Metastable Technetium 99 (99mTc) is a radioactive isotope that emits gamma radiation. Its radioactive decay half-life is 6.0 hours. After radioactive labelling with sodium pertechnetate (99mTc) solution, the technetium (99mTc) sestamibi solution obtained is used in single photon emission computed tomography (SPECT) for the diagnosis of myocardial, mammary and parathyroid disease. 0.05% of the administered dose is excreted in breast milk (Leide 2016, Liepe 2016). Breastfeeding interruption or close contact avoidance times are calculated so that the infant is not exposed to more than 1 millisievert (1 mSv = 0.1 rem) of radiation (ICRP 2008, Howe 2008, Stabin 2000). An adult receives between 5 and 10 mSv annually from environmental radiation. The main regulatory agencies which manage radioactive substances and experts consider that breastfeeding can occur immediately after a diagnostic test with Tc 99m sestamibi (ARSAC 2016, ICRP 2008, Howe 2008, Stabin 2000, Rubow 1994 and 1991). Some agencies (ARSAC 2016, ICRP 2008) consider it more prudent to discontinue breastfeeding for about 4 hours, expressing breast milk and instead offering milk previously expressed and stored in a refrigerator prior to testing.Milk expressed after feeding can be frozen and used after 10 radioactive half lives: 10 x 6.0 = 60 hours = 3 days (Hale 2017, p.1919). Close contact with the infant need not be avoided (Mountford 1999).