Nuclear Medicine · Supplementary · from Nuclear Medicine

Case 1: Thyroid Cancer Radioiodine Therapy

Patient Presentation

Demographics: 34-year-old female elementary school teacher

Chief Complaint: "I had thyroid cancer surgery last month and my surgeon says I need radioactive iodine treatment."

History of Present Illness: A 34-year-old female presents to the nuclear medicine department for radioiodine (I-131) therapy following total thyroidectomy performed 5 weeks ago. She was initially found to have a 2.3 cm thyroid nodule on ultrasound after her primary care physician palpated a firm, non-tender nodule in the right thyroid lobe during a routine exam. Fine needle aspiration biopsy revealed papillary thyroid carcinoma (Bethesda VI). She underwent total thyroidectomy with central neck dissection.

Surgical pathology revealed a 2.5 cm papillary thyroid carcinoma (classical variant) in the right lobe with extrathyroidal extension into the perithyroidal soft tissue and metastatic disease in 2 of 6 central compartment lymph nodes (largest metastatic deposit 0.8 cm). Margins were negative. BRAF V600E mutation was positive on molecular testing. The tumor was staged as pT3aN1a (AJCC 8th edition), Stage I (age <55). ATA risk stratification: intermediate risk.

She has been maintained on levothyroxine since surgery but was switched to liothyronine (T3) 3 weeks ago and then taken off all thyroid hormone replacement 2 weeks ago in preparation for radioiodine therapy. She has been following a low-iodine diet for 2 weeks. She reports symptoms of hypothyroidism including fatigue, constipation, cold intolerance, and weight gain of 3 kg.

Past Medical History:

• Papillary thyroid carcinoma (recently diagnosed)
• Total thyroidectomy with central neck dissection (5 weeks ago)
• No other significant medical history

Medications:

• All thyroid hormone replacement discontinued 2 weeks ago
• Calcium carbonate 500 mg TID (post-thyroidectomy hypoparathyroidism prophylaxis)
• Calcitriol 0.25 mcg BID

Social History:

• Elementary school teacher
• Non-smoker, no alcohol
• Married, no children (desires future pregnancy)
• No radiation exposure history

Family History:

• Mother: hypothyroidism (Hashimoto's)
• No family history of thyroid cancer or MEN syndromes

Physical Examination

• Vital Signs: BP 108/68 mmHg, HR 56 bpm, RR 14/min, Temp 36.2°C, SpO2 99% on room air, Weight 68 kg
• General: Fatigued-appearing female, periorbital puffiness, dry skin
• Neck: Well-healed thyroidectomy scar, no palpable masses or lymphadenopathy in lateral neck compartments
• Cardiovascular: Bradycardic, regular rhythm, no murmurs
• Extremities: Non-pitting edema of hands and feet, delayed relaxation phase of ankle reflexes
• Neurological: Sluggish deep tendon reflexes bilaterally

Workup and Results

Laboratory Studies (Pre-RAI therapy):

Imaging/Additional Studies:

• Pre-therapy diagnostic I-123 whole body scan: Uptake in thyroid bed (residual thyroid tissue); no distant metastatic uptake identified
• Neck ultrasound (post-operative): Small residual tissue in thyroid bed; no suspicious lateral neck lymph nodes
• RAI Therapy: 150 mCi (5.55 GBq) I-131 administered orally
• Post-therapy I-131 whole body scan (7 days post-treatment): Intense uptake in thyroid bed; faint uptake in two foci in right central neck (consistent with known nodal metastatic disease); no distant metastases; normal physiologic uptake in salivary glands, GI tract, and bladder

Clinical Image

Diagram illustrating the mechanism of radioiodine (I-131) therapy for thyroid cancer: iodine uptake via the sodium-iodide symporter (NIS), intracellular concentration, and beta-particle emission causing targeted DNA damage and cell death in thyroid tissue. Source: Educational illustration.

Diagnosis

Papillary Thyroid Carcinoma, pT3aN1a, Stage I, ATA Intermediate Risk, Status Post Total Thyroidectomy, Treated with I-131 Radioiodine Ablation

Key Diagnostic Criteria:

• Papillary thyroid carcinoma confirmed on surgical pathology with extrathyroidal extension
• Central compartment lymph node metastases (2/6 nodes positive)
• BRAF V600E positive (associated with more aggressive behavior)
• ATA intermediate risk stratification warranting RAI therapy
• Stimulated TSH >30 mIU/L confirming adequate preparation
• Elevated stimulated thyroglobulin (12.4 ng/mL) suggesting residual disease
• Post-therapy scan confirming uptake in thyroid bed and central neck nodes

Treatment Plan

1. I-131 dose administered: 150 mCi (5.55 GBq) -- appropriate for intermediate-risk disease with nodal metastases
2. Radiation safety precautions: Patient remains in isolation for 2-3 days; maintain 6-foot distance from others; separate bathroom; no close contact with children or pregnant women for 5-7 days; radiation safety officer to perform survey before discharge
3. Salivary gland protection: Sour candy/lemon drops starting 24 hours after RAI to promote salivary flow and reduce sialadenitis risk
4. Restart thyroid hormone: Begin levothyroxine 2 mcg/kg/day (137 mcg daily) on day 2 post-RAI; target TSH suppression to 0.1-0.5 mIU/L for intermediate-risk disease
5. Follow-up: Stimulated thyroglobulin and diagnostic whole body scan at 6-12 months to assess response; neck ultrasound every 6 months for first 2 years
6. Fertility counseling: Recommend delaying pregnancy for 6-12 months post-RAI; reassurance that RAI at this dose does not significantly impair fertility
7. Long-term monitoring: Annual thyroglobulin on suppressive levothyroxine; adjust TSH suppression target based on response to therapy

Key Learning Points

• Radioiodine (I-131) therapy exploits the sodium-iodide symporter (NIS) to selectively deliver targeted radiation to thyroid tissue and differentiated thyroid cancer cells
• Adequate TSH stimulation (>30 mIU/L) is essential for optimal RAI uptake; this can be achieved by thyroid hormone withdrawal or recombinant human TSH (rhTSH/Thyrogen)
• A low-iodine diet for 1-2 weeks prior to RAI therapy depletes the body's iodine pool, increasing the proportion of I-131 taken up by residual thyroid tissue
• Post-therapy whole body scan often reveals additional foci of disease not seen on pre-therapy diagnostic scans due to the higher therapeutic dose
• Stimulated thyroglobulin is a critical tumor marker for monitoring differentiated thyroid cancer; rising levels post-treatment suggest recurrent or persistent disease

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