The last article in this series examined the distribution of dose inside a patient when they are treated with a single therapy beam. The question is, how do we use this knowledge to treat a patient in the most effective manner? We want the patient’s tumor to receive the dose that the physician prescribes, but we do not want the dose to surrounding tissue to be too high. One way to achieve both of these goals is through arranging the angles of the beams entering the patient.
Suppose we had a patient with a tumor at a depth of 15 cm. For simplicity, we will assume that the patient is a cube of water with each side equal to 30 cm so that the tumor is in the exact center. (In reality, the majority of our patients are not cubes of water.) The doctor wants the tumor to get a dose of 100 cGy for each fraction. It is up to us to determine the best way to deliver that dose while sparing the normal tissue as much as possible.
Figure 1. A single 18 MV beam has a lower maximum dose than a 6 MV beam.
If we were to treat this patient with a single beam of radiation with an energy of 6 MV, the resulting dose distribution would look like the red curve in Figure 1. You can see that while the tumor is receiving the correct dose (100 cGy), the shallower normal tissue is receiving a much higher dose. If we were to use a higher energy, such as 18 MV, the dose distribution would look like the blue curve. Since the beam penetrates farther, the dose to shallower tissue is less, but still more than the tumor receives. In some cases this might be ok, but others will need a better plan.
Figure 2. Parallel opposed beams.
Figure 3. Adding another beam directly opposed from the first limits the maximum dose.
Figure 3 shows the dose distribution along the axes of the beam for a 6 MV beam. We can see that the dose to normal tissue is lower than with a single beam, but still slightly higher than the tumor dose. In addition, there are now two spots with a high dose.
Figure 4. Using higher energy beams will lower the dose to the normal tissue.
Figure 4 shows the difference in maximum dose for a 6 MV and an 18 MV pair of beams. Again, increasing the beam energy helps reduce the ratio of normal tissue dose to tumor dose.
Figure 5. A four field beam arrangement.
If we add a second pair of beams with axes perpendicular to the first pair, we have what is called (oddly enough) a four field arrangement, shown in Figure 5.
As Figure 6 shows, the highest dose throughout the patient is in the box where the four fields overlap. We can restrict the size of the box to be just larger than the tumor volume, and ensure that no other part of the body is getting as high a dose as the tumor. We can continue to add fields and improve the ratio of normal tissue dose to tumor dose, but at some point we get diminishing returns. Usually a treatment on a linear accelerator will have, at most, 11 to 13 fields. A Cyberknife or Gammaknife treatment might have over a hundred.
So which arrangement is best? The answer is patient specific. Usually, simpler is better. The more fields there are, the longer it takes to treat the patient. As treatment time increases, the more likely it is that the patient or the patient’s internal anatomy will move and take the tumor away from the beam of radiation. When we get to patient immobilization, I will show some strategies we use to minimize this factor, but it can never be completely eliminated. Also, adding more beams may decrease the maximum dose, but it also exposes more normal tissue to radiation. Therefore, the answer is to use as few beams as it takes to treat the tumor effectively while sparing normal tissue. That answer is definitely vague, and this is where treatment planning becomes an art.
All clinics in the United States have full time staff members known as dosimetrists whose job it is to find the optimal beam arrangement for each patient. (Other countries have equivalent positions, but with different job titles). Obviously, this can be a very time consuming process. Fortunately, most types of cancer can be treated in a standardized fashion. For example, most lung tumors can be effectively treated with a parallel opposed set of beams. Breast cancer can often be treated with two tangent fields that treat the breast from either side. Sometimes, however, extra fields must be added to effectively treat lymph nodes that are at risk of disease.
A lot goes into treatment planning, and beam arrangement is just one part. In the next installments, I will discuss how blocking the beam affects the dose distribution and we will move from there into more advanced techniques.
