Presentation on theme: "Radioiodine Treatment for Benign Thyroid Diseases Teofilo O.L. San Luis, Jr., MD, MPA Professor, Faculty of Medicine & Surgery, University of Santo Tomas."— Presentation transcript:
Radioiodine Treatment for Benign Thyroid Diseases Teofilo O.L. San Luis, Jr., MD, MPA Professor, Faculty of Medicine & Surgery, University of Santo Tomas Chief, Section of Nuclear Medicine, UST Hospital, Manila, Philippines Dean, Asian School of Nuclear Medicine
I 131 Treatment Indicated for Hyperthyroidism (Graves Disease, Toxic Nodular Goiter), recurrent disease, those with contraindications to anti-thyroid drugs & surgery Not indicated for: Destruction-induced thyrotoxicosis Thyroiditis (acute, subacute, postpartum) Thyrotoxicosis factitia Hyperthyroxinemia Used in shrinking euthyroid goiters or nodules
What happens inside the Thyroid Follicular Cell? I 127
What happens inside the Thyroid Follicular Cell with I 131 ? I 131
I 131 Treatment I 131 emits beta rays (0.61 MeV); max range in tissue = 2.4mm Beta rays destruction of thyroid follicular cells decreased thyroid cell mass reduced thyroid hormones over a period of time Variability in how long a time is the effect of I 131 ? how much cell mass is destroyed? how much thyroid hormone can still be produced?
Factors affecting I 131 effectiveness: Dose itself Over-all gland size (or weight) Uptake into the thyroid Transit through the thyroid Status of iodine sufficiency (or deficiency) Radiation sensitivity
FACTOR # 1: DOSE ITSELF Dose Calculation Dose = Gland Weight x uCi/gm x 100 % Uptake (24 hrs) Considerable variation in any of the entries Fixed Dose vs. Calculated Dose vs. Empirical Dose 5 mCi; 10 mCi; 15 mCi 100 – 160 uCi / gm 5,000 – 15,000 rads or 50 – 150 Gy Physician subjective factors: clinical contingencies, urgency for treatment, severity of disease, etc.) Regulatory requirements: out-patient vs. in-patient
FACTOR # 2: GLAND SIZE (WEIGHT) Variability in gland size (weight estimation) Clinical palpation estimate (based on experience) Thyroid scan image estimate (eye-balling) Thyroid scan planimetry RL = Length x Width, e.g. 6 cm x 2.5 cm = 15.0 LL = Length x Width, e.g. 5.5 cm x 3 cm = cm 2 x constant (thickness) 1.44 = 45.3 cm 3 or 45 gms Ultrasound volume (machine-derived) RL = Length x Width x Depth x correction factor LL = Length x Width x Depth x correction factor 6 x 2.5 x 2.5 x 0.52 = x 3 x 2.5 x 0.52 = gms
FACTOR # 3: UPTAKE INTO THE ORGAN Hyperthyroid uptake ranges from 30 – 90% Distribution: uniform vs. non-uniform 24 hr RAIU & scan as appropriate indicators Therapy uptake decreased by iodine contaminants (food, drugs) FACTOR # 4: TRANSIT THROUGH THYROID (Biologic half-life) Hyperthyroid patients: very rapid thyroidal turnover of iodine = short biologic half-life Depleted thyroidal iodine pool (in 15% of pts) Rapid transit decreased thyroid radiation dose (but increased blood & marrow radioactivity) effectiveness?
FACTOR # 5: STATUS OF IODINE SUFFICIENCY (or DEFICIENCY) Intra-thyroidal iodine stores In iodine sufficient areas: 10 – 20 mg (mean: 15 mg) In iodine deficient areas: 2 – 5 mg (mean 3.5 mg) Competition of stable iodine vs. radioiodine Iodine-poor diet list, etc FACTOR # 6: RADIATION SENSITIVITY Difficult to measure Prior use of anti-thyroid drugs (PTU but not Methimazole) confers radio-resistance treatment failure vs use higher I 131 dose
I 131 Treatment Outcomes Effects on Thyroidal Physical Configuration Decrease in global size (for diffuse enlargement) Decrease in nodule number & dimensions (for nodular enlargement) Effects on Thyroidal Function Desired: Euthyroidism Inevitable: Hypothyroidism Unintended: Persistent hyperthyroidism Unfortunate / Inadvertent: Thyroid storm
I 131 Treatment Outcomes Clinical Thyroidal Outcomes Hypothyroidism: early (within 1 st year): 20%; 2 – 3% / annum; 50% within decade; +5% every 2 decades Clinical Non-Thyroidal Outcomes Risk for carcinogenesis: very little Other effects: hardly, if at all Non-Clinical Outcomes Safety & Efficiency Economic (Cost-Benefit)
I 131 Treatment: Summary I 131 has definitive role in the management of hyperthyroidism and other benign thyroid conditions, as appropriate Radiation effects influenced by several factors Effects on thyroidal physical configuration & function with projected variable outcomes I 131 therapy is safe, efficient, cost-beneficial