A recent thread of mine.
My Mother Died - Miscellaneous and Personal Stuff I Must Share - Straight Dope Message Board
I remember an episode of PBS’s NOVA which details how certain cancers spread…metastasize…but the metastases essentially remain dormant due to a restrictor hormone from the main tumor. When the main tumor is neutralized due to surgery, chemo, or radiation, the loss of restrictor hormone stops holding the inert metastases in check, leading to a new outbreak of cancer.