C++ How can I use std::string::find to be case insensitive?

Question

I am currently using string::find to search for words stored in a priority queue. How could i make my search case insensitive? This is my current code:

vector<TreeNode> matches;
for (int i=0; i<wordQueue.size(); i++) {
    if (wordQueue.top().key.find(a) == 0){
        matches.push_back(wordQueue.top());
    }
    wordQueue.pop();
}

This is TreeNode:

struct TreeNode {
    string key;
    int data;
}

wordQueue is a priority queue populated with TreeNode datatype:

priority_queue<TreeNode> wordQueue;
void holdWord(TreeNode t) {
    wordQueue.push(t);
}