Where did this come from?

I've been obsessed lately with tracking the actual heredity of this malignancy. Obviously, I inherited it from my father who ostensibly inherited it from his mother. From a scientific point of view I can't state the latter explicitly because genetic testing was never done on my grandmother who died of carcinoma of the stomach in the early 60's. Nor can I empirically prove that HER mother, (my great-grandmother) who died in the first decade of the 20th century, at the age of 46, from what was anecdotally in the family called a "tapeworm" (recently discovered death certificate noted carcinoma), carried the gene mutation either. For some things I require a Harvard double-blind study to provide me with evidence. This is not one of them.
I know it's important for families to recognize this and prevent death when possible. But everything seems to link to descendancy: find those who may be at risk to negate needless suffering ... yes this I understand. But I'm currently faced with tracing this back beyond my great-grandmother, find HER siblings' descendants to warn them, and of course there's that irrational part of myself that wants to find the actual ORIGIN of the mutation. As if it is some kind of native or gypsy curse that I can have reversed with the wave of a properly greased etheric hand.