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I-131 UPTAKE IN A SOFT TISSUE CYST
Authored By: Joanna Fair and Keith Fischer.
Patient: 74 year old female
History: Previously treated papillary thyroid cancer.
Image Size:[small][as-submitted]

Fig. 1
5-mCi whole-body I-131 imaging.

Fig. 2
Post-therapy 100-mCi whole-body I-131 imaging, 1.5 years previously.

Fig. 3
AP radiograph left femur.

Fig. 4
Lateral radiograph left femur.

Fig. 5
Annotated AP and lateral left femur radiographs.

Fig. 6
Sagittal T1 and STIR MR images of the left thigh.

Fig. 7
Axial T1 with fat saturation MR image of the proximal thighs.
Image Size:[small][as-submitted]

Findings:

Radiopharmaceutical:  5.0 mCi I-131 sodium iodide p.o.


Findings:

Whole-body I-131 imaging (Fig. 1) demonstrates physiologic uptake in the salivary glands, bowel, and bladder.  There is an additional focus of uptake in the left proximal thigh, best seen on the anterior images.

Post-therapy imaging (100-mCi, Fig. 2) performed 1.5 years previously shows that the focus was also present at that time.

What would you do next?


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AP and lateral radiographs of the left femur (Fig. 3, Fig. 4, and annotated in Fig. 5) show a soft-tissue-density, rounded, lobulated lesion in the left anteromedial thigh.

What would you do next?

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Ultrasound with possible aspiration was initially recommended.  MRI (Fig. 6 and Fig. 7) was obtained, which showed a well-circumscribed lesion in the subcutaneous soft tissues of the left anterior thigh with fluid signal intensity and minimal rim enhancement. 
DDx: Soft-tissue metastasis from thyroid cancer versus I-131 uptake in a cyst.
Diagnosis: I-131 uptake in a cyst.
General Discussion: Full patient history:

This patient was initially diagnosed with papillary thyroid cancer 1.5 years previously; at that time, she underwent a thyroidectomy and treatment with 100-mCi of radioactive iodine.  She returned for routine followup.

Discussion:

One potential pitfall in interpretation of I-131 whole-body imaging is uptake within cysts.  The mechanism is likely that the I-131 enters the cyst during the initial circulation of the radiopharmaceutical.  As the I-131 clears from the other soft tissues, it remains trapped within the cyst, causing persistent activity on delayed images.

References: Carlisle MR, Lu C, McDougall IR. The interpretation of 131I scans in the evaluation of thyroid cancer, with an emphasis on false positive findings. Nucl Med Commun. 2003;24:715-735.

Brachman MB, Rothman BJ, Ramanna L, Tanasescu DE, Adelberg H, Waxman AD. False-positive iodine-131 body scan caused by large renal cyst. Clin Nucl Med 1988; 13: 416-418.
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Additional Details:

Case Number: 130574Owner(s): Joanna Fair and Keith FischerLast Updated: 02-07-2013
Anatomy: Other   Pathology: Benign Mass, Cyst
Modality: Conventional Radiograph, MR, Nuc MedAccess Level: Readable by all users, writable by NucMed Certifiers
Keywords: trnm, cyst, i-131, iodine, thyroid cancerACR: 27300.37900, 40000.36900

Case has been viewed 35 times.
Certified by Keith Fischer on 06-26-2009

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