There are many things that happen for the first time, many that happen for the first time in the field of medicine, and many thing that can happen for the first time in Canada (even if they happen somewhere else before then).
This was the first time this type of surgery was done in Canada, so it was "a" first. (The next day, another new surgery could have been done for the first time, making that another first.)
They referred to it as "a" first, because there are many other types of first that can exist.
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