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Radiation: Power to Cure

A treatment primer

It seems to me that using radiation therapy to treat cancer is a magical science; Star Trek phaser invisible beams emanate from Jules Verne machines to vanquish evil. Even its language is that of futuristic forces as 3-dimensional conformal tomographic linear accelerated photon waves rupture malignant genes promising mysterious deliverance from neoplasm.  The history of radiation therapy in the last 100 years, from accidental discovery which killed early researchers, to an astonishingly precise tool with millimeter accuracy, is that of advanced physics, experimentation and, above all, power.

Radiation therapy innovation has always been toward more voltage and more accuracy.  The original radiation treatment consisted of low voltage units, which delivered radiation not much more powerful than exposing the patient to a radioactive rock.  The problem was the same as with a dull flashlight or candle; the light spreads out and cannot be focused.  This means extensive radiation exposure to normal tissue and devastating side effects.

With time physicists and physicians developed techniques to deliver increasingly powerful, high voltage, beams.  As power was increased, radiation beams got thinner, spread out less, and penetrated deeper into tissues without releasing their energy.  Thus, while 30 years ago radiating part of the lung using orthovoltage generators producing 200 kilovolts, meant radiating the skin, muscle, chest wall, ribs, normal lung and heart, now linear accelerator (linac) units produce megavoltage radiation to as much as 25 MV. Combined with computer guidance, megavoltage radiation can treat an area less than a half-inch in size, deep inside the body and leave other tissues almost untouched.  Radiation Oncologists can chose the type of radiation needed to treat a particular patient’s cancer based on its location, while giving healthy tissue minimal exposure.  These techniques reduce complications, shorten the time of treatment and deliver more effective cancer killing doses.

Conventional radiation treatment uses fixed high voltage beams to deliver a small amount of radiation daily, usually over a couple weeks.  For specific diseases, there are several ultra high power, super accurate methods for delivering external radiation, which use sophisticated computer planning to aim high-energy radiation sources.  The planning process, known as “simulation,” utilizes CT or MRI scans of a tumor to study how much radiation needs to be delivered to kill the cancer.  The total amount of radiation needed to destroy the specific type of cancer is decided, the shape of the cancer is analyzed, as irregular tumor shapes require equally complex radiation fields, and treatment is planned to avoid vital tissues.

The most commonly used advanced radiation method is 3-dimensional conformal radiation therapy (3D-CRT), which relies on computer software to guide multiple fixed radiation beams to maximum benefit and safety. This is a highly effective method, especially treating cancers with simple shapes not very close to sensitive normal tissue.

Intensity-modulated radiation therapy (IMRT) was developed as an improvement on simple fixed radiation beams.  This also relies on computer programs to guide the treatment, but in addition the beams, which may move, are shaped by hundreds of tiny radiation shields called collimators, inside the machine, to create a complex radiation field.  Another modification of IMRT is to place the radiation generator at the end of a robotic arm so that it moves as needed to produce complicated radiation patterns (CyberKnife).  The goal of IMRT, as always, is to improve killing of the cancer by delivering high doses of radiation and reduce injury to normal tissues. IMRT has been used most in prostate and head/neck cancers.

Stereotactic Radiosurgery (SRS) fires intense radiation into small tumors in a single or a few treatments.  SRS is used in the brain or spinal cord where extreme accuracy is required. Multiple different types and brands of equipment may be used including linac, Gamma Knife, or CyberKnife.  These different techniques have similar results, but have not been directly compared, thus the choice often depends on which machinery a particular center has available.  Stereotactic Body Radiation Therapy (SBRT) uses similar equipment to deliver intense local treatment to small tumors in parts of the body, such as in the lung or liver. Finally, proton beam machines utilize particles instead of photon waves, and can treat with ultra-accuracy small tumors, particularly in the brain, spinal cord or eye.  

The medical specialty of Radiation Oncology and radiation therapy are critical to the treatment and cure of many cancers.  This complex technology, a true product of the atomic age, saves or improves the lives of millions every year. Developments in information technology, nuclear physics and our understanding of cancer promise continuing improvements in this vital area of health care in the years to come.

 

As published in Sunrise Rounds.

 

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