First In Vivo Monitoring of Helium-Ion Radiation Therapy With Secondary Ions

Ion beam radiation therapy offers steep dose gradients and high biological effectiveness required for the treatment of complex cancer cases. While the vast majority of ion beam therapy centers currently use protons and carbon ions, there is renewed interest in helium ions due to their unique physical and radiobiological properties. All ion-beam treatments are subject to beam-range uncertainties, mainly due to potential changes in patient morphology. In vivo treatment monitoring of secondary ions could potentially provide feedback on treatment quality, enabling dose reduction in healthy tissue or escalation of the tumor dose.

This work presents the first in vivo monitoring of a patient undergoing helium-ion therapy for a solitary fibrous tumor. The method is based on tracking secondary ions emitted from the patient as a natural byproduct of ion beam radiation therapy.

The comparison of 2 measured secondary-ion distributions confirmed high treatment reproducibility for the reported patient. However, significant differences between the 2 fractions were detected at the border of the skull base and the sinus sphenoidalis, which could originate from potential interfractional cavity filling.

We successfully performed the world's first in vivo monitoring of innovative helium-ion therapy. In the future, the observed signals will need to be validated in patients who receive regular control computed tomography scans. Moreover, Monte Carlo simulations and phantom measurements will help establish a robust link between changes in the secondary-ion distribution and clinically relevant changes in dose.