British Journal of Radiology (2003) 76, 913-915
© 2003 British Institute of Radiology
doi: 10.1259/bjr/14541259
Seed migration in prostate brachytherapy: a re-implant case report
N Di Muzio, MD
1
B Longobardi, PhD
2
A Losa, MD
3
P Mangili, PhD
2
L D Nava, MD
3
P Rigatti, MD
4
R Calandrino, PhD
2
E Villa, MD
1 and
G Guazzoni, MD
3
IRCCS S. Raffaele, Departments of 1 Radiochemotherapy, 2 Medical Physics and 4 Urology, Via Olgettina 60-20132 Milano and 3 Casa di Cura Ville Turro, Department of Urology, Via Stamira d'Ancona 20-20127 Milano, Italy
Correspondence: Dr P Mangili
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Abstract
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Seed embolisation to the lung is a possible risk following permanent prostate brachytherapy. The purpose of this work is to analyse a seed migration case and to suggest methods to reduce such occurrences. With this aim, the clinical history of the patient who experienced seed migration, the implant technique and the pre- and post-plan procedures have been investigated. The massive seed migration has been detected in the patient by means of a pelvic X-ray and a CT-scan of the thorax. The use of loose seeds, the implant technique and the presence of unfavourable anatomical characteristics, have been recognised as possible causes of this event. The use of linked seeds embedded in vicryl sutures for the peripheral portions of prostate, and the development of an implant technique based on both transverse and longitudinal ultrasound guidance are proposed in order to reduce seed migration.
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Introduction
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Several cases of seed embolisation after prostate brachytherapy (BT) have been published [1–6] since 1991. The exact mechanism of this event is unclear, although it might be due to migration through the periprostatic veins to distant sites. The incidence of seed loss published by a few reports ranges from 1% to 30% of patients undergoing prostate BT. In these cases the reported mean number of sources lost from the implanted region is less than two seeds per patient. Seed migration is very likely to occur within the first month after implant, probably on the day after BT [3]. The use of strand seems to limit the percentage of seed embolisation to only 0.7% of the patients [6]. We report the massive seed migration experienced by a young patient due to both the anatomical shape of the gland and the implant technique. The resulting under-dosage in a well-defined area led us to subject the patient to a second implant.
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Materials and methods
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The patient was 54 years old, with T1c prostate carcinoma and a prostate specific antigen (PSA) value of 5.6 ng ml-1. Multiple biopsies performed in the peripheral, transitional and central zones (technique previously described [7]) revealed the presence of a Gleason 3+3 grade neoplasm in the peripheral zone (right lobe, 1° sextant and left lobe, 6° sextant) and in the transitional zone, bilaterally. The uroflowmetry showed a maximum flow of 20 ml s-1 and no post-voiding residual volume; the International Prostate Symptoms Score (IPSS) score was 4. The patient refused both radical prostatectomy and external beam radiotherapy. In February 2000, he underwent 103Pd ultrasound (US)-guided transperineal interstitial permanent prostate BT as monotherapy with a prescribed minimum prostate dose of 135 Gy.
On the day of the implant, the patient underwent a transrectal US prostate evaluation after urethral catheterization. Transverse images of the prostate were recorded every 0.5 cm from the base to the apex. The definition of the planning target volume (PTV) entailed the enlargement of the prostate US volume to 3 mm in all directions except the posterior limit (where the gland is close to the anterior rectal wall). The rationale for additional margins is related to the possibility of clinically occult extraprostatic spread of carcinoma. Davis and colleagues [8] found that low-risk patients had a radial extraprostatic cancer extension distance of less than 3 mm and would therefore receive an adequate radiation dose coverage with BT alone [9]. To achieve this goal, it is necessary to implant portions of periprostatic tissue [10]. The treatment was planned with 109 103Pd loose seeds, in a modified peripheral loading scheme, placing about 35% of the total sources within the most peripheral region of the gland, mainly in the subcapsular area and in periprostatic tissue, achieving a PTV dose coverage of 98%.
The implant was performed under transverse US guidance; the needle longitudinal positions had been checked under fluoroscopy.
The day after BT, the patient underwent a pelvic X-ray showing only 95/109 seeds in the implanted region, which meant there had been a smaller dose within the PTV since the beginning of treatment.
3 weeks after the implant, the patient was imaged with MRI of the pelvis and a CT scan of the thorax: the CT showed the presence of several sources in the lungs, while the post-plan evaluation revealed a significant under-dosage in the anterior part of the transitional zone, bilaterally. Because of the unsatisfactory dosimetric results [11] (D90=110.7 Gy and V100%=77%, where D90 is defined as the minimum dose covering 90% of the prostate volume and V100% as the percent of the prostate receiving 100% of the prescribed dose), the well-defined cold area, where biopsies had revealed tumour, was re-implanted after 2 months with 29 seeds of 103Pd using 135 Gy as the reference dose for the re-treatment.
