When preparing the 6th edition of Intermediate Physics for Medicine and Biology, I like to scan the literature for new medical advances. While revising the chapter on nuclear medicine, I found some fascinating information about an isotope that was not mentioned in the 5th edition of IPMB: lutetium-177.
First, the physics. Lutetium (pronouced loo-tee-shee-uhm) is element 71 in the periodic table. Below are the energy level and decay data. The primary mechanism of decay is emitting a beta-particle (an electron), transmuting into a stable isotope of hafnium. The maximum energy of this electron is about 500 keV. Two other possibilities (each happening in about one out of every ten decays) is beta decay of 177Lu to one of two excited levels of 177Hf followed by gamma decay. The two most common gamma photons have energies of 113 and 208 keV. Lutetium-177 produces few internal conversion or Auger electrons. The average energy of all the emitted electrons is about 150 keV, which have a range of about 0.25 mm. The half-life of 177Lu is roughly a week.
Next, the biology and medicine. Lutetium can be used for imaging (using the gamma rays) or therapy (using the electrons). While the dose arising from all the electrons does not make this isotope ideal for pure imaging studies (technetium-99m might be a better choice), the gammas do provide a way to monitor 177Lu during therapy (in this way it is similar to iodine-131 used in thyroid cancer therapy and imaging). Such a combined function allows the physician to do “theranostics” (a combination of therapy and diagnostics), a term I don’t care for but it is what it is. 177Lu can be bound to other molecules to improve its ability to target a tumor. For instance, it is sometimes attached to a molecule that binds specifically to prostate specific membrane antigen. The PSMA molecule is over-expressed in a tumor, so this allows the 177Lu to target prostate tumor cells. One advantage of using 177Lu in this way—rather than, say, using radiotherapy with x-rays directed at the prostate—is that the 177Lu will seek out and irradiate any metastasizing cancer cells as well as the main tumor. Clinical trials show that it can prolong the life of those suffering from prostate cancer.
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