After the CT simulation is done, we create a computer plan. This is like a computer program that controls the treatment machine, adjusting the size of the radiation beams, from which angles they enter enter your body, how intense they are, and how long the beams are turned on for. This program is designed for you alone. Each time you will have a radiation treatment, your own individualized computer plan will be run.
Making the plan requires a few steps, and a lot of quality assurance.
Step 1: the radiation oncologist draws / contours
I sit down at the computer and looks at the simulation CT scan. A CT scan is like a set of pictures through the pelvis, each image is a slice about ⅛ inch (3mm) thick. When you stack these images on top of each other, you get a 3-dimensional picture of the pelvis. If there are any other scans available, like an MRI scan, they are also loaded into the computer and blended with the CT scan. Next, on every CT slice, I will draw an outline around the prostate gland and also anywhere tumor may be breaking through the prostate gland lining. I will then add an extra “safety margin” which is about an extra ¼ inch (5 – 6mm) around what I have already drawn, to help compensate for any patient wiggling, any cancer cells that may have broken through the prostate gland lining, any daily change in prostate gland shape, and any day-to-day variability in how the therapist gets you lined up on on the treatment table.
Step 2: the dosimetrist draws more and makes the treatment program
The dosimetrist checks my drawings and my dose prescription. He outlines all the normal organs in the pelvis, for example the skin, bladder, rectum, base of the penis, and hip bones.
Now after we’ve done all this outlining the planning computer knows where the prostate is, where all your normal organs are, what radiation dose we want the prostate gland to receive, and what the maximum dosages are that the healthy organs can tolerate. With some further guidance and wizardry from the dosimetrist, a computer plan is created that meets all these goals.
Step 3: the radiation oncologist checks the dose coverage
The radiation oncologist looks at the proposed arrangement of beams. He looks at a report showing how much radiation the tumor and each organ will receive. If he likes it, he approves the plan. If he doesn’t like it, he collaborates with the dosimetrist on how they can make the plan better or safer.
Step 4: the physicist does a dummy run on a phantom
Once the plan is approved by the radiation oncologist, it goes to the physicist for quality assurance, QA. The physicist is the one who makes sure that what we see on the computer plan is exactly what will happen in your body!! Obviously a vital step. The physicist will run the treatment program on a phantom, a piece of plastic shaped kind of like a body, but with a whole series of radiation detectors inside. He makes sure that the dummy is receiving the proper dose to the proper area, and all within a very tight margin of error.
This whole series of steps can take 1 week or so. These steps occur in the background, and are not seen by the patient. Obviously they are very important step! Once the plan is approved by the physicist you are almost ready to start treatment! Just a “virtual sim” to perform first.
Q) How can I be sure that the radiation is actually hitting my prostate gland, and it’s the proper dose?
A) Because of all the quality assurance steps and double checks that go into making the treatment computer plan. This includes having your treatment performed on a dummy phantom that has internal radiation sensors. In addition, the radiation machine has sensors that confirm the proper amount of radiation is being given.
Q) Do you treat the whole prostate gland?
A) Yes. There may be areas of prostate cancer in other parts of the prostate gland that we are unaware of, so it is routine to treat the entire prostate gland.
Q) Do the radiation beams pass through the body to get to the prostate?
A) Yes. We can control what direction the beams enter the body from, to avoid having beams pass directly through the bladder and rectum. Also, many small beams are used, and they come from different directions and all intersect on the prostate gland. One part of your pelvis may have only 1 beam passing through it and it will not be affected much, but the prostate gland will have 7 or more beams criss-crossing through it and will get the full dose.