phoenix5 logo This is an archived copy of an original page from The Prostate Cancer InfoLink site that went off-line in February, 2001. It is reproduced at Phoenix5 with the permission of Vox Medica.
More Prostate Cancer Pages at Phoenix5             About this archive

prostate cancer info link
click here to go to Where To Begin? click here to go to Diagnosis men click here to go to Treatment menu click here to go to Support & Help menu click here to go to Home menu
The information which follows is the opinion of the named author(s).
It does not necessarily constitute the opinion of The Prostate Cancer InfoLink or of CoMed Communications, Inc.

Three-Dimensional Conformal Radiation Therapy (3DCRT) for Prostate Cancer

Part 1 of 2
[divided only to help load time - go to Part 2]

Jeff M. Michalski, MD, Carlos A. Perez, MD, and James A. Purdy, PhD
Radiation Oncology Center, Mallinckrodt Institute of Radiology,
Washington University Medical Center, St. Louis, Missouri

Originally Received July 6, 1996; Last Revised July 8, 1996

Introduction | What is 3DCRT? | How is 3DCRT planned?
The following are found in Part 2
How is 3DRCT delivered? | What are the side effects of radiation therapy? | Is 3DCRT better than conventional radiation therapy? | What studies are currently ongoing investigating 3DCRT for prostate cancer? | What will the future hold for 3DCRT and the treatment of prostate cancer? | References | Editorial comment


This article is arranged as a mini-"FAQ" -- a short list of answers to frequently asked questions for patients with prostate cancer and their families. It certainly is not an exhaustive scientific literature review. Comments and suggestions are welcome, and can be sent to the authors by using the e-mail address links above.

Radiation therapy has been used to treat prostate cancers for decades. It plays an important role in the curative management of patients with early stage disease and also for the palliation of symptoms from advanced or metastatic disease.

Technical developments have made incremental improvements in the delivery of high dose radiation therapy for the curative treatment of prostate cancer. In the 1970s, medical linear accelerators were designed and developed that could deliver high doses of radiation therapy to deep parts of the anatomy, while sparing the superficial tissues. Despite the high energy X-ray beams that spare superficial normal tissues, organs adjacent to the cancer often received large volumes of high dose radiation therapy. This large treatment volume was required to assure that the cancer was always treated. If too small radiation fields were used to avoid treating the normal tissues, there was a risk that the radiation would not adequately treat the cancer [1, 2]. Many radiation oncologists, and their patients, would accept the extra radiation dose to the normal adjacent organs in order to assure adequate coverage of the prostate target volume. This "extra" radiation is partly responsible for the side effects and complications that accompany high dose curative radiation therapy. In order to keep the complication rates at an acceptable level, most physicians would keep the total radiation dose to a moderate and safe range. There is evidence that higher radiation doses may cure more prostate cancers but this would be at too great a risk with conventionally fractionated and planned radiation therapy [3, 4].

Figure 1. A two-dimensional pelvic X-ray demonstrates the bony anatomy but not pelvic organs.

Over the past decade there has been a significant increase in the availability of new and powerful computers that are making better targeting of radiation therapy possible. In the past, physicians had to rely on low contrast, two-dimensional X-ray images to target radiation therapy for the treatment of prostate and other cancers. These X-rays films illustrated the bony pelvis but neither the prostate nor normal tissues. Now cross-sectional imaging modalities such as computed tomography (CT) or even magnetic resonance imaging (MRI) allow clinicians to see inside a patient's body. Using these high quality imaging techniques and powerful image workstation computers, doctors are now able to visualize radiation therapy targets and direct radiation therapy beams directly at the tumor (and tissues at risk of having cancer) while sparing adjacent normal critical structures.

Figure 2. The pelvic CT scan can demonstrate human anatomy in cross section. The organs of the pelvis can be readily identified on these CT slices.

What is 3D conformal radiation therapy?

The goal of three-dimensional conformal radiation therapy (3DCRT) is to have the prescribed radiation dose distribution be shaped like or "conform" to a target volume. In the past, radiation therapy plans were reviewed in a single plane. The distribution of radiation dose above and below the plane was assumed to be adequate. This assumption was valid if there was not significant change in the shape or contour of an organ or tumor outside of that single plane. Unfortunately, human beings are not regularly configured for simple radiation therapy techniques. Three-dimensional conformal radiation therapy allows a three-dimensional and volumetric appreciation of a target volume and normal tissues that are not dependent on arbitrary and regular geometric shapes.

A volumetric CT scan acquired with the patient in the position identical to the one he will be in for his radiation therapy treatments is used for 3DCRT planning. On each slice of a CT scan, normal and target volume contours are defined by a radiation oncologist or a medical dosimetry assistant. These contours are then reconstructed and displayed (rendered in 3D) on a computer video monitor. This 3D visualization allows the radiation oncologist to choose various radiation therapy beam arrangements to maximally cover the tumor with radiation and minimize exposure to the normal structures.

