Biodamage Via Shock Waves Initiated by Irradiation With Ions

Just two doors down the hall from my office in Hannah Hall of Science at Oakland University is the office of my friend Gene Surdutovich. Gene teaches physics at OU, and is an international expert on the interaction of heavy ions with biological tissue. Recently he and his collaborators have published a review describing their work, titled “Biodamage Via Shock Waves Initiated by Irradiation With Ions,” in the journal Scientific Reports (Volume 3, Article Number 1289). Their abstract states

Radiation damage following the ionising radiation of tissue has different scenarios and mechanisms depending on the projectiles or radiation modality. We investigate the radiation damage effects due to shock waves produced by ions. We analyse the strength of the shock wave capable of directly producing DNA strand breaks and, depending on the ion’s linear energy transfer, estimate the radius from the ion’s path, within which DNA damage by the shock wave mechanism is dominant. At much smaller values of linear energy transfer, the shock waves turn out to be instrumental in propagating reactive species formed close to the ion’s path to large distances, successfully competing with diffusion.

Except for the British spelling, I enjoyed reading this paper very much.

Russ Hobbie and I discuss the interaction of ions with tissue in the 4th edition of Intermediate Physics for Medicine and Biology, in the context of proton therapy.

Protons are also used to treat tumors. Their advantage is the increase of stopping power at low energies. It is possible to make them come to rest in the tissue to be destroyed, with an enhanced dose relative to intervening tissue and almost no dose distally (“downstream”) as shown by the Bragg peak in Fig. 16.51.

Surdutovich and his colleagues note that the Bragg peak occurs for heavier ions too, such as carbon. What is really fascinating about their work is one of the mechanisms they propose. They note that

while the Bragg peak location answers a question of where most of the damage occurs, we are raising a question of how the damage happens… We investigate the effects that stem from a large inhomogeneity of the dose distribution in the vicinity of the Bragg peak on biological damage.

Their hypothesis is that when a carbon ion interacts with tissue, energy is abruptly deposited

within a cylinder of about one nm radius, which is so small that the temperature within this cylinder increases by over 1000 K by 10⁻¹³ s (we will refer to it as the ‘‘hot cylinder’’). This increase of temperature brings about a rapid increase of pressure (up to 1 GPa) compared to the atmospheric pressure outside the cylinder. Such circumstances cause the onset of a cylindrical shock wave described by the strong explosion scenario. The pressure rapidly increases on the wave front and then decreases in the wake.

I find this mechanism to be fascinating. The theory is noteworthy for its multiscale approach: analyzing events spanning a wide range of time, space, and energy scales. Interestingly, their theory also spans multiple chapters in Intermediate Physics for Medicine and Biology: Chapter 1 about pressure, Chapter 3 on temperature and heat, Chapter 4 on diffusion, Chapter 13 on sound, Chapter 15 about the interaction of radiation with tissue, and Chapter 16 about the medical use of radiation. They conclude

The notion of thermomechanical effects represents a paradigm shift in our understanding of radiation damage due to ions and requires re-evaluation of relative biological effectiveness because of collective transport effects for all ions and direct covalent bond breaking by shock waves for ions heavier than argon. These effects will also have to be considered for high-density ion beams, irradiation with intensive laser fields, and other conditions prone to causing high gradients of temperature and pressure on a nanometre scale.

This paper appears in an open access online journal, so you don’t need a subscription to read it. Be sure to look at the paper’s supplementary information, especially the movie showing a molecular dynamics simulation of the shock wave distorting a DNA molecule. And if you read very closely, you will find this nugget appearing in the acknowledgments: “We are grateful to … Bradley Roth who critically read the manuscript.”

By the way, this fall Gene is scheduled to teach PHY 325, Biological Physics, using the textbook….you guessed it….Intermediate Physics for Medicine and Biology.