In 1895 Willhelm Roentgen discovered a type of light that was not only invisible but appeared to pass straight through materials as if they weren’t there. The demonstration of this was the now famous image of the hand of Roentgen’s wife.
This new type of radiation was dubbed “x-rays” and the name stuck. Almost 120 years later, x-rays are still used every day for the diagnosis of a huge range of diseases and also for the treatment of cancer in what is today referred to as radiation therapy. It’s commonly referred to as radiotherapy but this was thought to be confusing as it has nothing to do with radio waves!
Light, in the normal sense of the word, is the visible part of the electromagnetic spectrum, covering wavelengths in the range 700 nm (red) to 400 nm (violet). X-rays are at the short wavelength end of the spectrum, below 1 nm. It’s more useful at this point to change from a wave description of light to a particle description and talk about the energy of an individual photon. X-ray photons have energies in the range 1 keV to 10 MeV, and what’s important about this energy (compared to the eV energies of visible photons) is that it’s high enough to ionize matter; a single photon can break the bonds that hold an electron in its orbit within an atom.
This highlights one aspect of the quantum nature of light and matter. The fact is that, perhaps contrary to common sense, a large number of low energy photons do not equal one high energy photon. In this particular case a single 1 keV x-ray photon can do something that one thousand 1 eV visible photons cannot – eject an electron from an atom. Incidentally, this quantum interpretation of light-matter interactions won Albert Einstein the Nobel Prize in 1921 (and not, one might think, his theories of either Special or General Relativity).
Ionization is the process behind a Geiger counter – radiation passes through the gas in the Geiger counter detector causing ionization that is converted to the ‘clicks’ one hears. But it’s also the process by which x-rays become a useful therapeutic agent. Target x-ray photons at a cancer cell and, amongst a whole host of possible reactions, you can cause strand-breaks in the DNA. This is important because healthy DNA is essential to the life of a cell. DNA is amazingly good at repairing itself, due to the double helix structure and matched nucleotides. However, x-rays can cause a double strand break, which is much harder for the cell’s repair mechanisms to deal with. A double strand break usually results in cell death. This is the aim of radiation therapy – use targeted x-rays (other types of ionizing radiation are also used) to preferentially kill cancerous cells within the body.
The big problem for radiation therapy is that an x-ray photon doesn’t know the difference between a healthy cell and a cancerous cell. If you are firing an x-ray beam at a tumour from outside the body (which the most common type of treatment) there are a lot more healthy cells to hit than cancer cells. So why is radiation therapy so successful?
It comes down to careful treatment planning and accurate treatment delivery. Modern radiation therapy uses sophisticated treatment planning calculations to deliver as much of the radiation to the tumour and as little to healthy tissue as possible. This is done by directing carefully shaped radiation beams from multiple directions.
A linear accelerator can delivers x-ray beams from any angle and the vast majority of radiation therapy treatments worldwide (around 80%) use this kind of x-ray beam. The picture above shows a treatment plan for prostate cancer. Overlaid on the black-and-white CT-scan of the patient (yet another use of x-rays) are several x-ray beams aiming at the prostate. This multi-direction technique ‘smears out’ the radiation delivered to healthy tissue while concentrating it on the cancerous area. There can be the temptation to minimize the dose (1) to healthy tissue but this can mean that not all the cancer cells are killed, resulting in a recurrence of the cancer later, or even metastases, where the cancer spreads to another part of the body. Going too far the other way, a high dose will indeed kill all the cancerous cells but will also produce significant side effects due to damage to healthy tissue. One of the primary concerns in radiation therapy is therefore finding the ‘sweet spot’ where the tumour is destroyed with the minimum of side effects. Determining the ideal treatment for each patient is a truly multidisciplinary activity, requiring knowledge of the physical, chemical and biological processes involved when an x-ray beam is aimed at a human body.
The science of using x-rays for medicine has come a long way since the late 19th Century and researchers and clinicians continue to improve methods for both diagnosis and treatment. Radiation therapy is at the same time a routine and amazing technique. Routine because almost half of all cancers are treated using ionizing radiation, amazing because we can manipulate x-ray beams to provide images with exquisite detail and deliver curative treatments with surgical precision. There’s clearly more to light than meets the eye!
1 – In most medical treatments, dose describes the amount of stuff (e.g. drug) that is administered. In radiation therapy, dose is shorthand for “absorbed dose” and refers to how the radiation interacts with matter (it’s a measure of the end-point, not the starting point). Absorbed dose is effectively the energy deposited by the radiation as it passes through the patient. It is something that can be easily measured and equated with the biological effect – the more energy deposited, the more DNA double-strand breaks and the more cells killed. The absorbed dose can therefore be used as a predictor of treatment outcome and therefore medical physicists in cancer clinics (assisted by research and calibration laboratories) put a lot of effort into accurately measuring the absorbed dose for radiation therapy treatments before the patient goes on the couch.
Malcolm McEwen is the Discipline Lead of the Ionizing Radiation Standards group within the Measurement Science and Standards portfolio of the National Research Council Canada. The IRS group develops primary air kerma and absorbed dose standards for x-rays, gamma rays, and electron, photon and neutron beams. These standards form the basis for calibration services that are used by all Canadian cancer clinics to determine the output of radiation devices used for diagnosis and treatment. His particular research interests focus on primary measurement standards and dosimetry protocols, particularly applied to radiation therapy.