When we speak about radiation oncology, on the biological level. What we’re trying to do is kill a cancer cell. What one has to realize is radiation is effective only in microscopic disease for the most part, and it’s not effective during the entire time period that the radiation is being given. The reason is, if you look at the biology of cancer, and you think about the dividing cell, the majority of a cancer mass does not divide at any one time, only about 10% of the cancer is dividing, so if you look at the cell cycle, what you see is that the majority of cells are what we call the G0 phase in a tumor mass, 90% of the cells are actually resting. For radiation to be effective, these cells have to be recruited out of their resting phase and have to be actually dividing because radiation will kill a cell when it’s either in the mitotic phase or beginning from the resting phase or the G2 phase.
The clinical implication of this is that one sees the maximum effect of radiation, not during the time of radiation but within three to four weeks after a patient completed radiation therapy. The reason we give a small dose initially of 180 to 200 rads is that lower doses, the cell has the ability to repair the damage of the radiation and that is called the sublethal effect. That is this little slope you see here. As you get over 200 centigrade, you begin to see the higher the dose, the more likely the cancer cell will be killed. Radiation therapy is based upon what we call the Compton effect, what the Compton effect is, is that when a photon or an x-ray hits a cell, it will go ahead and eject the nucleus from the outer shell of the cell of the atom producing what we call a scatter photon, and what you have to remember is x-rays are like energy or photons. This is the way radiation works. Radiation doesn’t work in large masses because the majority of cancers have a hypoxic out nucleus or have necrosis.
When angiogenesis takes place, and there is a vascularity, what happens is the vasculature oxygenates the center of the tumor mass and what we have found over the years, is the more oxygenated the cancer mass is, the greater the likelihood the cancer will die and here you have a comparison of what happens when cell are in a toxic background or no oxygen and when there is oxygen present. As you can see, treating cells with radiation what you will see is that there is a greater ability to kill the cancer cell when oxygen is present. This is important clinically because when a patient goes for radiation therapy, we like the patient not to anemic and to be transfused with a hematocrit over 30. This is the basis for it. When you send a patient to radiation oncology, they always give you a dose, 10 years ago what we used to report is dose in rads, and what rads was radiation absorbed dose. A patient would get for example, 4 to 5 thousand rads of external beam radiation therapy. However, the terminology has changed and what we now talk about rather than rads is centigrade and centigrade is if you pick up an older chart, you convert rads to centigrade by remembering that one rad is equivalent to one centigrade, 100 centigrade is equivalent to one grade, so if a patient receives 40 grade, they have received 5,000 centigrade or 5000 rads, they are all equivalent.