IMRT for Prostate Cancer
We use IMRT to treat many types of cancer, although this page will only focus on its use for prostate cancer. We have two treatment machines that provide IMRT: a Tomotherapy unit, and a Varian linear accelerator equipped with a multileaf collimator.
Radiation therapy and surgery are the two most popular treatments for prostate cancer. Over the years, radiation has evolved and has become more precise. The higher precision beams focused on the prostate gland have reduced side effects and allowed higher dosages to be used, with the hope for higher cure.
The competition between radiation, brachytherapy, and surgery involves a simple goal: men want a treatment that has a high chance of curing the cancer, but a low chance of side effects. Treatment can cause impotence, incontinence, or rectal damage, and that can be a very high price to pay for treating a cancer that often causes no symptoms and that men tend to die with rather than die from. Unfortunately, there is currently no magic bullet treatment that can cure the cancer without causing a risk of side effects. Hope has come though, to make external radiation treatments better.
How Does IMRT Work?
IMRT represents an evolution of radiation technology, from standard to 3D to IMRT. The evolution in technology offers the possibility of better cure rates with less side effects.
- Standard radiation involved starting with plain xrays of the pelvis. Lines were hand drawn on each xray film to make “radiation fields”. Lead blocks were then created which matched the hand drawings. Usually, four radiation beams were used, entering the body from the front, back, and both sides.
- 3D-conformal radiation involved starting with a CT scan. The prostate, rectum, and bladder were circled on a computer screen which showed the CT images. Any number of radiation beams could be used, and the computer shaped the beams to precisely match the contour of the prostate. Beams could be angled so that they missed most of the bladder and rectum, but passed through the prostate.
- IMRT is even more computer intensive than 3D. Every beam is broken down into tiny “beamlets”, and each beamlet can be given a different dose. This results in beams with different intensities across their surfaces. Often we’ll use as many as 36 beams for a treatment. Although the beams are all different in shape and intensity profiles, once they all converge on the prostate you are left with a nice high dose covering the prostate gland, and a lower dose hitting the rectum and bladder.
Do we really need this technology to treat prostate cancer?
Yes. Studies consistently show that the higher the radiation dose is, the better the chance of curing prostate cancer. A radiation dose of 66 Gray (Gy) will result in a 65% chance of being cancer-free at five years, but a dose of 80 Gy may result in a 90% chance, for early stage cancer. The possibility of rectal damage has prevented radiation oncologists from using high dosages of radiation in the past. Now, with IMRT, giving 80 Gy has become safer and more common. Dose is the most important variable in curing a given stage of cancer.
Drawbacks of IMRT
With IMRT, a whole different set of skills is needed for the physician, dosimetrist, and physicist. Expertise in this new technology is important to improve cure rates and reduce side effects. With the ability to pinpoint comes the need to know precisely what you want to aim at.
A technical issue. Because so many different “beamlets” converge on the prostate, there will be some areas where the dosage is higher or lower in the prostate, instead of being nice and even.
It can take as long as 30 minutes to give an IMRT treatment, versus 5 – 10 minutes for standard radiation. This is because there tend to be more beams which are also smaller and have a lower intensity than with regular radiation.
Cost and Resources
A radiation oncology department may need to invest millions to get a new IMRT machine, planning software, and increased personnel. It takes more manpower to design a radiation plan and to deliver the treatment.
What good is “pinpoint” radiation if what you’re aiming at moves around? The prostate can move up to 1/2 inch in the body in all directions. With greater accuracy comes the need to ensure that the target has not moved. We use a daily ultrasound (the BAT) to establish the prostate’s location. Other techniques are tomotherapy or inserting a rectal balloon.
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What’s the Best Radiation Dose?
We’re seeing that 81 Gy with IMRT is pretty effective and has a fairly low side effect rate for treating prostate cancer. Usually, that dose is given at a slow rate of 1.8 Gy daily over 45 weekdays. That is nine weeks! Or nine and a half weeks including the simulation and treatment design time. Radiation becomes not just a treatment, but a lifestyle over that length of time.
Recent studies show that prostate cancer has a low alpha-beta ratio, meaning that the cancer is very responsive to larger daily treatment dosages. At CTCA, we use a dose of 2.25 Gy daily over 33 days for a total of 74.3 Gy. Because of the increased effectiveness of these higher daily doses, it is actually equal to 81 Gy if it was given in 1.8 Gy daily treatments. (This calculation assumes a conservative alpha-beta ratio of 3.1.) Please see the effective dose spreadsheet for these equivalences.
As time passes, and we and others are assured that this dosage is very safe, we will gradually increase the dosage. For now, for cases where a higher dosage is desired it is probably more prudent to add some brachytherapy. Brachytherapy, which is a permanent or temporary radioactive seed implant, allows extra radiation to be given right inside the prostate gland. I combine IMRT (2.25 Gy x 20 days) plus high dose rate brachytherapy (6.5 Gy x 3 doses over 24 hours) for a dose that is equal to 87 Gy if it was given in 1.8 Gy daily treatments. Patients with larger, higher Gleason score tumors may especially benefit from these further dose increases.
With very exact treatments like IMRT, the therapists must make sure that the patient’s body is set up in the treatment machine the exact same way each day. There are marks on the outside of the body to help line up laser sights, to ensure that the pelvis in centered in the radiation field. But what happens if the prostate moves inside the body? Because it does. The prostate lies on top of the pelvic floor muscles, in front of the rectum, and underneath the bladder. These are three anatomical structures than can change their positioning and the prostate will move up to 1/2 inch in various directions based on their whims. The ways to solve this problem are to either:
- Take the movement into account, and treat a bigger area. (= more side effects)
- Limit the movement of the prostate, for instance by placing a balloon into the rectum every day and inflating it, which pushes the prostate up against the pelvic bone. (= even more changes to your lifestyle over the 9 1/2 weeks)
- Setting up the treatment according to the prostate location instead of the body position. (= elegant)
The balloon is actually a nifty idea, but not one that I personally am anxious to use as a doctor or would be as a patient. Instead we use tomotherapy which takes a CT scan of the prostate area each day prior to treatment. The treatment table is adjusted so that the radiation treatment is aligned to the prostate gland.
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