cancer cells, and this interaction destroys those cells.

But here’s the problem: producing neutrons typically requires nuclear reactors or large proton accelerators with spallation targets—systems far too big and complex to be housed in a hospital's radiation therapy department.

Here is the genius

This is where PhoNeS comes in: by adding an additional tungsten (or lead) target to the linac, photons are converted into neutrons (for the curious, this process is called Giant Dipole Resonance). The neutrons are then slowed to the correct energy using a heavy water moderator. (For the same curious minds: heavy water is used because neutron interaction with deuterium is safer for patients. In contrast, when neutrons interact with hydrogen in regular water, gamma radiation is emitted.) Finally, the neutron field is optimized and amplified with a smart reflection system—hence the graphite blocks. While beryllium would have been a more efficient reflector, it was not a feasible option for an in-hospital, scalable solution.

The PhoNeS modular solution also had another advantage: it didn’t require permanent modifications to the linac. The linac could be used in regular photon mode for conventional treatments and then converted into a neutron machine in just a couple of hours when needed.

The setbacks

Unfortunately, the PhoNeS project encountered significant challenges:

• The neutron flux we could produce was one or two orders of magnitude lower than what was needed for actual patient treatment. Simply put, there weren’t enough neutrons to treat patients in a reasonable/acceptable amount of time.
• A higher flux could have been achieved by running the linac in electron mode, but embedded machine protection systems prevented high currents in this mode.
• The medical industry was moving toward machines with lower photon energies. PhoNeS performed best with linacs operating at 21 or 25 MV, was acceptable at 18 MV, but struggled with the now-standard lower energy ranges of 6 to 10 MV.

Research into PhoNeS is still ongoing, exploring its potential for research applications, but the dream of in-hospital BNCT has not yet materialized.

My personal story (for the two of you curious enough to read it)

I worked on the PhoNeS project for almost 2 years. We traveled across Italy, and even crossed the border into Austria, to test our prototypes on a range of medical linacs (yes, including Elekta’s linacs — though at the time, twenty years ago, I had no idea I would one day work for Elekta. Life is always a surprise!)

The mastermind behind the project was Professor Gianrossano Giannini, who was then teaching nuclear astrophysics at the University of Trieste. I was directed to Gianrossano’s group by another brilliant mind of the era, Professor Luciano Bertocchi, Director Emeritus of the IAEA's International Centre for Theoretical Physics.

“Don’t do your thesis at CERN or other big labs where they’ll sit you down to analyze a small set of data so someone else can win the Nobel Prize,” Bertocchi told me. “Choose instead a small, innovative project where you can learn hands-on, make a direct contribution, and truly learn physics. Go talk to Giannini; he always has some new idea no one else has thought of.”

And there I was

And so, there I was — after meeting the prerequisite of passing the nuclear astrophysics course with honors (check!) and agreeing to work tirelessly (check!), including weekends (check!), on a project with no guarantee of success (well, it was a success for me, even though we never went clinical!).

Professor Giannini was a constant source of surprises. One day, we would discuss the physics of erupting volcanoes; another day, he would teach me about muon tomography of archaeological sites. He even had a passion for Leonardo da Vinci and the hidden messages in his paintings. At first, I couldn’t see how all of this connected to the PhoNeS project and the idea of delivering neutrons to hospitals willing to experiment.

As time went on and I got to know Giannini and his team better, I realized how new ideas spark from creating unexpected connections, made possible by a diversity of interests. We may not have been “The Boys of Via Panisperna,” but Gianrossano’s bubbling ideas and genius left a measurable impact.

How to measure neutrons in medical linacs

To estimate the yield of neutrons from the PhoNeS prototype, we had first to benchmark the neutron production in linacs. We performed measurements both with open and closed jaws, at the maximum dose rate available for the machine. Bubble neutron detectors were placed at various distances from the target. (Same bubble dosimeters I recommended as a Christmas gift idea, to measure neutrons when flying on commercial airplanes)

Bonus Story

When Pierluigi and I got robbed of 5 liters of heavy water

We were traveling from Trieste to Rome by train to perform neutron production measurements with a medical linac at Gemelli Hospital. We had some bottles of heavy water in our luggage, which we stored in the baggage area near the door of the train car. Our seats were in the center of the car.

When we arrived in Rome, the train was quite crowded, so it took us a couple of minutes to reach the door and retrieve our luggage. Pierluigi's bag was missing. We had a moment of panic, but then we saw a man running away along the platform, carrying our bag. We started running after him, yelling, "Hey, stop!" The man saw us, and as we got closer to him (physics joke: the thief was probably a neutron, as he was slowed down by the heavy water), he dropped the bag and escaped.

I can only imagine how amusing it would have been to go to a police station to report the theft: "Hello, we’re two nuclear physicists, and we were just robbed of 5 liters of heavy water." Imagine the reaction of the police!

About me

I’m passionate about radiation and radiation safety, and I lead these efforts at a top MedTech company. My experience includes working with the European Commission and international physics laboratories, where I developed my expertise in nuclear physics (without causing any explosions!). With a PhD in applied nuclear physics, I’ve published research in peer-reviewed journals and enjoy crafting content that makes complex topics in science, safety, and security accessible and engaging—because everyone loves a good science story!

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