Bone-seeking radiopharmaceuticals (radionuclides) occupy a valuable niche in the palliation of painful bone metastases.

Isotopes:
- 89Sr (strontium-89)
- 153Sm (samarium-153)
- 32P (phosphorus-32)

They work by binding with high affinity to hydroxyapatite in regions of rapid bone turnover near osteoblastic metastases, delivering therapeutic doses of localized beta radiation, with a tissue penetration measured in millimeters.
The precise mechanism of analgesia is unknown but is probably not dependent solely on cell kill. Rather, analgesia may also be a function of inhibition of lymphocyte-associated cytokines or alterations in osteoclast and/or osteoblast activity.
Analgesia may begin within 3-7 days, but more typically begins within one to two weeks after administration. Analgesia will last from 2 to 6 months; treatment may be repeated.
Symptom improvement is noted in 60-80% of patients, with complete analgesia in 20-30% of responders. Radiopharmaceuticals may delay onset of pain in preexisting, clinically silent metastases.

Procedure:
The radiopharmaceutical is delivered in the outpatient setting by a single IV injection or orally (32P only). Administration requires no special monitoring.

Patient selection:
Multiple painful bone metastases, demonstrated by bone scan and/or plain X-ray, corresponding to site(s) of pain and an expected survival of >12 weeks. Evidence supporting efficacy in prostate and breast cancer is substantial; data for other tumor types is limited.

Contraindications:
- preexisting myelosuppression (WBC < 3.0 K and PLT < 60-100 K)
- oncological urgencies/emergencies (in which radiopharmaceuticals will be of no benefit), e.g. actual or impending spinal cord compression or pathologic fracture
- renal insufficiency (relative contraindication)
- evidence of disseminated intravascular coagulation (relative contraindication)
- pregnancy

Adverse effects:
- Marrow suppression. Reversible, moderate neutropenia and thrombocytopenia manifested by approximately 30-70% drop in leukocyte and platelet counts is a predictable side effect. Depending on the specific agent this begins 2 to 4 weeks following administration, with a nadir between weeks 4 to 6; bone marrow recovery occurs by weeks 8 to 12.
- Pain flare. Increasing pain occurs in 10-20% of patients, usually within the first week of administration. It is transient and may be predictive of a good therapeutic response.

Adapted from Reisfield GM and Wilson GR. Palliative Care Network of Wisconsin. Fast facts and concepts #116. Radiopharmaceuticals for painful osseous metastases. Internet. Accessed on January 25, 2016.