This is a technical subject that may be of interest to some people. Treatment planing is the process of designing a computer plan that directs the treatment machine to use certain size radiation beams, aimed from certain angles, each beam giving a portion of the dose, with the goal of treating the prostate gland with a specified dose of radiation and reducing the dose to surrounding healthy organs like rectum and bladder.
Mapping is also known as CT-Simulation. Typically a cushion (Vac-Lok) is used which molds to your body and helps keep you still during there treatment. We then do a plain CT scan through the pelvic area. Marks are placed on your skin. During the radiation treatments you will be placed in the exact same position that you were in for the CT-simulation. Some people ask why we can’t just use an old CT scan, it is because we have to do a CT scan with you in the exact same position that you will be in during your daily radiation treatments!
We will usually ask that you have a comfortably full bladder for the simulation and for every radiation treatment.
Some doctors ask that you use an enema before the simulation and before every radiation treatment.
We may do a second CT scan with a urine catheter placed into your bladder so that we can see where the urethra (urine passage) is.
On the same day as the CT-simulation we will usually have you undergo an MRI scan of the prostate. The MRI shows the prostate gland better than the CT does, and it can sometimes show the tumor(s) within the prostate. Later on we will merge the MRI scan with the CT-simulation scan, which is called fusion. If you have had a separate CT with a urine catheter inserted we will also merge in that scan.
Computer Treatment Planning
Treatment planning is where we create the computer treatment plan, i.e determine how many radiation beams will be used, from which angles they will enter your body, and what size and dose each beam is. The computer plan will then control the CyberKnife machine during your treatment sessions. Here are the steps we do:
Step 1) Your CT-simulation and MRI scan are fused together so that they are exactly superimposed.
Step 2) The radiation oncologist draws his target. This is called “contouring”. On each CT / MRI slice, he draws a line around the target he wants treated. This would normally be a line around prostate gland. The normal organs like the urethra, bladder, and rectum are also contoured. If there is an area of the prostate that needs extra dose (called a boost) this is also drawn. Finally, a small safety margin of less than a quarter inch is added around the prostate. This additional margin is also treated with radiation. We add a margin in case any cancer cells have escaped outside the prostate gland, or in case there is any type of targeting uncertainty.
In this picture, the inner red line is the prostate gland, the outer red line in the safety margin. The orange zone inside the prostate is getting extra dose (the boost). Yellow is the bladder, and blue is the rectum. The white region between the prostate and the rectum is the SpaceOAR, pushing the two apart!
Step 3) The radiation oncologist prescribes a radiation dose to the prostate, and specifies how much dose the surrounding healthy organs can safely tolerate.
Step 4) The dosimetrist creates a computer plan.
Everything within the orange line is getting the prescribed dose (7.25 Gy x 5 days = 36.25 Gy). Everything inside the red line is getting an extra 10% dose. By the time you get out to the yellow line, the dose has dropped off to 50% of the dose. The light blue area in the urethra, and the dark blue area is the rectum. This is a CT-simulation scan. Notice how it looks different from the MRI in the image above.
Step 5) The radiation oncologist checks the plan to make sure it will accomplish what he wants. If there is a problem, then he sends it back to the dosimetrist with suggestions on how to make it better.
Step 6) The approved radiation plan is sent to the PhD physicist (a doctor of physics). He makes sure that the treatment plan will run properly on the CyberKnife machine and will deliver exactly the right dosage to exactly to right location.
Homogenous versus Heterogenous Dosing
Homogenous dosing means that you try to treat the entire prostate gland very evenly, all to the same dose, with as little dose variance as possible. This is the typical way that stereotactic radiation is planned for prostate cancer treatments.
However, prostate tumors tend to lie around the outer edges of the prostate gland in the peripheral zone, sometimes the cancer is only on one side of the gland, and sometimes we know quite confidently where the tumor is in the prostate gland. Surely we should give extra radiation dose to these areas when it makes sense to. In addition, the urethra (urine passage) travels through the center of the prostate so it makes sense to give the urethra less dose if possible.
Intentionally giving different radiation dosages to different regions of the prostate gland is called heterogenous dosing. This is what I prefer to do. It does require a very precise treatment machine like CyberKnife and extra computer planning time on my part and also for the dosimetrist. The way we place the dose in the prostate with heterogenous dosing also makes the plan look closer to what we can achieve with high dose rate (HDR) brachytherapy. Below is a dose map achieved with brachytherapy by placing 25 catheters into a prostate gland, leaving them in place for 24 hours, and giving a series of radiation treatments by having a radioactive source go into each catheter. We think CyberKnife is a kinder gentler alternative to HDR.