Appendix 15 Answers to Chapter 15

Abstract

1. (d) 131I is used to treat thyroid cancer and hyperthyroidism by ablation. Grave’s disease is the most common cause of hyperthyroidism. 2. (a) The intramuscular injection of nonradioactive B12 is given to preload the liver and to saturate receptor sites in plasma so that the portion of radioactive B12 absorbed will be passed through the urine. 3. (d) 32P chromic phosphate is a bluish green colloid and is used for intracavitary therapy. 32P sodium phosphate is a clear solution that is used to treat polycythemia vera and bone pain from metastasis and is administered intravenously. 32P is a beta emitter. 4. (b) As with all radioactive materials, 89Sr should be assayed prior to administration. However, the NRC allows administration of 89Sr unit doses without assay because not all dose calibrators assay them accurately. 5. (b) See explanation for question 3. 6. (c) 32P chromic phosphate is useful in treating malignant ascites. 32P sodium phosphate is used to treat polycythemia vera. 89Sr (as well as 32P sodium phosphate and 153Sm-EDTMP) is used to treat malignant bone pain by localizing where there is bone mineral turnover. 131I may be used to treat Grave’s disease. 7. (c) 89Sr chloride can be administered by direct venous access, but as it is a beta emitter, it is a better option to access the vein with an intravenous line and check the patency of the system before injection. If the dose is infiltrated into the tissue surrounding the vein, the tissue will be unnecessarily irradiated. 8. (b) 89Sr and 32P are beta emitters and will be effectively shielded with the plastic syringe. Using a lead syringe shield will cause bremsstrahlung. 9. (b) See explanation to question 6. 10. (b) Intrinsic factor is a glycoprotein. It is secreted by the gastric mucosa, and without it vitamin B12 cannot be absorbed. If a Schilling test shows a B12 deficiency, the test can be repeated with the intrinsic factor to determine if the lack of it is the cause of malabsorption. 11. (b) The radioactivity in the urine represents the amount of vitamin B12 absorbed. The labeled B12 is excreted through the urine, so if any urine is not collected, the percent excretion will be falsely low. 12. (c) The results from the first 24-h sample collected during Stage I are abnormal (normal is >8–10%), and the result from Stage II is 12%, which is normal. This indicates that the addition of intrinsic factor has corrected the malabsorption, meaning the patient has pernicious anemia, which is a vitamin B12 deficiency resulting from lack of intrinsic factor. There are other causes of B12 which include low intake, pancreatic insufficiency, and various medications. 13. (d) To calculate the percent excreted, one uses the following formula: