Student Issue – Ultrasound for Prostate Localization

CT/MRI fusion planning

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References

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Expand

Prostate localization is a tricky business. The use of CT alone is not accurate enough for prostate treatment planning, and bony anatomy alone does not provide enough information to ensure the best quality treatment for the patient. An MRI may enhance soft tissue, but it is not as effective. However, when the two techniques are "fused," they show a more accurate picture of the patient's internal anatomy because the images are superimposed on each other. With a CT/MRI fused image and daily ultrasound, a patient could receive lower radiation doses to normal tissue and higher doses to the tumor. The patient would also benefit from reduced exposure of weekly port films.

Though CT and MRI have been used for target volume calculations separately, fusion produced by the two is more accurate. The combination of ultrasound guidance with the fused image improves accuracy of prostate localization.

Breakdown

The use of ultrasound for localization was first used in the medical field to detect brain tumors. Ultrasound has made great strides in technology, advancing from the use of sound wave generators, cathode tubes and Polaroid/35mm film photography to the introduction of the microchip. The advancement in ultrasonography is built on the work of Hiroki Watanabe, William Boyce, Brian Peeling and Hans Henrik Holm, who conducted early experiments with ultrasound probes and recording devices.

CT imaging has been used for target volume calculations and allows the oncologist and the dosimetrist the ability to design target volumes by creating contours. The use of bony anatomy alone for localization does not account for the daily movement of the prostate. It was once thought conceptually that the prostate did not move. In actuality, the prostate moves by a few millimeters - a great deal of difference when the goal of therapy is to deliver a tumorcidal, while sparing normal tissue. A target volume is designed with margins that allow for this movement, using portal verification films of bony anatomy to localize the prostate. These volumes then must take into account the normal tissue that will be included in the field, which may include the bladder and the rectum adjacent to the prostate.

The use of MRI can provide good differential of soft tissue. With CT/MRI fusion, physicians and dosimetrists can provide a more accurate depiction of the prostate gland and the surrounding normal tissue dimensions, thereby creating a better target volume. The fused image is created when a CT image is superimposed with an MR image. The fused image provides a good representation of corresponding bony and soft tissues.

Analysis

A CT image illustrates the upper prostate and seminal vesicles, but the lower extent of the gland is difficult to distinguish from adjacent normal structures. The urogenital diaphragm, for example, is difficult to visualize on a CT. A CT scan provides more accurate geometric and physical information of the anatomy and is more applicable for complex bone/air heterogeneity. The latter is essential for dose calculations and designing compensators.

In comparison, an MRI has better tissue contrast and higher sensitivity to a disease process that alters tissue contrast. An MRI can also acquire images in any plane and generally carries no health risks.

When the information gathered from each individual modality is fused, there is a higher degree of detail provided to the physician and the dosimetrist when calculating dose and determining treatment plans. With the addition of a daily ultrasound for localization, there is improved quality of treatment. As the ultrasound is performed, the fused CT data displays each CT slice and then correlates with each ultrasound slice. The information creates a real-time image between the two. The information can then be used to verify that the ultrasound localization contours match the contours of the CT.

There are advantages and disadvantages with the daily use of ultrasound for prostate localization, one advantage being reduction of X-rays from weekly portal verification films. The prostate is localized on a daily basis with ultrasound, therefore reducing or eliminating the need for port films. The dosimetrist can plan an increased dose, an optimal isodose distribution to the tumor and reduce normal tissue exposure.

A disadvantage to using ultrasound is the inability to get a good image on large or obese male patients. With a large patient, the image produced is generally not clear, making the bladder, prostate and rectum hard to see. There may also be distortion when trying to locate the interface lines of each of these organs. The therapist usually resorts to taking a portal verification film.

Another disadvantage to the technique is patient positioning. An error that occurs between patient positioning for MRI and positioning for CT could cause miscalculations in the data acquired from the two diagnostic images. Also, concerns surround cost and patient comfort. The average cost of a CT is approximately $110 to $800, while MRI images range from $700 to $1,000. Claustrophobia may also be a major deterrent, as the length of time needed to complete the procedure is greater than the time necessary for just an MRI scan.

Ultrasound-directed localization for prostate positioning is as safe and as accurate as CT scanning of the prostate for conformal external beam therapy. Combining ultrasound with CT/MRI fusion planning and the application of this technology to current techniques will allow the reduction of treatment margins in all directions. This should diminish treatment-related morbidity and facilitate higher doses with an end result of improved tumor control.

Patient setup verification on the basis of bony anatomy does not reflect actual prostate position. Ultrasound target alignment with fusion planning provides a real-time prostate localization system that may make it possible to measure prostate position variations and reduce field margins. Technological advancement in this system improves treatment for patients and enhances the ability to achieve the goal of therapy. The use of fusion planning with daily ultrasound localization reduces side effects to the bladder, prostate and rectum.

— Ronnie Orban is a radiation therapy student at Texas State University, San Marcos. Questions and comments can be directed to editorial@rt-image.com.