Physics

Radiopharmaceuticals

Strontium-89

  • St-89 is calcium analog that preferentially localizes to osteoblastic areas
  • Most effective for metastatic lesions from prostate and breast cancer.
  • decays to yttrium-89 principally by radioactive beta-particle emission
  • Sr-89 has a long therapeutic effective half-life, which may nearly equal its physical half-life of 50.5 days
  • repeat dose can be administered at 3-month intervals if necessary
  • Painful flair episodes occur in 10-20% of the patients

Samarium-153

  • Sm-153, however, is the only agent in its class targeted to bone via chelation to the aminotetraphosphonate EDTMP (ethylenediaminetetra- methylenephosphonic acid).

Characteristics of Radiopharmaceuticals for Bone Metastasis [1], [2]

Radioisotope Strontium-89 Samarium-153
Characteristics
beta energy 1.46 MeV 0.81 MeV
gamma energy -- 0.103 MeV
maximum range (mm) 7.0 2.5
Half-life (days) 50.0 1.9
Response Rate (%) 70-80 65
Clearance from blood Rapid by Renal 6 h by Renal
Indications prostate, breast and lung prostate, breast, lung, and osteosarcoma
Results
Preference for Mets 10 x 5 x
Response Time (days) 7-20 7-20
Duration of Response (mo) <= 6 3-4
Side-effects
Extent of Myelosuppression 1+ 3+

Characteristics of Radiopharmaceuticals for NHL

Trade Name Bexxar Zevalin
Generic Name Tositumomab Ibritumomab
Mechanism
Radioisotope Iodine-131 Y-90
Receptor binds to the CD20 antigen binds to the CD20 antigen
Characteristics
beta energy low-energy beta 0.75-0.94 MeV
maximum range (mm) 5
Half-life (days) physical 2.67
Indications refractory NHL refractory NHL
Complications
Dose limiting severe thrombocytopenia and neutropenia
Side-effects hypersensitivity reactions, including some with fatal outcome cutaneous and mucocutaneous reactions, some with fatal outcome
Others Deaths have occurred within 24 hours of Rituximab infusion which Zevalin uses

Radioisotopes

Emittion (not decay) half-life MeV use
Radium-226 Gamma 1622 years 0.83
Cesium-137 Gamma 30 years 0.662 GYN implants
Gold-198 Gamma 2.7 days 0.412
Iridium-192 Gamma 74 days 0.38 HDR machines
Iodine-125 Gamma 60 days 0.03 prostate
Palladium-103 Gamma 17 days 0.02 prostate
Strontium-90 Beta 29.8 years but biological half-life is much shorter 0.54 (2.27) Eye/IVBT

Clinical Radiation Generators

Contact Therapy (Endocavitary machine)

  • 40 to 50 kV
  • SSD of 2.0 cm or less
  • 0.5 to 1.0 mm aluminum is used to harden the beam.
  • useful depth of 1 to 2 mm.
  • completely absorbed by 2 cm of soft tissue.

Superficial Therapy

  • x-ray produced at potentials range from 50 to 150 kV.
  • 1-6 mm of aluminum is used to harden the beam.
  • SSD is between 15-20 cm.
  • useful depth of 5 mm (~90% depth dose).

Electron Therapy

energy loss rate of electrons

in water 2 MeV/cm
in lead 2 MeV/mm
Bibliography
1. Friedland J. Local and systemic radiation for palliation of metastatic disease.
Urol Clin North Am. 1999 May;26(2):391-402, x. Review. PMID: 10361561
2. Ilora G Finlay, Malcolm D Mason, Mike Shelley. Radioisotopes for the palliation of metastatic bone cancer: a systematic review. Lancet Oncol 2005; 6: 392–400.
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