Basically, x-rays consist of nothing more than light having a very short wavelength; as a result, x-rays are not visible to the human eye. The following principle applies: The shorter the wavelength, the more energy is conveyed 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 through interaction with the molecules of the body, by which process they continually lose intensity. (This is similar to a light beam in fog.) The maximum dosage is delivered just under the skin, such effect being based on the recruitment of scattered radiation, which only occurs starting with the skin. As the radiation continues to penetrate tissue in the direction of the tumour, the radiation dosage declines steeply over an exponential curve. Tumours, which are typically located deep within the body, are therefore exposed to a smaller dosage than tissue situated upstream in the path of the beam, and organs located behind the tumour (e.g. spine, optic nerve, brain parts) 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 tumour located at a depth of 20 cm receives significantly more radiation than the tumour itself, but the tissue behind the tumour is still exposed to a considerable amount of radiation. Increasing the photon energy by technical means flattens this exponential dosage loss. It therefore only represents a trade-off between tissue damage in front of and tissue damage behind the tumour, but there is no fundamental improvement.

Essentially, there is no substantial difference in the radiation tolerance of healthy tissue and that of tumour tissue. The tolerance dosage for a 50% probability of side effects ranges from 5 Gy to about 60 Gy in healthy tissue, while tumours require a dosage of 30 Gy to 85 Gy for sterilization. In fact, tumours often require a higher dosage than the surrounding normal tissue—such as in healthy lungs or the intestine--is able to tolerate (Figure 2). Therefore, tumours can only be treated by a high local dosage, which in practice is always limited by the dosage deposited in the surrounding healthy tissue and thus by the side effects induced.

The immediately triggered side effects (such as intestinal bleeding) may cause damage to the skin, as well as pneumonia and later arteriosclerosis. Additionally, there is the risk of a subsequent cancer occurring, because the genetic material of healthy cells is also damaged by the ionizing radiation. Experts calculate there is a probability of about 1% for each remaining year of life, that a patient treated with x-rays will develop a "radiation-induced" cancer during the course of his or her lifetime. This represents a substantial risk for children, who still have the major portion of life before them.

Today, the problem is solved to a certain extent by precisely irradiating the tumour from different directions. The x-rays then overlap precisely in the tumour tissues and increase the effect, while the healthy surrounding tissue is exposed to a single beam. Figure 3 shows equivalent schematized dosage distributions in a cross section of the body. Although the diagram demonstrates the considerable overlap effect in the area of the tumour, it is also evident that much of the surrounding tissue is simultaneously exposed to sublethal radiation dosages. Moreover, the physical properties of x-rays mean a dosage is always deposited in the tissue behind the tumour, exposing the organs therein situated. Accordingly, IMRT (Intensity Modulated RadioTherapy) is a more modern radiation delivery method in which the contour and intensity of the x-ray beam is continuously modified, even as the x-ray tube rotates while the tumour is being irradiated. Although a good overlapping effect is achieved, there is no change with respect to the fundamental problem because IMRT cannot overcome the physical limits of x-ray radiation. IMRT does not substantially reduce the radiation which impacts healthy tissue, since it merely changes the pattern of the damage. As such, the unfavourable ratio of useful radiation to damaging radiation remains unchanged.