Radiation therapy planning based on 3D CT has revolutionized radiation oncology, and U.S. researchers believe they've found another benefit: the elimination of retrograde urethrography when planning treatment for prostate cancer.
Retrograde urethrography has traditionally been used in planning for radiation therapy treatment of prostate cancer to define the location of the prostatic apex. Careful localization of this anatomical structure helps radiation oncologists define the radiation field more accurately and avoid irradiating healthy tissue.
However, retrograde urethrography involves threading a catheter into the urethra for the release of contrast, which is invasive and uncomfortable for patients and involves morbidities, according to a research team led by Dr. Melisa Boersma from the University of Texas Health Science Center in San Antonio. The researchers sought to determine if 3D CT treatment planning could obviate the need for the technique, and their study was published in the May 1 issue of the International Journal of Radiation Oncology, Biology, Physics (Vol. 71:1, pp. 51-57).
Boersma et al started with a population of 15 patients who underwent two sequential CT planning studies: first, CT without contrast, followed by retrograde urethrography with contrast. A radiation oncologist then used a CT-based 3D treatment simulator (AcQSim, Philips Healthcare, Andover, MA) to draw contours on both scans, defining important landmarks such as the location of the prostatic base and apex and the "beak" of contrast material on the urethrograms.
On contrast scans, the prostatic base was defined as the point where the prostate abutted the contrast-outlined bladder, whereas on noncontrast CT, the interface was defined by the differential between solid tissue and urine, the authors wrote. They compared anteroposterior (AP) and lateral views from both CT scans with the contours marked on digitally reconstructed images.
For a common point of reference between the two scans, they drew a horizontal line on the AP films at the most inferior aspect of the ischial tuberosities, which are often used to define the lower border of radiation beam fields for treatment. They then measured the distance from the reference line to marked contours of the prostatic base and apex for both contrast and noncontrast studies. This enabled them to compare the relative accuracy of both techniques in defining the prostate.
Boersma et al found that measurements of the prostatic apex made from either of the two scans varied by a mean of 3.8 mm (range 1.3-8 mm); likewise, measurements of the prostatic base also varied by a mean of 3.8 mm. They pointed out that this degree of variation was within the limits of radiation treatment planning, which allows for a 1.5- to 2-cm margin when setting the inferior field edge of the area to be irradiated.
"It would appear that the prostatic apex would have been included in the treatment portals, regardless of which scan had been used," the authors wrote.
To further confirm their measurements of the location of the prostatic apex, the authors compared their data to a group of 57 patients who received brachytherapy implants under ultrasound guidance. Their study of this group further confirmed the ability of imaging to correctly delineate anatomical structures in the prostate, they wrote.
In conclusion, Boersma et al stated that their study confirmed that noncontrast CT treatment planning was accurate enough to eliminate the need for retrograde urethrography.
"With the use of modern 3D-CT planning systems capable of sagittal, coronal, and axial reconstructed imaging, the identification of the apex is relatively simple," they wrote. "Our results have confirmed that it is not necessary to put a patient through the trauma of urinary catheter placement for contrast delivery."
By Brian Casey
AuntMinnie.com staff writer
May 12, 2008
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