I-131 retention in benign cysts: Examples and a mechanism

Objectives There are numerous reports in the medical literature of false positive foci in post-therapy I-131 images in thyroid cancer patients. These include: mucocele, pleural effusion, pericardial effusion, pleuro-pericardial cyst, renal cysts, and ovarian cysts. Goal is to understand the underlying mechanism of these false positive uptakes.

Methods Focal uptake was observed in post-therapy I-131 scans of 5 patients over the period from 2005-2011 at the University of Iowa. Subsequent imaging or history showed that the focal uptake was in a cyst. A two-compartment kinetic model was set up and solved numerically using Excel. A realistic input function was created using a gamma variate with a prolonged exponential tail.

Results The examples include: a pseudotumor in the major fissure of the right lung, a cyst in the liver capsule, a left renal cyst, right breast cyst and bilateral renal cysts. The compartmental model assumed immediate complete mixing in the plasma and interstitial space along with gradual equilibration with the fluid in a cyst. Activity slowly built up in the cyst fluid (Cc) until it reached the same concentration as the plasma and interstitial fluid (Cpi). Thereafter Cc decreased more slowly than Cpi, resulting in markedly greater concentration in Cc than in Cpi at late times.

Conclusions False positive foci of uptake are occasionally seen in I-131 post-therapy scans in fluid collections or cysts. The mechanism of accumulation of activity in these collections is passive and does not require any active uptake of radioiodine