As an example, let’s take the situation with secondary deposits from cancer of the mouth in lymph nodes in the neck. When the lymph nodes are 4-6cm in diameter, the dose needed to eradicate the cancer is about half as much again as when the secondary deposits are so small that the nodes are normal in size. In spite of the higher dose, the enlarged nodes are also much less likely to be cleared. Nodes bigger than 6cm cannot be cleared of cancer with a safe dose of radiation. The dose that would be needed would completely destroy some of the surrounding normal tissues. The alternative of surgical removal of such big glands is likely to be a better option than radiation. This is not the same as saying that a safe dose of radiation would be useless. It could shrink such big nodes temporarily but would be very unlikely to cure the cancer altogether. In fact, it is generally true that the dose needed to temporarily control symptoms is much less than the dose needed to completely destroy a cancer deposit. Therefore it is often possible to safely control symptoms by radiation when it would not be possible to cure the cancer by it.
Just as the average dose that is safe for each normal tissue is known, so the average dose that will destroy each type of cancer is known. Some types of cancer are more sensitive than others. For example, one type of cancer of the testis—seminoma—is much more likely to be cured by radiation than other types of testicular cancer. On average, lymphoma in lymph nodes is more sensitive to radiation than secondary cancer in lymph nodes. Ewing’s sarcoma of bone is much more sensitive to radiation than other primary bone cancers. Thus the type of cancer is important in predicting whether it is likely to be cured by radiation and what dose will probably be necessary.
*263/40/1*