The re-implant planning was based on both the MRI results (to recognise cold spot) and the new US volume study performed on the day of the procedure, using the urethra (viewed by the insertion of the catheter) as a landmark for the 3D reconstruction. The re-implant seed distribution was previously studied on the MR images in order to cover only the cold area. On the day of the re-implant, the same area was identified on the new US volume study and the intraoperative planning was performed following the previously defined needle loading pattern. 3 weeks after the second implant, the patient was imaged to obtain the final post-plan. As it was nearly impossible to distinguish between sources administered in the first implant and those in the second one, the quality of the treatment was evaluated based on the global dose coverage as if it had been obtained in a single procedure. The MRI evaluation showed a global prostate dose coverage (V100%) of 98.4% (D90=206.3 Gy; V150%=90.8%) (Figure 1
), and a Vu 150% for the urethra equal to 65% (Du1=358.2 Gy, where Du1 is the minimum dose covering the 1% of the urethra volume). In order to split the contribution of the two treatments in the re-treated area, the doses delivered in the first brachytherapy session were subtracted, point by point (2 mm apart), from the final dosimetry, obtaining consistent values for the re-implant dosimetry. To date the patient has no biochemical evidence of disease or acute pulmonary symptoms and had early side effects comparable with those experienced by non-retreated patients. The follow-up (FU) is too short to confirm either the clinical outcome of the treatment or possible late effects associated with lung tissue irradiation.
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Figure 1. MRI-imaged axial slice of the prostate gland. (a) Post-implant dosimetry reveals a cold area in the transitional, subcapsular zone; (b) post-implant dosimetry after the re-implant confirms the optimal dosimetric coverage of the anterior part.
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Results and discussion
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The two possible consequences of vascular seed migration are inadequate dose coverage of the prostate and potential adverse effects for tissue irradiation at the site of embolisation. There have been no reports of such a large number of seeds lost from the prostatic region (14 seeds — 13% of total). The second transrectal US volume study showed the presence of an anterior enlarged periprostatic vein plexus and a longitudinally-shaped gland with a sharp reduction of height from the mid-prostate to the apex. Both these anatomical characteristics, together with the use of loose seeds and the implant technique, could explain the massive seed migration in our patient. Using fluoroscopy to control the longitudinal needle position with respect to the 0.0 plane, it can be difficult to check if and where, in the sagittal plane, the seeds are misplaced. It is probable that, during the first implant, the seeds were released too close to or possibly inside the venous plexus, owing to either an incorrect definition of the 0.0 plane or a longitudinal seed dragging (Figure 2). To overcome this kind of problem, we have developed an implant technique based on both transverse and longitudinal US guidance. The needle insertion and the seed releasing are continually monitored by means of a longitudinal US scan, which allows the operator to check on-line the position of the seeds with respect to anatomical structures, reducing errors during the implant.
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Figure 2. Sagittal, longitudinal section of a "pear-shaped" prostate, with needles in the correct positions to release seeds in the pre-planned anterior subcapsular zone. When a longitudinal ultrasound "real-time" control is not performed, seeds could be misplaced in the venous plexus due to: 1) 0.0 plane definition at fluoroscopy or transverse ultrasound control can be doubtful; 2) too rapid retraction of the needles can drag seeds.
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As a number of studies suggest a correlation between dosimetry parameters and biochemical control [12–14], our philosophy is to perform a re-implant in patients that present an inadequate dose coverage in a well-defined cold area. To date, three of our patients (3/134) have undergone re-implants. For the case under discussion the decision was to re-implant in the localized transitional zone cold area where cancer was detected by biopsies. To the best of our knowledge there are no series published about re-implantation or indications when and how to perform it. We considered the cold area as not treated; this area became the target of the second implant in which we planned to deliver the same prescribed dose as in a primary therapy. The physical global dosimetry showed an acceptable dose coverage of the entire gland and the point by point analysis revealed that more than 95% of the points received a dose higher than 135 Gy. To date, the side effects experienced by the patient in the early FU were comparable with the other non re-implanted patients. The low value of PSA (0.2 ng ml-1) after 24 months supports the choice of treatment. The re-implant in this patient improved an unsatisfactory treatment with regard to dose coverage of the prostate gland without an apparent increase in early side effects. A longer FU is required to draw final conclusions.
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Conclusions
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Seed migration can affect the dosimetrical outcome of a prostate BT implant with modified peripheral loading pattern of the sources. The use of linked seeds embedded in vicryl sutures (strands) for the peripheral portions of the implant, and the use of a technique that allows on-line US monitoring of the source release could reduce radioactive seed implant migration.
Although several post-implant analyses have been published suggesting a correlation between dosimetry parameters and biochemical outcome, the management of a case of inadequate implant is still an open issue. We decided to perform re-implant on this patient because the underdosage and the disease (as revealed by histopathological exam) were located in the same area.
Our experience revealed that in selected cases with a well-defined cold area, the re-implant is technically feasible, safe and permits an acceptable dose coverage of the entire target to be achieved. The short FU (24 months) does not allow any conclusions to be drawn about the final oncological outcome. The patient had a pre-operative PSA value of 5.6 ng ml-1 and now his level of PSA is 0.2 ng ml-1. This trend led us to think that the treatment has had a satisfactory therapeutic effect.
Received for publication July 23, 2002.
Revision received October 29, 2002.
Accepted for publication March 10, 2003.
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References
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Abstract
Introduction
