Design
This is a discussion of radiation therapy us- ing radiopharmr~~euticals, not of thc use of vari- ous nuclides as sealed sources or pieces of wire.
The behavior of radiopharmaceuticals used for radiation therapy must be very wcll understood to avoid radiation close t o othcr than the in- tended areas.
When radiation therapy is the intended use of a radiopharmaccutical, the design criteria change slightly. We arc still intending to use easily produced, available, inexpensive radio- nuclides of high specific activity. Most impor- tant is that the target-t<)-nontarget ratio be cx- tremely high in order to minimize the danger to other organs. Most rddiopharlnaceuticals in use now do not have the high ratio needed to satisfy this criterion. Since we do not intend t o minimize the radiation dose to the target but rather to maximize it, different radiation char- acteristics must be sought. The dose should be delivered fairly quickly, so the effective half- lifc should be short, primarily because of the physical half-life. The material should be a beta emitter with no external radiation, so that the dose can be localizcd in the patient and so that his attendants and visitors do not get an un- wanted dose. If the material cannot be made to remain at the site of localization, it wust be re- moved from tbc body quickly, as by hydrating the patient or by using cleansing enemas.
131 I
By far the most radiation therapy in nuclear medicine is performed with
19'I.
Iodine therapy is practiced on patients suffering from hyper- thyroidism, who are not terminal patients being treated palliatively but are people who have long lives ahead of them. They must be treatedcarefully to avoid the possibilities of can- cer induction and genetic damage. Under no circumstances should a prcgnant woman be treated because indinc crosses the placental barrier and can accidentally treat the thyroid of the fetus, leaving it athyroid. Iodine therapy is also used to treat thyroid cancer, usually after surgery. It is uscful for ablating remain- ing thyroid tissue and for treating metastatic thyroid tissue. The iodine is used in the iodide ion form, with no added carrier. The thyroid may be stimulatcd before the dose is admin- istered to thyroid cancer patients. The iodine is a normal constituent of the thyroid and its hormones, so the mechanism for uptake for thcrapy is the same as that of other iodine in- corporation into thyroid hormone. The many gamma rays of r:nI create a radiation hazard to the people surrounding thc patient. li51 has also been usrd in thyroid therapy.
32P sodium phosphate
Phosphorus 32 is a pure beta emitter with a 14.3-day half-life. It has been used in the soluble sodium phosphate form for the treat- ment of scveral hematologic conditions such as leukemia and polycythemia vera. It is admin- istered either orally or intravenously and con- centrates in the blood cell precursors of the marrow where there is rapid proliferation of cells. There is some evidence that thc treatment can cause leukemia in people who do not have it and that chemotherapeutic and othcr treat- ments may be prcfcrred (Table 6- 1).
Colloids
Phosphorus 32 in the insolublc colloidal form of chromic phosphate and gold 198 as the col- loid have been used to treat effusions, both of 91
92 Basics of rudiopharmucy
Table 6-1. Therapeutic uses of radionuclides
Nuclide Chemical form Target organ Indications
1311 I- Thyroid Thyrotoxicosis, thyroid cancer,
thyroid Cancer metastasis
32 p Sodium phosphatc Bone marrow Leukemia, polycythemia vera
3 2 p Chromic phosphate Body cavitics Malignant effusion
I "Au Colloidal gold Body cavities Malignant effusion
the synovial membrane in rheumatoid arthritis and of the peritoneal cavity often after incom- plete surgery. The colloid, diluted in saline to fill the space, is instilled into the cavity in ques- tion. The dose is delivered on the surface of the cavity to which the colloid adheres. In perito- neal instillation it may be that lSHAu confers less harmful doses to other structures in the patient; because of its shorter half-life, it spends more of its lifetime in the correct cavity and less as a colloid that has passed through the diaphragmatic surface and made its way into the liver.
32P,
on the other hand, is a pure beta emitter, whereas the gold emits a gamma ray at 410 kev, which presents a hazard to the sur- rounding tissues. All the colloids behave simi- larly, though not identically because of particle size differences. It is possible to image the cavity into which the32P
colloid is to be in- stilled by giving a tracer dose of YsmTc sulfur colloid prior to the :izP procedure.Handling therapy patients
ASSURANCE OF RADIOISOTOPE DOSAGE Some physicians will prescribe the exact mCi dose and chemical form of the radionuclide to be used for therapy. At other times, it may be necessary for the technologist or radiopharma- cist to assist with the calculation required for arriving at the required mCi dose. For example, in treating the thyroid with radioiodine, data from a previous radioiodine uptake study and estimation of gland size either from a radioiso- tope scan or palpation can be used to estimate the required number of mCi of l3II to be ad- ministered to provide a given radiation expo- sure dose. A previous study of YemTc sulfur colloid can sometimes be used to help estimate the required number of mCi of colloidal
32P
or lDflAu.
