Jan says:
This recommendation puzzles me just a little bit. Tom Hale suggests that
waiting 5 half lives assures that the substance has virtually disappeared from
the milk, but you are saying that waiting 2 half lives is sufficient? That
would be much nicer for moms, but is this only for Tc99m or would it be for all
radioactive substances?
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I stand by the recommendation of the ICRP, they are the maximum authority in radiation protection. However note that their recommendation is related to the time before it is advisable to resume breastfeeding. Tc99m not only leaves the body due to decay (physical half life, the one equal to 6hs) but also because it is excreted by the body.  This last mecanism of "decay" also contributes and makes the actual "lifetime" of Tc99m in the body shorter. I assume that the 12 hours window for breastfeeding is related to that. Also, the amount of Tc99m used for diagnosis is very small (I don't know the exact amount). ICRP recommendations for other radioisotopes are very restrictive: in the case of some Iodine isotopes, they say breastfeeding should be stopped altogether.

However, I think that your reasoning regarding the 1/2 life and stored milk is right. It may be reasonable keeping the milk for longer than 2 half lives, since you don't have the biological component of removal working for you here. I defer to Dr. Hale's advice in the case of pumped milk. But still, I do believe that the milk can be safely stored and used later.

In the case of feeding milk directly from the breast, I think that the ICRP advice is sound and breastfeeding can restart 12 hours after the medical procedure.

Regards,

Veronica Garea
Grupo de Apoyo a la Lactancia Materna de Bariloche

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