Alpha PRRT: advancing treatment of neuroendocrine tumours.

Alpha versus beta — the physics in NET context

Lu-177 DOTATATE delivers beta particles. Beta particles travel about 2.5 mm in tissue and deliver a moderate amount of energy along that path. The 2.5 mm range is well-suited to small-to-medium soft-tissue lesions because the radiation reaches not only the directly bound cell but neighbouring cells through the so-called crossfire effect. This is one reason Lu-177 PRRT works as well as it does in NET liver metastases. PRRT success rate data from the NETTER-1 trial reflects this.

Ac-225 emits alpha particles. Alpha particles travel only 40 to 100 micrometres in tissue — about the diameter of a single cell or two — but deposit far more energy per unit path length. The energy density is roughly 100 times higher than beta. The biological effect is dominated by direct DNA double-strand breaks, which are difficult for cells to repair regardless of oxygenation status or radioresistance phenotype.

The clinical implication: alpha PRRT is well-suited to the disease state where beta PRRT has stopped working. The cells that survive beta PRRT are typically the ones at the centre of large tumours (where radioresistance from hypoxia matters), or the small micrometastases that beta did not fully treat, or cells that have repaired beta-induced damage. Alpha radiation, with its different damage profile and shorter range, addresses these populations in ways beta cannot.

What the evidence shows

The clinical evidence for alpha PRRT in NETs is from case series, not from randomised trials. This is important to be honest about. The most influential early experience comes from Kratochwil and colleagues at Heidelberg, who first reported Ac-225 DOTATATE outcomes in patients who had progressed on Lu-177 PRRT.^[1] Subsequent series from Heidelberg, from Indian centres, and from the IAEA international experience have broadly converged on a few observations:

• Approximately 50 percent of carefully selected patients achieve some form of disease control (partial response or stable disease), with a smaller proportion achieving partial response.
• Median progression-free survival following alpha PRRT is in the range of 9 to 12 months in published series.
• Quality-of-life and hormonal-symptom outcomes appear to be meaningful in responders.
• Toxicity is more pronounced than with Lu-177 PRRT, particularly renal and haematologic toxicity, requiring careful selection and monitoring.

Randomised trials of alpha PRRT in NETs are now in design or early enrolment. Until those data mature, the case-series evidence is what is available. This is why the conversation with each patient about expected benefit and risk has to be specific and honest.

Who is eligible

How a course is delivered

An alpha PRRT course typically consists of 3 to 4 cycles of Ac-225 DOTATATE delivered every 8 to 12 weeks. The dose per cycle is in the range of 25 to 40 MBq. The longer interval between cycles (compared with Lu-177's 8-week interval) reflects the need for adequate marrow recovery between alpha doses.

Between cycles, the team monitors:

• Renal function — eGFR and serum creatinine; this is the most carefully watched parameter.
• Blood counts — full count weekly for the first three weeks after each cycle.
• Hormonal markers (chromogranin A, urinary 5-HIAA) — for biochemical response tracking.
• Imaging — Ga-68 DOTATATE PET-CT typically after cycle 2 or 3 to assess radiographic response.

Side effects, honestly

Alpha PRRT has a more pronounced side-effect profile than Lu-177 PRRT. The honest picture:

• Bone marrow suppression — anaemia, thrombocytopenia, leukopenia. More pronounced and more durable than with Lu-177 PRRT. Severe (Grade 3-4) cytopenia in a meaningful minority of patients.
• Renal toxicity — clinically the most concerning. Cumulative dose to the kidneys must be carefully tracked. Patients with pre-existing renal compromise are usually not candidates.
• Xerostomia — significant in a subset of patients due to alpha-particle delivery to salivary glands. See our guide on managing xerostomia after Ac-225.
• Fatigue — common, generally manageable.
• Gastrointestinal effects — nausea, anorexia, diarrhoea — managed symptomatically.

The Helsinki Declaration framework

Because the evidence base for alpha PRRT in NETs is from case series rather than randomised trials, FMRI offers it under the framework of the World Medical Association's Helsinki Declaration on ethical principles for medical research involving human subjects.^[2] In practical terms, this means:

• The patient receives a written summary of what the published evidence does and does not show.
• The patient receives a clear statement that randomised trial evidence is not yet available.
• Realistic alternatives are reviewed and offered.
• Informed consent is obtained in writing, including the patient's understanding that this is an early-evidence therapy.
• Outcomes are tracked and reported as part of the centre's case series, contributing to the wider evidence base.

This framework is not unique to alpha PRRT — it applies wherever a clinically reasonable therapy is offered ahead of randomised-trial evidence. The framework exists to protect patient autonomy and to ensure that the evidence base grows responsibly. We bring the Helsinki framing into the consultation explicitly, in writing, because patients deserve to know exactly what kind of evidence they are deciding on.