Commonly used sources
131I is the most commonly used unsealed source for treatment of malignancy. It has a physical half life of 8.0 days, decaying through beta emission to 131Xe. Most (90%) of the beta particles have an energy of 606 keV, although energies may range from 69-807 keV. Gamma photons of 364 keV (80%) and 723 keV (20%) are also released after beta emission. Its biological half life is 120 - 140 days, resulting in an effective half life of 7.6 days. The specific activity of iodine-131 is 4,600 TBq/g.
131I must be stored in a secure lead pot during transport to reduce exposure. 131I may be absorbed through the skin, inhaled or ingested, making area surveys after treatment necessary. It is excreted in the urine. Patients are typically treated as inpatients and isolated for 2 - 3 days until their activity has reduced to safe levels. Sex should be avoided for at least one month and pregnancy for at least 4-6 months. Female patients must also have a pregnancy test 24 hours before receiving 131I.
Because 131I is taken up preferentially by the thyroid gland and malignancies which arise from it, 131I is most commonly used in the treatment of thyroid cancer following surgery. It may also be used to treat an overactive thyroid, either due to Graves' disease or a thyroid adenoma.
Dosage for benign thyroid conditions is typically in the order of 3 - 8 MBq per gram of thyroid tissue. For thyroid cancer, activity levels of 2.75 - 7.5 GBq are administered depending on treatment goals. 2.75 GBq may be used for treatment of thyroid bed, whereas doses > 7.5 GBq may be necessary for metastatic disease.
131I is administered orally.
131I is a fission product that can be separated from the other fission products relatively easily. It can also be created through neutron bombardment of 130Te.
89Sr is used for widespread osteoblastic bony metastatic disease which can not be safely treated with external beam radiotherapy. 89Sr has a physical half life of 50.5 days, and mostly decays (99.99%) by negative beta emission of mean energy 0.58 MeV (max 1.46 MeV). It may also release gamma photons (0.01%) of energies up to 0.91 MeV. The average soft tissue range of the beta particle is 2.4 mm.
89Sr has a short half life in the blood, and is excreted quickly through the kidneys. After a single void, the risk to the general public is minimal due to the short range of emitted particles and the low gamma emission probability. Patients should sit to urinate for at least two weeks post administration, and wash soiled clothes separately. If the patient is hospitalised, staff should use gloves when handling the patient. Catheter bags should be emptied promptly, wearing gloves.
Dosage and Administration
89Sr is administered intravenously as strontium chloride. The administered dose for 89Sr is about 2 MBq/kg. Patients do not require hospitalisation, although they may experience a flare of pain between 3 and 21 days following the dose. Maximum effect is seen up to 4-6 weeks
89Sr may be produced through several means. It is a fission product and may be harvested from spent fuel rods, although this requires separation from 90Sr which is more prevalent. It can also be produced through neutron bombardment of 88Sr.
153Sm-lexidronam is a compound administered intravenously for widespread osteoblastic bone metastases (similar to 89Sr). It has a physical half life of 1.9 days, decaying via beta emission (with a broad spectrum of energies), with a mean energy of 0.23 MeV (max 0.81 MeV). 29% of decays release an additional gamma photon of maximum energy 0.103 MeV, which allows imaging of 153Sm distribution in the body. Beta particles have an average range of 0.6 mm in tissue.
Unbound 153Sm is excreted via the kidneys rapidly. Coupled with a short half life, this gives it improved radiation protection over 89Sr; however recommendations are similar. Patients should be discharged following one (or preferably two) voids. Similar guidelines to 89Sr exist with urination and catheterisiation.
Dosage and Administration
153Sm is administered intravenously. The administered activity is typically 37 MBq/kg. Like 89Sr, patients do not require hospitalisation and a flare of pain is possible.
153Sm is produced through neutron bombardment of 152Sm in a reactor.
Less commonly used sources
32P is another intravenously administered radiopharmaceutical, usually used for polycythaemia rubra vera or bony metastases. It decays by negative beta emission with a single energy of 1.71 MeV and range of 3.0 mm. The half life is 14.3 days. Notably, 32P has no gamma emissions. As an important element in biological processes, phosphorus has a poorer distribution than the more commonly used 153Sm and 89Sr isotopes. Haematological toxicity is more common.
90Y is a short lived byproduct of 90Sr decay, with a half life of 2.67 days. It decays through negative beta emission to 90Zr, with 99.99% of emissions containing a single energy of 2.28 MeV. It is used in embolising microspheres for liver metastases, or for treatment of severe rheumatoid arthritis of the knees. 90Y is harvested from 90Sr, from which it exists in a secular electronic equilibrium due to the massively different half lives.