Radiation is not a very selective poison. It will cause all kinds of cancers, not specifically rare brain tumors. In particular, leukemia is infamously related to exposure to fission products, which we know from studies on survivors of the attacks at Hiroshima and Nagasaki. I actually checked the notes (hah!) I have from radbio and brain cancer didn't make the top 5 (resp. leukemia, skin, breast, bone, lung), though it certainly can happen.
This. Radioiodine can cause thyroid cancer but other than that when you're looking at a cluster of rare cancers don't look at radioactivity.
On the other hand, we have had many cancer clusters that make the most sense as if they were infectious agents. Now, cancer is obviously no infectious--but we already know HPV is responsible for a lot of cancer, including strains that produce no symptoms. It's quite reasonable to think there might be other viruses that are carcinogenic but otherwise asymptomatic and thus staying below the radar.
Indeed. "If the radiation dose is low and/or it is delivered over a long period of time (low dose rate), the risk is substantially lower because there is a greater likelihood of repairing the damage. There is still a risk of long-term effects such as cancer, however, that may appear years or even decades later."
