Clinical investigation
Marker seed migration in prostate localization

Presented in part at the 44th Annual Meeting of the American Society of Therapeutic Radiation Oncology, 7 October 2002, New Orleans, LA.
https://doi.org/10.1016/S0360-3016(03)00328-6Get rights and content

Abstract

Purpose

Marker seed location was analyzed to test the hypothesis that there is no intraseed migration within the prostate, a premise fundamental to the technique of marker seed localization of this organ. Despite increasing interest in the use of implanted seeds as fiducial markers for gland location, there are few data available with which to evaluate the validity of this technique, particularly over the entire course of external beam radiation therapy.

Methods and materials

Between May 2001 and December 2001, after obtaining fully informed written consent, 9 patients with early stage prostate cancer were enrolled on an institutionally reviewed protocol. Patients had four to five marker seeds implanted into the prostate under transrectal ultrasound guidance before definitive radiotherapy. The porous gold seeds were each 1.2 × 2.0 mm in dimension. Seed locations from orthogonal radiographs based on the initial simulation and weekly orthogonal films were digitized using a CMS Focus planning system, thereby facilitating the determination of intraseed spacing. The digitization of the isocenter from each orthogonal pair of radiographs was used to determine digitizing error for seed localization. Pubic symphysis, bilateral femoral heads, and isocenter were also digitized and will be analyzed at a later date.

Results

Overall, the average migration of all the seeds in the patients was 1.2 ± 0.2 (SD) mm. The greatest average movement of any seed in any patient was 1.9 mm over the entire 7-week course of radiotherapy. The smallest average movement was 0.6 mm. The greatest change in intraseed spacing in any of the patients during the full course of therapy was 6.6 mm. One seed in 1 patient was lost at the start of the third week of therapy and censored from analysis. Digitizing error in seed localization was calculated to be 0.20 ± 0.03 (SD) mm.

Conclusions

As an aggregate, there is negligible seed migration within the prostate over the entire course of definitive radiotherapy. However, there are small, detectable movements in individual seed locations, perhaps resulting from topographic changes in the gland secondary to seed placement, anatomic changes in bladder and rectum, or treatment itself. With respect to seed migration, prostate marker seeds represent an accurate and reliable surrogate of gland location during a full course of radiotherapy.

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.

Section snippets

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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