Everyone who has cancer drinks water.
Therefore, water causes cancer.
It’s surprising how many people reason like this....
The way to do this is to amass an enormous amount of data about a large group of people who have brain cancer and look for unique things they have in common.... your cause may be found there. This study looked at just one possible cause for 29 years, not a good approach IMO.
There are multitudes of studies like that, and, in general, they are not very helpful. For one thing, at the threshold of significance--P-value < 0.05--one out of twenty statistically significant findings is just plain false. For another thing, those studies only look at statistical correlations; I think we are all aware by now that correlation is not causation. For example, if a study finds that obesity correlates with the incidence of a specific kind of arthritis, does it mean that obesity causes arthritis, that arthritis causes obesity, or that some underlying condition causes both?
Although many many physicians conduct correlative studies such as you described, they really are very poor science. While physicians love to publish these studies as if they are definitive, they are, at best, indications that there may be something worth researching further.