In a first, arc therapy beats tricky cancer while sparing nearby tissue

A team of experts in the U.S. has successfully used a technique called step-and-shoot spot-scanning proton arc therapy (SPArc) to treat a patient’s adenoid cystic carcinoma for the first time. SPArc works by targeting tumours with protons while sparing surrounding tissue. The findings of the team, at the Corewell Health William Beaumont University Hospital, were published in the June 2025 issue of the International Journal of Particle Therapy.

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The team compared the results of three techniques: SFO-IMPT (the current standard of care), step-and-shoot SPArc, and fully dynamic SPArc (simulated with computers). The SPArc methods reduced radiation delivered to the brainstem (by 10%), optical chiasm (56%), oral cavity (72%), and spinal canal (90%) over SFO-IMPT. "This is definitely significant,” Narayana Subramanian, lead consultant, Head and Neck Surgical Oncology at Aster International Institute of Oncology, Bengaluru, said. “In tumours of the head and neck, limiting dose delivery to organs at risk is a major consideration of treatment planning. When these organs receive a higher dose of radiation than they can tolerate, they undergo irreparable damage.”While fully-dynamic SPArc performed better, the difference between the two arc styles was small, the team wrote. In dynamic SPArc, the dose’s energy and point of delivery is constantly adjusted, whereas in step-and-shoot the machine follows a pre-programmed pattern.The researchers wrote that dynamic SPArc is still under development and awaits regulatory clearance and integration with existing oncology information systems.Multi-technique protocolThe first person to receive step-and-shoot SPArc was a 46-year-old woman whose parotid-gland cancer had spread alo