Once the radiation exposure dose is set and the mCi dose required to produce this exposure dose is estimated, the dose must be measurcd out and dispensed to the patient. Some clinics have policies that require two individuals to check each other to assure that both the calcu- lations and the radioisotope measurements are correct. Great care is always taken to assure (1) that the correct mCi amount is measured out and administered, ( 2 ) that the dose is given to the correct patient, and (3) that appropriate radiation safety considerations are met. Radia- tion therapy, though usually less traumatic to a patient than surgery, is a procedure with con- sequences similar to surgery. To make a rnis- take with a therapy dose is a very serious rnat- ter similar in magnitude to operating on the wrong patient. Thus, it is often wise to request verification of calculations and measurements from a radiation physicist or nuclear medical scientist.
RADIATION SAFETY CONSIDERATIONS
The possibility of personnel radiation expo- sure during the drawing, handling, and mea- surement of therapy dose should be carefully considered. The same precautions required for handling diagnostic dosages are used. How- ever, often a second trained person is avail- able to survey the operation and monitor the radiation exposure levels with a hand-held sur- vey meter.
With doses of l3'I larger than 30 mCi, hos- pitalization of the patient is required. The pa- tient is placed in isolation until the body burden is less than 30 mCi. During this period, special procedures are followed to minimize radiation exposure to nurses and other health care per- sonnel. Also, special procedures for disposal of radioactive body wastes and clothing are fol-
Radiution therapy with radiopharmaceuticals 93
Fig. 6-1. Radioiodine solutions are being given to patient in lead-shielded cup and disposdblc straw (or pipette if straw is not available). Radiopharmacist discusses procedure with patient so that patient coopcration is assured.
lowed so that radiation contamination of the hospital environment is avoided.
TALKING WITH THE PATIENT
Each person who deals with a patient under- going radiation therapy with a radionuclide has the responsibility to help make the procedure safe and effective by relating kindly and care- fully to the patient. Patient cooperation is often best achieved by clear and precise cornmuniea- tion with the patient. We have found that if we explain exactly what we are doing and why, the patients usually feel more at ease with the process,
When administering oral radioiodine solu- tions, often the solution is dispensed in a cup within a lead shield (Fig. 6-1). Absorbent pa- pers are used to guard against spillage.
Tf
the reasons for use of this are explained, the patient usually will not be upset by what appears to be a strange procedure.Fig. 6-2. Radioiodine container is rinsed with water two t o three times. Patient is asked to drink washing t o assure that total dose is taken.
After explaining the procedure t o the patient, question him as to whether he has a settled stomach. If a paticnt is nauseated, it is wise to postpone oral doses of radionuclides. Also, instructions are given to assure that the patient takes all of the dose. The cup may he rinsed two to thrce timcs with water to assure that the whole dose is swallowcd (Fig. 6-2). Avoid rinsing the cup with saline solution or warm watcr because this can induce nausea. Once the dose is administered, it is expedient to release the paticnt s o that exposurc to self and other individuals in the nuclear medicine clinic is minimized. Radioactive patients interfere with
counting instrumentation by increasing and causing unpredictahle fluctuations in back- ground radiation levels.
Suggested readings
Brucer, M.: From surgery without a knife t o the awmic cocktail. Vignettes in Nuclear medicine, No. 2, St.
Louis, 1966, Mallinckrodt Chemical Works.
Larose, J . H.: Radionuclidc therapy. In Early, P. J . , Raz- xak, M. A . , and Scdee, D. B., editors: Textbook of nuclear medicine technology, St. Louis, 1975, The C. V.
Mosby Co.
Silver, S . : Radioactive nuclide in medicine and biology, Philadclphia, 1968, Lea & Febigcr.
Werner, S. C.: Radioiodinc. In Werner, S. C . , and Ingbar, S. H., editors: The thyroid, New York, 1971, Harper
& Row, Publishers.
CHAPTER 7