Planar scintigraphy does not always detect migrated seeds after transperineal interstitial permanent prostate brachytherapy with 125I radioactive seeds
A Sugawara, E Kunieda, J Nakashima, H Nagata, H Asakura, M Oya, N Shigematsu
125i, brachytherapy, migration, prostate cancer, seed
A Sugawara, E Kunieda, J Nakashima, H Nagata, H Asakura, M Oya, N Shigematsu. Planar scintigraphy does not always detect migrated seeds after transperineal interstitial permanent prostate brachytherapy with 125I radioactive seeds. The Internet Journal of Urology. 2010 Volume 8 Number 2.
Seed migration after transperineal interstitial permanent prostate brachytherapy is sometimes observed, particularly when the implant is performed with loose seeds, and the most frequent site of seed migration is the lung [1-6]. An 125I seed measures 4.5 mm in length and 0.8 mm in diameter; its small size favors potential displacement mostly from the periprostatic insertion site to the adjacent prominent periglandular venous plexuses. Pulmonary seed migration is thought to occur by a mechanism in which seeds implanted or eroded into the periprostatic venous plexus migrate hematogeneously to the iliac veins, inferior vena cava, right heart, and finally the lungs . Seed migration has the potential to result in a reduction in radiation dose to the prostate and, moreover, unwanted seed-related sequelae, such as an acute myocardial infarction . A current standard technique for detecting migrated seeds is a postoperative conventional radiograph, in which a migrated seed is visualized as a radiopaque foreign body. However, a radiographic examination does not always detect the embolized seeds, especially when they have migrated to the lung base or to areas overlapped by bones. Furthermore, seeds in motion in the intracardiac region may not be visible on radiographs  .
Recently, a scintigraphic technique for detecting migrated seeds after transperineal interstitial permanent prostate brachytherapy with 125I was proposed [9, 10]. One supposed merit of the scintigraphic technique is its excellent sensitivity for detection of seed migration. Previous investigators have reported 100% sensitivity with scintigraphy, while conventional radiography showed a sensitivity of 35% . Moreover, scintigraphic detection is a cost-effective method that can monitor seed location with no additional irradiation.
However, few published data exist on the scintigraphic technique in the detection of migrated seeds, and it has not been fully validated in larger studies. One drawback of this technique is that it must be performed within a limited period of time after seed implantation; when the radioactivity of a migrated seed has decreased substantially, the scintigraphic technique has some difficulty detecting it. Other possible limitations come from the low photon energy of 125I; when a migrated seed is surrounded by dense tissue, such as bone, the scintigraphic technique has some difficulty detecting it because the bony tissue may attenuate mostly low-energy photons emitted by 125I. The purpose of this retrospective study was to demonstrate and discuss the benefits and limitations of the scintigraphic technique for detecting migrated seeds after transperineal interstitial permanent prostate brachytherapy with 125I seeds.
Materials and Methods
Ten consecutive patients underwent transperineal interstitial permanent prostate brachytherapy with 125I seeds (OncoSeed, model 6711; Amersham) with a Mick applicator (Mick Radio-Nuclear Instruments, Bronx, NY) at our institution in 2007, between February and the beginning of March. Four (40%) of the 10 patients exhibited seed migration on postimplant conventional radiographs (orthogonal chest radiographs, an abdominal radiograph, and a pelvic radiograph) at day 1 or day 18 and were informed of the seed migration. Three (75%) of the four patients who consented to participate in the study were examined by scintigraphy. The mean age of the three patients was 64 years (range, 61-66 years). All three patients had T1cN0M0 (AJCC TNM classification, 2002) adenocarcinoma of the prostate with a Gleason score of 6 (3+3). The mean prostate-specific antigen level was 5.61 ng/mL (range, 4.79 to 6.75 ng/mL). The mean preimplant prostate volume measured by transrectal ultrasound examination was 26.4 cc (range, 24.6 to 28.4 cc). The prescribed dose was 145 Gy. The 125I source strength was 12.07 MBq per source on the day of the operation. A total of 249 seeds were implanted in all three patients. The mean number of seeds implanted per patient was 83 (range, 79 to 90). Postimplant conventional radiographs showed that each of these patients had one migrated seed, for a total of three seeds (1.20%) that had migrated in all three patients. One experienced a seed migration to the pelvis (
Figure 1A and Figure 1E are reproduced from Figure 3 and Figure 4, respectively, of the below article with permission.
The present study shows that planar scintigraphy does not always detect migrated seeds. In Case 1, anterior scintigraphy of the pelvis could not detect a seed that had migrated to the right ischial bone, and axial and coronal SPECT images of the pelvis barely showed the seed, although in Case 2 and Case 3, anterior scintigraphy was able to detect seeds that had migrated to soft tissues successfully (the lung and the liver, respectively). The reason why, in Case 1, anterior scintigraphy could not detect the migrated seed is not clear. One possible explanation is that, in Case 1, 125I photons emitted from the migrated seed might have been largely attenuated by bone tissue. As a result, anterior scintigraphy could not detect the migrated seed, and axial and coronal SPECT images of the pelvis barely showed it. In contrast, in Case 2 and Case 3, 125I photons emitted from the migrated seeds may have been less attenuated by soft tissues (the lung and the liver, respectively) compared to bone tissue. Therefore, anterior scintigraphy was able to detect the migrated seeds successfully.
It has been reported that bone tissue attenuates 125I photons much more strongly than does soft tissue because of the dominance of the photoelectric effect . Previous investigators have confirmed a high attenuation coefficient in bone relative to soft tissues . These theoretical explanations support our findings. However, further studies will be needed to confirm our theory.
The present study demonstrates that the scintigraphic technique would have some difficulty in detecting a seed that had migrated to hard tissue, such as bone. In Case 1, without SPECT images, the scintigraphic technique could not detect the seed that had migrated to the right ischial bone. This means that routine whole-body planar scintigraphic scans alone would be insufficient to detect a migrated seed with a scintigraphic technique of higher sensitivity. It is thought that routine whole-body SPECT scans would also be needed. Moreover, it is highly doubtful that, without radiographic information, we could correctly detect a seed that had migrated to the right ischial bone with the scintigraphic technique alone or including SPECT, because axial and coronal SPECT images barely showed it. It is thought that radiographic information is still necessary to detect a migrated seed with a higher sensitivity.
The scintigraphic technique may be a useful method for detecting seed migration after transperineal interstitial permanent prostate brachytherapy with 125I seeds. However, it has a limitation in that it has some difficulty in detecting a seed that has migrated to bone tissue.