# Post-Therapy I-131 Whole Body Scan Interpretation

## Overview

The post-therapy I-131 whole-body scan is performed 5 to 7 days after therapeutic I-131 administration for differentiated thyroid cancer. It takes advantage of the higher administered activity compared with diagnostic doses, providing improved sensitivity for detecting disease. In 10 to 25% of patients, the post-therapy scan detects metastatic disease not identified on pre-therapy diagnostic scans. This information is critical for staging, prognosis, and determining the need for further therapy.

## Technique

Whole-body planar imaging in anterior and posterior projections is acquired using a large-field-of-view gamma camera. The energy window is centered on the 364 keV photopeak of I-131, and high-energy collimators are required because septal penetration occurs with low- or medium-energy collimators. SPECT/CT of the neck and chest is strongly recommended for precise anatomic localization of findings. The scan speed is typically 5 to 10 cm per minute for whole-body imaging. While imaging at 5 to 7 days post-therapy is standard, some centers image at 48 to 72 hours with acceptable image quality.

## Normal Biodistribution on Post-Therapy Scan

### Expected Uptake Sites

The thyroid bed shows residual thyroid tissue or remnant, which is an expected finding after thyroidectomy. The salivary glands, including the parotid and submandibular, demonstrate uptake due to NIS expression in ductal cells. The nasal mucosa shows physiologic uptake from secretion and NIS expression. The oral mucosa and oropharynx accumulate activity from salivary excretion. The stomach concentrates iodide in the gastric mucosa. The small and large bowel show activity from gastrointestinal secretions and swallowed saliva. The urinary bladder demonstrates activity because renal excretion is the primary route of elimination. The liver shows uptake from metabolism of iodinated thyroid hormone analogs and deiodination.

<image>Normal post-therapy I-131 whole body scan showing expected biodistribution in salivary glands, nasal mucosa, GI tract, and urinary bladder with thyroid bed remnant uptake</image>

## Abnormal Findings: True Metastatic Disease

### Cervical and Mediastinal Lymph Nodes

Focal uptake in the lateral neck or superior mediastinum suggests lymph node metastases. SPECT/CT is essential to distinguish nodal disease from physiologic salivary or esophageal activity. Level II through IV and level VI cervical nodes are most commonly involved.

### Pulmonary Metastases

Pulmonary metastases may appear as diffuse bilateral pulmonary uptake in a miliary pattern or as focal lung uptake. Micronodular pulmonary metastases may not be visible on CT but still demonstrate RAI uptake. This subset represents the most favorable group for RAI therapy, with excellent long-term outcomes particularly in young patients. Macronodular metastases larger than 1 cm may lose RAI avidity over time.

### Bone Metastases

Focal uptake in the skeleton, most commonly the spine, pelvis, and ribs, indicates bone metastases. These foci must be distinguished from contamination and physiologic activity. RAI-avid bone metastases carry a better prognosis than RAI-refractory bone lesions.

### Other Distant Sites

Brain, liver, and soft tissue metastases are less common. Any unexpected focal uptake outside the normal biodistribution sites warrants further investigation.

## Common False Positives

### Physiologic Variants and Artifacts

Thymic uptake occurs in young patients due to thymic rebound after withdrawal-induced hypothyroidism. Breast tissue shows physiologic NIS expression, particularly in lactating women, which is another reason RAI is contraindicated during breastfeeding. Contamination from skin or clothing exposed to saliva, sweat, or nasal secretions produces spurious foci that resolve after the patient showers and changes clothes. Esophageal activity from swallowed saliva mimics mediastinal lymph node uptake and can be cleared by having the patient drink water and repeating spot views. The nasolacrimal duct may mimic anterior facial uptake.

| False Positive Category | Examples | How to Distinguish |
|---|---|---|
| Physiologic/secretory | Salivary glands, nasal mucosa, GI tract, bladder | Normal biodistribution pattern; symmetric |
| Contamination | Skin, clothing, hair | Resolves after shower/clothing change |
| Swallowed saliva | Esophageal activity | Clears with water; repeat imaging |
| Thymic rebound | Anterior mediastinal uptake (young patients) | SPECT/CT localizes to thymus |
| Breast tissue | Bilateral breast uptake | NIS expression; symmetric |
| Pathologic non-thyroid | Warthin tumor, struma ovarii, renal cyst, effusion | SPECT/CT; clinical correlation |

### Pathologic False Positives

Salivary gland tumors such as Warthin tumor or pleomorphic adenoma may concentrate iodide. Struma ovarii, a mature ovarian teratoma containing thyroid tissue, produces pelvic uptake. Renal cysts show uptake from stagnant urine containing excreted I-131. Pericardial or pleural effusions accumulate iodide in transudative fluid. Inflammatory lung disease including bronchiectasis or active infection may demonstrate uptake. Meckel diverticulum concentrates iodide in ectopic gastric mucosa. Hepatic hemangioma may show increased blood pool activity.

