International Journal of Radiation Oncology*Biology*Physics
Clinical investigationMarker seed migration in prostate localization
Introduction
Organ motion during the course of external beam radiation treatment for prostate cancer has been the subject of intense study for many years. More recently, with increasing interest in dose escalation and conformal treatments, organ movement, and hence target localization has taken on new importance. Several investigators have reported their experience with implanted radiopaque prostate seeds as fiducial markers for gland location 1, 2, 3, 4, 5, 6, 7, 8, 9. Although this technique seems to hold promise, fundamental questions about its validity remain unaddressed. For example, to what extent is there intraseed movement and how is that movement, if any, related temporally to treatment? In an attempt to answer these questions, we conducted a pilot study to quantify and characterize the movement of marker seeds placed within the prostate during a course of radiation therapy.
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Patients
After obtaining full informed written consent, 9 patients with histologically confirmed adenocarcinoma of the prostate without radiographic evidence of metastatic spread and a Karnofsky Performance Status ≥70 were enrolled in an institutionally reviewed protocol between May 2001 and December 2001. All patients were candidates for definitive local radiation therapy. Table 1 shows the patients’ characteristics. Ineligible patients either had an allergy to the gold marker material, prior pelvic
Results
Table 2 is a representative sample of the raw data coordinates from patient D for Week 2 of treatment. From these coordinates, the distance between marker seeds was calculated for each orthogonal pair weekly. Table 3 is an example of the data from Week 2 of treatment of the distance between marker seeds in Patient D. Next, the average change in distance between the marker seeds relative to the simulation radiograph was computed for all seeds in each patient. Table 4 is an example of the
Discussion
Numerous investigators have established the parameters of prostate organ motion during radiotherapy 10, 11, 12. Factors such as bladder and rectal filling, retrograde urethrograph, and even respiratory movement (5) have been implicated as causal agents in the uncertainty of gland location relative to bony anatomy. Because bony landmarks of the pelvis are the customary reference used in prostate radiotherapy for patient setup and target localization, the search for an alternative technique of
Conclusions
When analyzed as an aggregate, there appears to be little marker seed movement, on the order of 1 mm, provided the initial implantation successfully fixes the marker seed. Individual marker seeds, however, can have small, detectable movements throughout the duration of radiotherapy treatment of approximately 2 mm. These small movements may be due to topographic changes in the gland secondary to seed placement, anatomic changes in bladder and rectum, or treatment itself. Further investigation
Acknowledgements
The opinions contained above are those of the author and do not reflect the opinions of the Department of the Army or Department of Defense. The Walter Reed Clinical Investigation Committee and the Human Use Committee approved the research. All subjects enrolled into the study voluntarily agreed to participate and gave written informed consent. Funding for the study was provided by the Department of Clinical Investigation, under Work Unit #01-46001.
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