Once the beams are selected, the fields are shaped to conform to the shape of the target volume in that projection. This shaping can be done by the physician using computer drawing tools, like a computer mouse, or by automatic generation of a block contour by the computer. This process is repeated until a volumetric radiation dose distribution is conformed to the shape of the target volume. The dose distribution is then analyzed using graphical tools, such as dose volume histograms (DVHs), or 3D dose displays. DVHs are analytical tools that depict the volume of each organ or target tissue and the dose that it receives. They help the radiation oncologist know if the target volume is adequately covered and that the volume of normal tissues are not receiving excessive radiation doses. 3D dose displays allow a visual appreciation of how the radiation doses are spatially distributed.

The process of implementing 3DCRT is slightly different from standard radiation therapy. Patient positioning is critical, and for this reason immobilization devices are routinely used. These devices are aids to the radiation therapist for repositioning the patient on a daily basis for seven or more weeks of outpatient therapy. 3DCRT often employs multiple treatment fields -- as many as six or eight. Treating this many uniquely shaped fields is done most efficiently and accurately with a beam shaping device called a multi-leaf collimator (MLC). An MLC is made up of many small leaves (1 cm projection at the radiation therapy linear accelerator isocenter) that are driven by independent motors to create the desired shape of the radiation field. These devices avoid the use of the heavy lead alloy blocks that were used in the past. (Note: These blocks are still used today in most radiation therapy clinics and even in some clinics using 3DCRT. However, the use of lead alloy blocks for 3DCRT requires increased time with less efficiency in treatment delivery.)

How is 3D conformal radiation therapy planned?

After your radiation oncologist decides that it is appropriate for you to be treated with 3DCRT, he or she will schedule a CT scan for 3DCRT planning.

Step 1: Immobilization

As described above, patient positioning is a critical part of active 3DCRT. A device is made that will allow the radiation therapist to reposition you accurately on a day to day basis. These devices come in a variety of different forms and may be made of a foam that hardens as the result of a chemical reaction or from a thermally sensitive plastic that hardens as it cools to room temperature. Once the device is made, a treatment planning CT scan is acquired with the patient in his immobilization device.

Figure 3. A patient with prostate cancer lying in an immobilization device known as an alpha cradleTM. This device helps improve the accuracy with which radiation therapy treatments can be delivered.

Step 2: CT acquisition

Although physically similar to a diagnostic CT scan, the treatment planning CT is acquired with the patient in his treatment position. Sometimes the use of contrast media is necessary to help visualize the prostate. The CT scan is acquired with "slices" taken at 2-5 mm intervals. After the scan is acquired, the patient's skin and immobilization cast may be marked with ink to assist in subsequent treatment setup.

Figure 4. Patient is undergoing a CT scan for treatment planning.

Figure 5. A large number of CT slices are acquired for 3DCRT. This data will be used to develop the radiation therapy treatment plan.

Step 3: Target volume and normal tissue definition

This is a critical and time-consuming task for the radiation oncologist. He or she will identify normal and tumor-containing tissues on relevant slices of your treatment planning CT scan. A medical dosimetrist will often assist the doctor in defining some of the more obvious tissues, such as the skin and bones.

After the doctor defines the gross tumor (usually the whole prostate because of the risk of multi-focal disease) he/she will sometime add a safety margin to account for microscopic spread of disease. We know from surgical data that cancer cells can spread outside the prostate or to the seminal vesicles even when the tumor is felt to be confined to the gland. The safety margin for cancer spread (called the clinical target volume or CTV) helps the radiation oncologist be sure that the areas where cancer cells may have spread will not be underdosed. In some cases the seminal vesicles will not be treated if the calculated risk of cancer spread to this tissue is very low. If there is a high risk of extracapsular disease or seminal vesicle invasion because of a high PSA or Gleason score (or because there is definite extension of disease into the seminal vesicle), the seminal vesicles will be included in the clinical target volume.

Figure 6. On each slice of the treatment planning CT scan, the physician and his assistant, a medical dosimetrist, will identify the prostate and seminal vesicles as well as the normal tissues at risk of radiation injury, including the bladder, rectum, and femoral heads.

Another margin needs to be added to the clinical target volume to account for setup variation and internal organ motion. Because patients are living and breathing human beings, we cannot control their movements down to the exact millimeter. We don't want to risk being "too conformal" and miss treating the prostate gland or any cancer within it. The position of the prostate and seminal vesicles changes daily with the degree of bladder and rectum filling. Likewise, even with assistance of an immobilization device, there is still some variation with daily treatment setup. A margin of 5-10 mm is generally added to the CTV to accommodate for the variation in positioning of the target volume and patient. After the target volumes and normal tissues are defined and contoured on the CT scan, the next step is virtual simulation.


Where to Begin?    |    Diagnosis    |    Treatment    |    Support    |    Home Page

The content in this section of the Phoenix 5 site was originally developed by CoMed Communications (a Vox Medica company) as part of The Prostate Cancer InfoLink. It is reproduced here with the permission of Vox Medica.

Go to Phoenix5 Main Menu