Is stage 1 rectal cancer very serious?

It can be..: This is where the cancer does not go past the muscle layer. Surgery can cure this but there are risks. Surgery can be through the abdomen with or without surgery on the anus. The first way might need a temporary or permanent colostomy, an opening for expulsion of waste. The other way requires a permanent colostomy. Surgery may be done that just involves the anus, but that risks recurrence.

Yes: All colon and rectal cancers can be serious especially if not diagnosed or treated promptly, and good follow up is not maintained.