A Potential False Positive Finding on 99mTc Red Blood Cell Gastrointestinal Bleeding Scintigraphy

Abstract

Turkish Abstract

Introduction

Tc-99m labeled RBC scintigraphy is a commonly employed method in the evaluation of especially lower gastrointestinal bleeding. On Tc-99m RBC images gastrointestinal bleeding site can be seen as an initial focus of increased radiotracer activity. On subsequent images the intensity of the activity increases and activity site migrates due to bowel movements. Although, this pattern is specific for gastrointestinal bleeding site, false positive findings do occur imitating gastrointestinal bleeding. We report a case of a patient who had total colectomy two years prior to the scintigraphy and minimal hematochezia ongoing for two days which was suspicious  for lower gastrointestinal hemorrhage.

Case Report

A 36 years old male patient was admitted to the hospital for minimal hematochezia ongoing for two days. The patient had a history of total colectomy because of familial adenomatous polyposis two years ago. Bleeding site was not detected by upper endoscopy. The patient was referred to our clinic for evaluation of bleeding with  Tc-99m labeled red blood cell scintigraphy. The modified in vivo technique used for labeling red blood cells with an activity of 20 mCi technetium-99m pertechnetate. Images were obtained with a dual headed gamma camera. Dynamic images were collected during the first 1 hour and static images (500,000 counts per image) were obtained at 2nd, 4th and 24th hours. Dynamic images revealed a small focus of activity in the lower midabdominal region  (Figure 1).Post-void anterior-posterior delayed images and lateral images con?rmed that the lower abdominal activity at dynamic imaging was actually located at the penil localisation (Figure 2) . The authors concluded that the images were not compatible with gastrointestinal bleeding.

Figure 1 A small activity at lower midabdominal region was observed and the configuration of the activity changed to a lineer pattern during dynamic imaging.

Figure 2 After the dynamic study, postvoid static images [anterior-posterior (c,d) and lateral (a,b)] were taken and they demonstrated that the finding was compatible with penil activity

Discussion

Patients with active upper gastrointestinal bleeding are mostly managed with upper endoscopy. Active lower gastrointestinal bleeding especially when intermittent and slower tends to be a diagnostic dilemma for clinicians. Tc-99m labeled red blood cell scans are routinely employed in patients with suspected lower gastrointesinal bleeding. As in our patient, a dynamic study is carried out for 60 minutes, which was followed by static images up to 24 hours. Angiography has been reported to have a sensitivity ranging from 30% to 47% and a specificity of 100 %.^1-2 Angiography is known to detect gastrointestinal bleeding at a rate of 1 ml/min. However, scintigraphy with T-99m labeled red blood cell scintigraphy can detect gastrointesinal bleeding at a rate as low as 0.04-1 ml/min. The study is useful for confirming bleeding and helps to localize the bleeding site. Because of blood pool activity, patients with suspected intermittent bleeding can be imaged over a period of 24 hours. On Tc-99m labeled red blood cell images gastrointestinal bleeding site can be seen as an initial focus of increased radiotracer activity. On subsequent images the intensity of the activity increases and activity site changes shape and position with bowel movements.A  pattern not conforming to normal physiologic and anatomic sites such as renal or bladder activity is usually spesific for active bleeding site, false positives do also occur. When the site of the activity is unclear, SPECT/CT with fusion may be helpful in detecting the location of uptake and aid in the diagnosis.^3-4

In our case, in the dynamic and late static images physiologic penil activity mimicking bleeding site was revealed. In male patients penil activity is one of the potential false positives in gastrointestinal bleeding scintigraphy and should not be misinterpreted as a focus of hemorrhage, because of slow filling pattern simulating an acute bleeding

Conclusion: For less experienced pyhsicians the unusual radiotracer accumulation might be quite confusing. Lateral views, late static images and when penil activity is suspected postvoid images may help for the clari?cation of the diagnosis. In this presented case, one of the potential false positives in gastrointestinal bleeding scintigraphy performed with Tc-99m labeled red blood cells is illustrated.

References

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2. Fiorito JJ, et al. The diagnostic yield of superior mesenteric angiography: correlation with the pattern of gastrointestinal bleeding. Am J Gastroenterol. 1989; 84(8): 878-81.
3. Turgeon DK, et al. Possible role of Meckel's Scan fused with SPECT CT imaging: Unravelling the cause of abdominal pain and obscure overt gastrointestinal bleeding. Case Rep Gastroenterol.  2008; 2: 83-90.
4. Dillman JR, et al. Utility of SPECT/CT with Meckel's scintigraphy. Ann of Nucl Med. 2009; 23: 813-5.