Basically, X-rays are nothing more than light with a very short wavelength; as a result, x-rays are not visible to the human eye. The shorter the wavelength, the more energy is transported and the more difficult it is to stop the radiation. This is why x-rays, unlike visible light, can penetrate virtually any solid material. However, x-rays are also absorbed as they interact with the body’s molecules, steadily losing intensity in the process. (Think of a light beam in fog.) The maximum dosage is delivered just under the skin because scattered radiation is recruited only at the skin. As the radiation continues to penetrate toward the tumor, the radiation dosage declines exponentially. Tumors are typically located deep within the body so they are exposed to a smaller dose of radiation than tissue situated upstream in the path of the beam, and organs located behind the tumor (such as the spine, optic nerve, and parts of the brain) always receive the downstream penumbra.
Figure 1 shows the depth dosage profile for radiation coming from the left. For example, the tissue in front of a tumor at a depth of 20 cm receives significantly more radiation than the tumor itself, but the tissue behind the tumor is still exposed to a considerable amount of radiation. Increasing the photon energy by technical means flattens this exponential dose loss. It merely results in a trade-off between tissue damage in front of and tissue damage behind the tumor, but there is no meaningful improvement.
Radiation tolerance: tumor tissue versus healthy tissue
Essentially, there is no difference in radiation tolerance between healthy tissue and tumor tissue. The tolerance dose for a 50% probability of side effects ranges from 5 Gy to about 60 Gy in healthy tissue, while tumors require a dose of 30 Gy to 85 Gy for sterilization. In fact, tumors often require a higher dose than the surrounding normal tissue in organs like the lungs or GI tract–is able to tolerate (Figure 2). Therefore, tumors require a high local dose of radiation; in practice, one is always limited by the dose administered to the surrounding healthy tissue and the resulting side effects.
Immediate side effects (such as intestinal bleeding) may damage the skin and cause pneumonia and later arteriosclerosis. There is also a risk of post-treatment cancer because the ionizing radiation also damages the genetic material of healthy cells. Experts calculate that for each remaining year the patient lives following X-ray treatment, there is a roughly 1% chance he will develop “radiation-induced” cancer. This represents a substantial risk for children, who still have most of their lives ahead of them.
Tissue damage can be minimized, but not prevented: radiation overlap and IMRT
Today, the problem is minimized by precisely irradiating the tumour from different directions. The X-rays overlap directly within the tumor tissues and increase the therapeutic effect while limiting exposure of the healthy surrounding tissue to a single beam.
Figure 3 shows equivalent schematized dose distributions in a cross section of the body. Although the diagram shows the considerable overlap at the tumor site, it is also evident that much of the surrounding tissue is also exposed to a sublethal dose of radiation. Moreover, the physical properties of X-rays mean that some radiation always affects the tissue and organs behind the tumor. A newer procedure, IMRT (Intensity Modulated RadioTherapy) continuously modifies the contour and intensity of the X-ray beam; the X-ray tube rotates while the tumor is being irradiated. Although a good overlapping effect is achieved, the fundamental problem remains; IMRT cannot overcome the physical limitations of X-ray radiation. IMRT does not substantially reduce the radiation exposure of healthy tissue, since it merely changes the pattern of the damage. The unfavorable ratio of useful radiation to harmful radiation remains unchanged.