The Precursor Loophole: Unlocking Fusion-Backed Cancer Therapies

SAN FRANCISCO – September 24, 2026 – For over three decades, oncologists have chased the holy grail of targeted radiotherapy: Terbium-149 (Tb-149). Discovered in 1950, the radioactive isotope possesses a rare combination of physical characteristics that makes it uniquely lethal to cancer cells while sparing healthy tissue. In 1996, researchers affiliated with the University Hospital of Geneva and CERN bluntly declared it "the radionuclide of choice in all aspects except production."

That production caveat has proven to be an intractable barrier. Because Tb-149 has a fleeting half-life of just 4.1 hours, manufacturing it in a centralized facility and shipping it globally is a logistical impossibility. By the time a dose clears quality control and boards a cargo plane, the therapeutic radiation has largely decayed into irrelevance. Consequently, Tb-149 has never progressed to human clinical trials, leaving a highly promising class of cancer treatments stranded in preclinical purgatory.

Today, Marathon Fusion, a deep-tech isotope production company, announced a novel strategy to break this deadlock. In a newly published research paper, Scalable Terbium-149 Production from Highly Enriched Gadolinium-150 Targets, the company outlines a decoupled supply chain that bypasses the decay problem entirely. By centrally manufacturing an effectively stable precursor isotope and distributing it to local medical facilities for final activation, Marathon believes it can unlock tens of thousands of Tb-149 doses annually—and eventually scale to millions using emerging nuclear fusion technology.

The Fluorine-18 Playbook: Bypassing the Half-Life Trap

Marathon's proposed pathway is an elegant hack of nuclear logistics, heavily inspired by the medical industry's most successful radioisotope model: the PET scan.

Every year, millions of PET scans are performed using fluorine-18, an isotope with a half-life of less than two hours. This is achieved not by shipping fluorine-18 across the country, but by centrally enriching oxygen-18—a stable, rare isotope—and shipping it to regional radiopharmacies. There, local medical cyclotrons bombard the oxygen-18 with protons, creating fluorine-18 just hours before it is injected into a patient.

Marathon's strategy applies this exact playbook to targeted alpha therapy. Instead of trying to distribute short-lived Tb-149, Marathon proposes centrally producing gadolinium-150 (Gd-150). While technically radioactive, Gd-150 has a half-life of 1.8 million years. For all practical supply chain purposes, it is perfectly stable. It can be stockpiled, packaged, and shipped worldwide via standard air freight without any meaningful loss of mass.

Once the Gd-150 targets arrive at a regional facility, they can be irradiated by standard medical cyclotrons to generate Tb-149 locally.

"Terbium-149 has been clinically interesting for a long time, we just haven't had enough of it to get drugs through clinical trials, or to feel like there will be a meaningful market even after approval," said Adam Rutkowski, CTO and Co-founder of Marathon Fusion. "Our work shows that there's a practical solution leveraging existing infrastructure to not only get Tb-149-based drugs through clinical trials, but to reach patients all over the world."

However, industry experts observe that this "existing infrastructure" will require some specialized upgrades. While more than 700 medical cyclotrons globally possess the 16.5- to 19-MeV proton beam energy required for this reaction, the vast majority are currently configured for liquid or gas targets. Bombarding Gd-150 requires a solid-target station and an automated robotic hot cell to rapidly dissolve the metal and purify the Tb-149. Rather than true bedside hospital production, this pathway will likely rely on a network of commercial radiopharmacy hubs located within a two-hour driving radius of major oncology centers.

The Alpha-PET Paradigm: Why Terbium-149 Matters

The pharmaceutical industry's desperate need for Tb-149 stems from the biological limitations of existing cancer-killing isotopes.

Currently, the darling of the targeted alpha therapy world is Actinium-225 (Ac-225). While highly effective at destroying tumors, Ac-225 suffers from the "recoil dilemma." When it emits its first alpha particle, the violent recoil energy severs the chemical bonds tethering the isotope to the targeting drug. This releases radioactive daughter isotopes—like Francium-221 and Bismuth-213—to circulate freely in the bloodstream, often accumulating in the salivary glands and kidneys. This off-target toxicity has led to severe side effects in clinical trials, such as intractable dry mouth and renal damage.

