Anyone, please clarify me with thyroid cancer?

my brother was diagnosed with ectopic growth of thyroid tissues all over his body blocking his spine which paralyzed him waist down. He was treated with radioactive iodine(RAI) and recovered, walk on his own for two or more months, then it happened again. Now he's bedridden and were trying to battle bedsores. was it really thyroid tissues? how can this tissues be not in their proper place? has this something to do with genes? was this acquired at birth? and why is RAI so expensive? I was adviced that if we can have another dose of RAI we might totally erradicted the tissues and recover my brother...

I am not a medical doctor nor an expert in this rare condition. I do medical physics research and do know about radiation therapy, however, so here's my best attempt to answer your questions.
The problem actually occurs *before* birth. Thyroid tissue can grow where it doesn't belong (which is what "ectopic" means) due to problems when we are still embryos. Imagine all the different tissues that later turn into different organs all lying next to each other, growing and moving into position, so unfortunately sometimes they end up where they shouldn't. Most cases of ectopic thyroid affect the throat, such as the thyroid growing into the base of the tongue.
The thyroid gland loves iodine, which it uses to make thyroid hormone. Yet no other part of your body can use iodine. This unusual fact makes it possible to use radioactive iodine to kill thyroid tissue. It's like a heat seeking missile, goes straight to the thyroid and nowhere else. That's why it is so effective and yet causes no side effects. Nothing is perfect, however. That means that in some people (as much as 25%), some thyroid tissue survives and grows back. That's why repeat treatment may be necessary, sometimes even 3 times.
As for the cost, well I guess that's a question for the drug and insurance companies. As you can imagine, it's not cheap to make, transport, store, and handle anything that is radioactive.
Good luck to your brother and your family.

I had thyroid cancer but much different to your brother. It spread only to a lymph node, and I had both the lymph node and the thyroid surgically removed. When thyroid cancer spreads to other organs it still thyroid cells.

I think Radioactive iodine needs to be ship from one of the few places it is made. Shipping it can be expensive. People with cancer may need several treatments to get kill all the thyroid cells. I had to go on a low iodine diet before treatment, so the thyroid cells absorb the radioactive iodine and kill it. Thyroid tissue is the only tissue that makes use of iodine.