BERKELEY, CA (UroToday.com) - Prostate cancer is a very common cancer in the United States (for the year 2012 there will have been approximately 241 740 new cases and 28 170 deaths). One modality of treatment is external beam radiation therapy. External beam treatments may be delivered through 3D conformal plans, multi-field intensity-modulated radiation therapy (IMRT), or arc therapy. IMRT plans and arc plans are able to deliver the prescribed dose to the tumor volume while limiting the dose to the surrounding organs at risk. In the case of prostate patients, the organs at risk include the rectum, bladder, and femoral heads. Whenever possible, for daily treatment, our site utilizes the Calypso beacons as a way to localize the prostate. The beacons allow for a continuous reading of the current position of the ever-moving prostate during treatment.
When a patient has bilateral hip prostheses, modifications to the planning process are necessary. For example, the use of Calypso® beacons is contraindicated in patients with hip prostheses. Furthermore, it is difficult to delineate the prostate and organs at risk between the prostheses due to streak artifacts. Also, the American Association of Physicists in Medicine Task Group 63 recommends not using beams that would enter through the prostheses, along with contouring the streak artifacts, and assigning them an appropriate density to allow for more accurate dose calculation.
Two prostate cancer patients with bilateral hip prostheses presented to our center for treatment. For each patient, 6 gold fiducial markers were implanted along the posterior boundary of the prostate to aid in the prostate/rectum delineation. A pelvic CT scan was performed, followed by a TomoTherapy® mega-volt CT (MVCT) scan. The MVCT had less streak artifacts, so the prostate was contoured on that scan and pasted onto the CT scan for reference. The contours were finished on the CT scan including contouring all the streak artifacts and the prostheses with a 5 mm margin.
The CT images were sent to the TomoTherapy® software for treatment planning. The streak artifacts were given the density of water, and the prostheses + 5 mm structures were given a directional block. The block prevents any beams from entering through the prostheses, which would cause uncertainty in the dose calculation. Beams may exit through the structures, however. One patient was prescribed 78 Gy in 39 fractions to the prostate target volume. For this patient the dose volume histogram showed 15% of the rectum received 65 Gy and 11% of the bladder received 70 Gy. The other patient’s prescription was for 44 Gy in 22 fractions to the pelvis (lymph nodes, seminal vesicles, and prostate) followed by a boost to the prostate tumor volume of 36 Gy in 18 fractions (total dose of 80 Gy). This patient’s plan sum showed 16.4% of the rectum received 65 Gy and 10% of the bladder received 70 Gy. At 2-year follow-up, both patients are without evidence of disease (PSA < 0.5).
Treatment planning for patients with bilateral hip prostheses is challenging. The delineation of organs is difficult due to the streak artifacts. These artifacts need to be accounted for in the dose calculation, and the beams should not enter through the prostheses. Most of the radiation is already prevented from entering through the anterior and posterior directions to reduce dose to the bladder and rectum. Not allowing the radiation to enter laterally creates few angles for dose to be delivered, which tends to create hot spots or dose streaking.
We had attempted to create 7-field IMRT plans for these patients, but the results were unacceptable. Dose streaking extended out to the body contour due to the avoidance of having any beams enter through the hip prostheses. We also attempted arc plans with avoidance sectors around the prostheses, but results showed vast dose streaking from anterior to posterior. The TomoTherapy® directional block conformed to the prostheses better, which allowed more beams to contribute dose, giving a more conformal plan without dose streaking. The planning process also required less time to optimize compared to the IMRT or arc plans that required a lot of fine tuning in an attempt to reduce the significant hot spots/cold spots/streaking. The TomoTherapy® plan was able to deliver 100% of the prescription to 95% of the tumor volumes while keeping the maximum dose under 110% and showed good sparing of the bladder and rectum. Planning and treatment for patients with bilateral hip prostheses may be easier to perform using TomoTherapy® versus other methods.
Written by:
Justin Kling, DC, CMDa and Kamal M. Patel, MDb as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations etc... of their research by referencing the published abstract.
Midwestern Regional Medical Center
Radiation Oncology
2520 Elisha Avenue
Zion, IL 60099 USA
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