Using 131I-NaI

 

Thyroid Diseases Treatable with 131I-NaI

- hyperthyroidism (Graves disease)

- toxic nodular goiter (Plummer’s disease)

- thyroid carcinoma (ranked in order of likelihood of 131I uptake)

1. Follicular

2. Papillary

the other two types of thyroid cancer, medullary and anaplastic, are not treatable with I-131

Quiz

The decay scheme below indicates that there are 14 gammas and 6 betas emitted from I-131. Therefore, True or False, 14/20 of the tissue damage is attributable to gammas and 6/20 to betas.

Answer

False for 2 reasons:

1. The LET (Linear Transfer Rate) for betas is much higher than for gammas; consequently they confer a much higher radiation dose

2. The fractions 14/20 and 6/20 imply that the % abundance of each of these 20 emissions is exactly 5%, which is not possible. In fact the % abundances vary from a fraction of 1% to almost 85%.

Correct answer is that ~90% of tissue damage is attributable to beta particles.

 

Typical Doses of 131I Compounds
procedure dose (mCi) route of administration
raiu, normal 0.005 oral
raiu & scan, substernal 0.100 oral
total body mets survey 5-10 oral
hyperthyroidism 5-15 oral
toxic nodular goiter 25 oral
thyroid Ca therapy 75-225 oral

Radiation Dosimetry of 131I- NaI

Following oral administration of 10 mCi dose of 131I-NaI for treatment of hyperthyroidism, 90% of dose to tissue is achieved by b- emissions. For a hyperthyroid patient treated with I-131:

Tissue absorbed radiation dose  (rads/10 mCi of I-131)
Thyroid 11,000
Testes 9.2
Ovaries 9.3
Whole body 16.0

 

Dose Determination for Therapy in Graves Disease

Method 1

                 Measure % uptake; estimate mass of thyroid (g)

            Dose = 60-100 uCi/g x mass (g) x 100% % uptake

disadvantage: since 60-100 uCi /g is a wide range, it is difficult to determine the appropriate factor for an individual patient. Use of this formula often results in incorrect estimate of the required dose, resulting in over- or under-dosing of patient.

Method 2

            A standard dose of 131I NaI is given orally to all patients
            (8 mCi to females, 10 mCi to males); these doses may be modified a litle based on %RAIU

 

Advantage:  adequately treats 85% of all Graves disease patients with 1 treatment.

Disadvantage: of the 15% who are refractory, 10% require a second administration of  131I; the other 5% require a  third dose of  131I.

 

  1. Day of administration no immediate effect
  2. 4-6 weeks patient begins to notice beneficial effects
  3. 12 weeks maximum beneficial effects observed
  4. 6 months few observable changes after this interval

 

Long-term Side Effect

As indicated in the following graph, the rate of hypothyroidism after the first year is 3%/year for all patients treated with 131I sodium iodide for Graves disease. They are treated with synthroid daily for the rest of their lives.

Rate of Induction of Hypothyroidism Following Therapy with 131I-NaI

 
Precautions to be Observed with High-dose I-131 Therapy Patient

1. Keep your distance to minimize personal radiation dose

2. Patient is assigned a private room

3. Everyone involved with patient must wear film badge

4. Gloves must be used by patient to handle telephone, bed controls

5. Housekeeping not allowed in room until room is released by RSO

6. No visitors allowed for at least 24 hr

7. No bed baths

8. Patient must stay in bed unless instructed otherwise

9. Absorbent pads taped to floor from toilet to bed

10. Patient must use disposable items for food service

11. Diagnostic blood samples taken by Nuclear Medicine

12. If patient dies, attending physician must be notified immediately

13. Room must be surveyed by RSO prior to release for next use.

14. Every participant in therapy must have thyroid counted 24 hr post dose

Patient Release Criteria

For Some Agreement States...Reading <5 mR/hr at 1 meter from patient’s chest, which is equivalent to a body burden <30 mCi of 131I.

For NRC States*...Reading <7 mR/hr at 1 meter from patient’s chest, which is equivalent to a body burden <33 mCi of 131I.
Note: NRC is no longer requiring hospitalization for 131I therapy