FLASH Radiotherapy Delivers Cancer Treatment in Milliseconds at Ultra-High Dose Rates

FLASH radiotherapy delivers therapeutic radiation doses in fractions of a second at ultra-high dose rates, potentially reducing radiation-induced injury in normal tissues without compromising anti-cancer effects. The approach could enable safer dose escalation and improved outcomes for selected tumors.

FLASH radiotherapy refers to the delivery of radiation at dose rates typically exceeding 40 Gy per second, in contrast to conventional radiotherapy, which is commonly delivered at approximately 0.5 to 5 Gy per minute. This difference represents a distinct time dose structure that may change how tissues respond to radiation. Rather than spreading dose delivery over minutes, FLASH radiotherapy compresses treatment into milliseconds, creating an ultra-brief exposure window that appears to alter radiobiological responses in ways that favor normal tissue protection.

Radiation therapy remains one of the most widely used and effective pillars of cancer care. Over the past decades, advances in imaging, treatment planning, and beam delivery have enabled increasingly conformal radiation, allowing clinicians to target tumors with greater precision and spare surrounding organs. Yet a central challenge persists: tumor control is often dose dependent, and the ability to escalate dose is limited by the risk of severe, sometimes irreversible, toxicity in healthy tissues.

Preclinical research has repeatedly shown that FLASH-RT can reduce damage to healthy tissues compared with conventional dose-rate radiotherapy. The most widely discussed explanation for this protective effect involves oxygen dynamics. Because radiation-induced tissue damage is strongly influenced by oxygen, the ultrafast delivery of radiation may rapidly deplete oxygen in normal tissues, causing a transient hypoxic state that reduces free-radical mediated injury. Normal tissues are generally well-oxygenated and capable of maintaining oxygen balance, while tumors often contain regions of chronic hypoxia and disordered vasculature, which may limit the protective benefit in malignant tissue.

Animal studies suggest that these effects may translate into reduced toxicity in organs that typically constrain radiation dosing, including the brain, lung, skin, and gastrointestinal tract. At the same time, the anti-tumor effect appears broadly preserved in many models, which is central to the promise of an improved therapeutic index. However, the biological picture is not fully settled. Immune effects, DNA damage response pathways, inflammatory signaling, and differences among tumor microenvironments may all contribute, and these mechanisms remain active areas of investigation.

Modern linear accelerators deliver radiation in pulses rather than as a perfectly continuous stream. In conventional radiotherapy, pulses are delivered at set frequencies and the overall treatment time extends over minutes, producing an average dose rate that is relatively low even if the instantaneous dose rate within each pulse is higher. In FLASH-RT, the overall dose is delivered in far fewer pulses over a dramatically shorter time, requiring much greater energy transfer per pulse and substantially higher average dose rates.

Delivering 8 Gy in conventional settings may take minutes, whereas in FLASH-RT it can be delivered in around 0.2 seconds. The difference in dose per pulse and energy load is enormous, and that creates a technological challenge: the beam must be stable, predictable, and measurable at ultra-high dose rates, while maintaining accuracy that is acceptable for clinical practice.

Accurate dosimetry is essential in radiotherapy, but FLASH conditions push measurement systems to their limits. The pulsed, high-intensity delivery can produce saturation, recombination effects, and nonlinear responses in detectors that perform reliably under conventional dose rates. Real-time monitoring becomes particularly challenging when treatment is completed in milliseconds, leaving minimal opportunity for correction if dose deviates from the prescribed value. To support FLASH radiotherapy, dosimetric systems must be able to characterize the delivered dose under these demanding conditions.

Radiation therapy is one of the most effective treatments for many kinds of cancer, helping to destroy tumours and save lives. Because these treatments use high-energy radiation directed at specific parts of the body, accuracy is critical — regardless of how small the dose is. Even small inaccuracies can affect how well a tumour is treated or increase the risk of side effects. Dosimetry measures how much radiation is delivered during medical procedures and ensures that patients receive exactly the dose prescribed. Reliable dose measurement is essential for safe and effective radiotherapy, diagnostic imaging and nuclear medicine.