<image>Common false positives on post-therapy I-131 scan: thymic uptake in a young patient, breast uptake, skin contamination, and esophageal activity from swallowed saliva</image>

## SPECT/CT: Added Value

SPECT/CT substantially improves the localization and characterization of foci seen on planar imaging. It distinguishes thyroid bed remnant from cervical lymph node metastases, differentiates esophageal activity from mediastinal lymph node disease, and identifies the precise anatomic location of bone metastases. SPECT/CT reduces the rate of equivocal findings by 30 to 50% and is recommended as a routine addition to planar whole-body imaging.

## Stunning Effect

There is ongoing debate about whether a diagnostic I-131 scan administered before therapy reduces the efficacy of the subsequent therapeutic dose. The proposed mechanism is that the diagnostic dose causes sublethal damage to thyroid cells, reducing their subsequent uptake of the therapeutic dose. Strategies to avoid stunning include using I-123 for diagnostic scans instead of I-131 (since I-123 emits no beta particles), using low diagnostic I-131 activities of 1 to 3 mCi when I-123 is unavailable, and proceeding to empiric therapy without a pre-therapy diagnostic scan in high-risk patients. The clinical significance of stunning remains debated and may be negligible with low diagnostic doses.

## Interpretation Framework

### Systematic Approach

A systematic interpretation begins with assessing image quality for adequate counts, proper positioning, and absence of motion artifact. The thyroid bed remnant is identified and its size and intensity estimated. The neck is surveyed for lateral cervical lymph node uptake. The mediastinum and lungs are evaluated next, followed by a systematic assessment of the skeleton. The abdomen and pelvis are evaluated with attention to distinguishing pathologic foci from gastrointestinal and urinary activity. The images are checked for contamination, including surface markers and clothing artifacts. Finally, any suspicious foci are correlated with SPECT/CT.

### Reporting Elements

The report should address thyroid bed uptake (presence, intensity, and laterality), cervical lymph node uptake (location, number, and laterality), distant metastases (location, number, and RAI avidity), exclusion of false positives (contamination cleared, physiologic activity identified), and comparison with the pre-therapy diagnostic scan and cross-sectional imaging.

<image>Post-therapy I-131 SPECT/CT fused images demonstrating a focal area of uptake in the left lateral neck localized to a level III lymph node metastasis, distinguished from salivary gland activity</image>

## Clinical Pearls

The post-therapy scan frequently upstages patients, and findings should be incorporated into the dynamic risk stratification.

SPECT/CT of at least the neck and chest should always be obtained to avoid misinterpreting physiologic activity as metastatic disease.

Esophageal activity is the most common cause of a false-positive "mediastinal" finding. Having the patient drink water before repeat imaging resolves it.

Star artifact from intense thyroid bed remnant activity can obscure adjacent lymph node metastases, and SPECT/CT helps overcome this limitation.

Diffuse hepatic uptake on the post-therapy scan reflects normal metabolism of iodinated thyroglobulin and should not be mistaken for liver metastases.

Young patients with diffuse RAI-avid pulmonary micrometastases have the best long-term prognosis with repeated RAI therapy.

## References

- Haugen, B. R., et al. "2015 ATA Management Guidelines for DTC." *Thyroid*, 2016.
- Oh, J. R., et al. "False-Positive I-131 Whole-Body Scan Results." *Journal of Nuclear Medicine*, 2012.
- Avram, A. M., et al. "SNMMI Procedure Standard for I-131 WBS." *Journal of Nuclear Medicine*, 2022.
- Amin, A., et al. "Added Value of SPECT/CT to Post-Therapy I-131 Scan." *Clinical Nuclear Medicine*, 2015.