About New Techniques and Procedures in Radiation Therapy
Radiation treatment has evolved over many decades. At one time, more than thirty years ago, the cobalt machine was high-energy and up-to-date equipment. This was simply a shielded housing that contained a radioactive source of co6o. There was a little opening that would allow the radiation to be emitted and thereby treat the patient. A device called a collimator, placed below the opening, allowed doctors to change the size of the field that they wanted to treat, and at that time, they could make only square or rectangular treatment fields. Doctors could modify the shape slightly by adding blocks of lead on a plastic tray below the collimator. To determine the treatment time, they had to use graphs and charts and they used a slide rule for necessary calculations. Patients were usually treated from only two directions because calculating dose and time for multiple fields was cumbersome, time consuming, and not very accurate using charts, graphs, and slide rules. Because doctors used only two fields or directions, the normal tissue in front and behind the tumor received almost the same dose as the tumor.
The development of high-energy linear accelerators and the use of computers allowed significant advancements. They gave doctors the ability to perform the very complex calculations required to determine the dose and time to treat with multiple fields. In addition, computer technology led to the development of the CT scan. The CT scan gives doctors a cross-sectional picture every three to five millimeters. To understand the images a CT scan produces, think of a bologna sausage with an olive in the center. Doctors can’t see the olive until they slice the baloney. Each slice is a cross section of the bologna, just as each CT scan picture shows a cross sectional slice of the patient.
With a CT scan and computer-generated treatment plans doctors gained the ability to see the tumor in relationship to the normal surrounding organs. This information is key to determining what dose each would receive. Another new development, 3-D conformal radiation therapy, allows doctors to shape the beam with poured lead blocks to conform to a more optimal treatment area. As a result, doctors are able to lower the dose to normal tissue and increase the dose to the tumor.
Knowledge begets knowledge. Computers became faster and the manufacturers of the linear accelerators produced multiple-leaf collimators. These have multiple fingers that can shape the radiation beam into any shape. These two further developments lead to intensity-modulated radiation therapy (IMRT). Essentially, IMRT is a progression of conformal treatments. These advanced treatment planning systems, combined with computer-controlled linear accelerators, enable doctors to deliver multiple fields, each with a different shape and each field with a different dose, thereby optimizing the tumor dose and minimizing normal tissue dose. For example, doctors can set the computer to deliver 200 centigrays to the tumor and keep the dose to a nearby organ to less than 10 percent of the tumor dose. The computer will then calculate how many fields, the shape of each field, and the dose to each field.
IMRT is not indicated for most radiation treatments. It focuses the beam on a confined area and most times doctors not only want to treat the tumor but they also want to treat the surrounding area because cancerous tumors have invasive tentacles into the adjacent tissue.
On other hand, early prostate cancer lends itself well to IMRT. The cancer is usually confined to the prostate gland, a relatively small organ, and doctors must deliver a high dose while sparing the rectum and bladder.