Cancer doesn’t run in my family or my husband’s. Only one of my ancestors had it: my paternal grandmother died of lung cancer when I was 11. But she was 64 and a long-time smoker, so her diagnosis didn’t really come as a surprise.

What did come as a surprise was my own metastatic melanoma diagnosis in August 2013 — and my daughter’s metastatic osteosarcoma diagnosis just one year later. I was 42 when I learned that I had cancer...