That's a really good question I've asked myself many times. First, how did I get it with nil family cancer history limited to non-TC's in distant relatives in their 80's. Second, how likely is that my 3 younger brothers and 8 nephews on my mother's side will get it? My two sons are adopted so they fall into the general population of random events.
I have read everything I can find on this, and call upon college courses in anatomy, physiology, embryology, genetics, statistics and probability. All cancers have as a root cause a cellular mutation, so the question is what causes that? And with TC, there are so few cases relative to the population that it's difficult to build a statistically significant (meaningful) case for any one cause. TC is probably caused by several things.
In my own mind, I lump cancer causes into 5 main catagories:
-Genetic or chromosomal defect. This is much more evident in many female cancers, but there are enough cases of father-son, brother-brother, uncle-nephew, and cousins that this is somewhat likely for TC.
-Environment. Exposure to chemicals which either by themselves are carcinogenic or when combined get through the blood-testis barrier or pass from mother to fetus (called teratogenic).
-Radiation. Radioactive isotopes many of us or our parents have been exposed to including atomic bomb test fallout, water and air radon, and industrial waste. Could also be electromagnetic but many of these have been researched and ruled out (so far) including radar guns, microwave ovens, cell phones, high voltage transmission lines, and TV and monitor screens.
-Viral. Some types of cervical cancer, for example, but never heard this associated with TC.
-Completely random event not caught in time by your immune system.
I believe my TC was triggered at my somewhat high-than-average age by long-term exposure to a huge variety of chemicals starting with my chemistry set when I was 7 right through industrial plants I've been through, insecticides, pesticides, herbicides, electronics PCB's, paints, stains, cleaners, 10 years as a volunteer firefighter, and on and on. Lots of those chems have been banned. It could also be that my mother was exposed at some point and she passed down an egg with a mutation. (She was an electrical engineer and worked with our share of German scientists after World War 2 for several years until conceiving me. God knows what they worked with).
That leaves purely genetic. I have found a couple of very limited studies that show links to a gene on an X-chromosome (female side). If true, are these also caused by environmental factors or are they truly random events? Who knows. Very sketchy data. In any event, this would factor into brothers, nephews, and cousins, not father-son.
So how about facther-son? I have not come across any research that points to a genetic defect on a Y-chromosome. That would steer me to believe that father and son(s) where exposed to the same or similar chemicals or elements. From where? A partial list might include agricultural, household, industrial emissions, water or air radon, landfill, and public or private water supplies.
Bottom line is that the stats say that sons, brothers, and nephews are 4-8 times as likely to get TC as the general population. But that's still a very low probability and undoubtedly reduced if living in different areas or having eliminated the exposures.
I don't know if this helps or not. It is rambling, but this was a good excuse to get some thoughts onto paper, so to speak.