MathNet 2014 Question 43

Let $P$ be outside a circle $\mathcal C$. The tangents from $P$ touch it at $A,B$. A line through $P$ meets it at $Q,R$. Let $S$ be a point on $\mathcal C$ such that $BS\parallel QR$. Prove that $SA$ passes through the midpoint of $QR$.