Terbium-149 bypasses this physiological hazard entirely. It decays directly into Europium-145 without emitting any dangerous secondary alpha particles, virtually eliminating the recoil toxicity risk.

Furthermore, Tb-149 offers a capability that oncologists refer to as "Alpha-PET." Alongside its destructive alpha emissions, Tb-149 produces a positron signal. This allows physicians to utilize standard PET imaging to watch the therapeutic payload accumulate in the tumor in real-time.

"We've seen a recent confluence of factors: progress in targeting driven by years of clinical research, evidence for the efficacy of alpha therapy demonstrated through dozens of clinical trials, and a vibrant ecosystem capable of bringing new treatments to market," said Kyle Schiller, CEO and Co-founder of Marathon Fusion. "If we can show that there's a scalable production pathway for terbium-149, an isotope clinicians have wanted to use for decades, that'll bring all the pieces together to take a leap forward in cancer treatment options."

Fusion’s First Real Market: Radiotherapy, Not the Grid

Perhaps the most fascinating element of Marathon's announcement is how it redefines the commercial roadmap for the nuclear fusion industry.

To manufacture the vital Gd-150 precursor, an abundant natural isotope called Europium-151 must be bombarded with high-energy neutrons. While existing high-energy proton accelerators and spallation facilities can produce enough Gd-150 to support tens of thousands of initial doses, scaling to the millions of doses required for a globally approved blockbuster cancer drug will require a much larger neutron source.

This is where fusion energy enters the equation. In deuterium-tritium (D-T) fusion reactions, 80 percent of the energy is carried away by 14.1 MeV fast neutrons. Marathon calculates that a compact, 6.8-megawatt steady-state fusion reactor could irradiate enough Europium-151 to supply 40 million annual doses of Tb-149.

For an industry that has historically struggled with the "always thirty years away" trope, medical isotopes offer an immediate, high-margin commercial lifeline. Fusion startups do not need to achieve net-electricity generation (Q>1) or build gigawatt-scale power plants to manufacture isotopes. Sub-critical, non-power-producing fusion neutron generators can be deployed in the near term, operating under significantly lighter regulatory frameworks than commercial power plants.

"Fusion reactors produce an extraordinary number of high-energy neutrons," said Dr. Jason Parisi, Principal Research Scientist at Marathon Fusion. "Electricity is the application everyone associates with fusion, but those neutrons can also be used to mass-manufacture medical isotopes, creating a new supply chain for this critical industry."

From Computational Theory to the Test Tube

Despite the elegance of the proposed pathway, significant scientific hurdles remain. The transition from computational modeling to physical reality hinges on a single, unverified metric: the nuclear cross-section of the Gd-150 to Tb-149 reaction.

Because Gadolinium-150 is an extinct primordial radionuclide, macroscopic quantities of the pure metal do not exist in nature. Consequently, no laboratory has ever been able to physically bombard a Gd-150 target to measure exactly how efficiently it converts into Tb-149. Marathon’s current projections rely entirely on established statistical nuclear-reaction codes, such as TALYS, which have been benchmarked against neighboring lanthanide isotopes.

Nuclear physicists note that while these computational models are highly sophisticated, they can occasionally miss unmeasured resonance peaks or miscalculate isomeric branching ratios. If the proton bombardment preferentially produces the metastable isomer of Terbium-149 rather than the therapeutic ground state, the practical yield could diverge from the theoretical promise.

Marathon Fusion is acutely aware of this technical risk, marking empirical validation as the immediate next step in their development timeline. The company must now utilize high-flux accelerators to transmute enough Europium to synthesize the world's first macroscopic Gd-150 test targets, allowing them to finally measure the reaction in a physical laboratory.

"With the model in place, it's now critical to move forward with experimental confirmation," Parisi noted. "If demonstrated, this would validate a previously overlooked route with all the right characteristics to solve Tb-149 supply. The important thing now is to test it."