Relative biologic effectiveness and oxygen enhancement ratio at various depths of a 910-MeV helium ion beam

Roadmap: helium ion therapy

Helium ion beam therapy for the treatment of cancer was one of several developed and studied particle treatments in the 1950s, leading to clinical trials beginning in 1975 at the Lawrence Berkeley National Laboratory. The trial shutdown was followed by decades of research and clinical silence on the topic while proton and carbon ion therapy made debuts at research facilities and academic hospitals worldwide. The lack of progression in understanding the principle facets of helium ion beam therapy in terms of physics, biological and clinical findings persists today, mainly attributable to its highly limited availability. Despite this major setback, there is an increasing focus on evaluating and establishing clinical and research programs using helium ion beams, with both therapy and imaging initiatives to supplement the clinical palette of radiotherapy in the treatment of aggressive disease and sensitive clinical cases. Moreover, due its intermediate physical and radio-biological properties between proton and carbon ion beams, helium ions may provide a streamlined economic steppingstone towards an era of widespread use of different particle species in light and heavy ion therapy. With respect to the clinical proton beams, helium ions exhibit superior physical properties such as reduced lateral scattering and range straggling with higher relative biological effectiveness (RBE) and dose-weighted linear energy transfer (LET_d) ranging from ∼4 keVμm^-1to ∼40 keVμm^-1. In the frame of heavy ion therapy using carbon, oxygen or neon ions, where LET_dincreases beyond 100 keVμm^-1, helium ions exhibit similar physical attributes such as a sharp lateral penumbra, however, with reduced radio-biological uncertainties and without potentially spoiling dose distributions due to excess fragmentation of heavier ion beams, particularly for higher penetration depths. This roadmap presents an overview of the current state-of-the-art and future directions of helium ion therapy: understanding physics and improving modeling, understanding biology and improving modeling, imaging techniques using helium ions and refining and establishing clinical approaches and aims from learned experience with protons. These topics are organized and presented into three main sections, outlining current and future tasks in establishing clinical and research programs using helium ion beams-A. Physics B. Biological and C. Clinical Perspectives.

Heavy-ion beam-induced reactive oxygen species and redox reactions

Quantification and local density estimation of radiation-induced reactive oxygen species (ROS) were described focusing on our recent and related studies. Charged particle radiation, i.e. heavy-ion beams, are currently utilized for medical treatment. Differences in ROS generation properties between photon and charged particle radiation may lead to differences in the quality of radiation. Radiation-induced generation of ROS in water was quantified using several different approaches to electron paramagnetic resonance (EPR) techniques. Two different densities of localized hydroxyl radical (•OH) generation, i.e. milli-molar and molar levels, were described. Yields of sparse •OH decreased with increasing linear energy transfer (LET), the yield total •OH was not affected by LET. In the high-density, molar level, •OH environment, •OH can react and directly make hydrogen peroxide (H₂O₂), and then possible to form a high-density H₂O₂ cluster. The amount of total oxidation reactions caused by oxidative ROS, such as •OH and hydroperoxyl radial (HO₂^•), was decreased with increasing LET. Possibilities of the sequential reactions were discussed based on the initial localized density at the generated site. Water-induced ROS have been well investigated. However, little is known about radiation-induced free radical generation in lipidic conditions. Radio-chemistry to understand the sequential radio-biological effects is still under development.

Protection of the skin of mice against irradiation with cyclotron-accelerated helium ions by 2-mercaptoethylamine

Mouse skin was exposed to doses of 1 250 to 3 000 rad using a helium ion beam with modal LET of 15 keV per micron. The skin reactions were evaluated for mice treated with a topical application of 10% MEA in a cream base or a placebo 15 minutes before irradiation. A comparison of the skin reactions indicated that the MEA treatment resulted in a DMF of at least 1.2. The implication for radiation therapy was discussed.

Tolerance of the spinal cord of rats to irradiation with cyclotron-accelerated heliumions

Spinal cords of rats were exposed to either single doses of helium ions ranging from 500 to 3500 rads, or to fractionated doses of helium ions ranging from 1300 to 6530 rads using 10 equal fractions given over 22 days. Both plateau and spread-out Bragg peak regions of ionization were used. For single doses there was no difference betweenplateau or spread-out Bragg peak ionization regions for production of paralysis; thetolerance dose for production of paralysis was about 1900 rads. For the fractionation experiment, there was also no difference between plateau or spread-out Bragg peak ionization regions for production of paralysis; the tolerance dose was between 5220 and 6530rads. Comparison of dose levels producing about 10% partial paralysis indicates that about 500 rads of dose is recovered between fractions. In both single dose and fractionated experiments, latency between onset of paralysis was between 20 to 24 weeks.