Monday, December 4, 2017

The Physics of Healing: Radiation Therapy

Few days are worse than a day when you hear the words, “I’m sorry, you have cancer.” Fear of the unknown, fear of pain, and fear of death all attend the moment when you learn the news, and nothing can prepare you for the shock of learning that your body has betrayed you. It can be difficult to know there’s something growing inside you that shouldn’t be there, and the urge to get it out can be overwhelming.

Sometimes there are surgical options, other times not. But eradicating the tumor is not always the job of a surgeon. Up to 60% of cancer patients will be candidates for some sort of radiation therapy, often in concert with surgery and chemotherapy. Radiation therapy can be confusing to some people — after all, doesn’t radiation cause cancer? But modern radiation therapy is a remarkably precise process that can selectively kill tumor cells while leaving normal tissue unharmed, and the machines we’ve built to accomplish the job are fascinating tools that combine biology and engineering to help people deal with a dreaded diagnosis.

Controlled Killing

Simply defined, radiation therapy is the application of specific kinds of ionizing radiation with the intent to treat a disease, which is usually but not always cancer. This differs substantially from using radiation to gather diagnostic information, as in the case of medical X-rays, CT scans, and nuclear medicine. All ionizing radiation has the potential to cause cellular damage, but in diagnostic radiology, doses are kept as low as possible to protect against cellular damage while still getting the diagnostic information needed. Radiation therapy, however, uses doses of ionizing radiation with the express intent of killing cells in as controlled a manner as possible.

To understand how radiation therapy works, it’s important to know a few simple facts about cancer. Cancer is not one disease, of course, but all cancers share a trait: uncontrolled cell growth. Cancer cells divide more or less continually and proliferate, often forming solid masses called tumors. Cancer cells also tend to be relatively undifferentiated cells; that is, they lack the specialized structure and function of normal cells.

These characteristics provide weaknesses that can be exploited for therapies. For cells to divide they must replicate their DNA, and while DNA is replicating, it’s particularly vulnerable to damage. Damage can come from powerful drugs like those used in chemotherapy, or by exposure of the cells to ionizing radiation. Damage the cell enough and it dies. Damage enough cells and the tumor starts to shrink.

Cancer cells are more vulnerable to damage from drugs and radiation because they are replicating more rapidly than the surrounding normal cells. But the normal cells are replicating too, and can incur collateral damage while the tumor cells are being targeted. Being able to spare the surrounding tissues from damage while killing the tumor is the goal of any cancer treatment, and this is where radiation therapy shines.

High Energies

Radiation treatment works by forming a very precisely shaped beam of ionizing radiation that can illuminate a tumor without exposing the surrounding tissue. The problem is that bodies are three-dimensional structures, and no matter which way you aim a beam, normal tissue will be either above or below the tumor, and will get dosed. Luckily, cells are sensitive to accumulated doses of radiation, so it’s possible to deliver a partial dose from multiple angles, limiting the damage to structures above and below the tumor. This is accomplished by precisely rotating the therapy beam axially around the patient with the tumor at the focus. Over time, the tumor builds up enough of a dose to start dying, while the surrounding normal tissues are spared lethal doses.

The machines used to deliver external beam radiation therapy can be immensely complicated. Not only do they have to generate extremely powerful beams of ionizing radiation, they have to control, filter, and shape the beam. They’ve also got to position the beam with extreme precision so that the treatment plan developed by the radiation oncologist and the medical physicist is correctly executed. On top of that, the machine has to have multiple redundant layers of safety interlocks to protect the patient and technicians from potentially lethal exposures.

Options for sources of ionizing radiation vary, with different sources offering different therapeutic options based on the energy of the photons they produce. Orthovoltage X-ray machines are essentially high-power X-ray tubes that generate beam energies in the 200 to 500 kiloelectron volt (keV) range. Like diagnostic X-ray tubes, orthovoltage X-ray tubes work by accelerating electrons into a tungsten target to create powerful beams of photons.

Further up the energy scale are the linear accelerator machines, or linacs. These can provide either X-ray beams or electron beams in the 4 to 25 Megaelectron volt (MeV) range. Where the electron source in diagnostic and orthovoltage X-ray tubes is generally a simple hot cathode design, linac electron beams are produced by injecting electrons from a tungsten filament into a long waveguide. A magnetron produces a standing RF wave inside the waveguide which accelerates the electrons to huge kinetic energies. The electron beam can be used directly, or the beam can be used to strike a tungsten target to produce a high energy beam of X-rays.

Staying in Shape

One of the most interesting parts of a radiation therapy machine is the collimator. Collimation controls the shape of the beam and limits unwanted exposure. A diagnostic X-ray machine’s collimator is simple — two sets of lead or tungsten leaves set 90° to each other can be moved in or out of the beam and produce a rectangle of various sizes. A radiotherapy beam needs to be able to match the irregular profile of a tumor, so a multileaf collimator (MLC) is used instead. An MLC has a large number of tungsten plates that can be moved in and out of the beam to control its size and shape. The MLC is set to project a beam based on the two-dimensional profile of the target tumor as seen from a certain angle, which changes as the beam is rotated around the patient.

MLCs are highly engineered mechanisms. Not only must every leaf be precisely and accurately positioned to match the therapeutic plan, it must do so while being bathed in high-energy photons. The leaves have to interlock and overlap so there’s no leakage between leaves, but thermal expansion must not be allowed to jam the leaves. The leaves also have to be thin enough that the “pixelation” of the edge of the beam is minimized.

These aren’t the only therapeutic modalities available for radiation therapy, of course. Some external beam therapies use a fixed radioisotope source like cobalt-60 rather than an accelerator, and some therapies use beams of particles like protons to kill cancer cells. But no matter the physics behind the treatment, the engineering that goes into controlling beams of lethal radiation to kill only what needs killing is something to admire.

[Featured images source: Varian Medical Systems]

Filed under: Featured, Medical Hacks

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