Converting region $\int_{0}^{2} \int_{0}^{x} f(x,y) dydx$ from rectangular form to polar form

Question

How can I convert the region in

$$ \int_{0}^{2} \int_{0}^{x} f(x,y) dydx $$

(which is basically a right triangle contained in the first quadrant) to a polar form? Im sure that

$$0 \leq \theta \leq \frac{\pi}{4}$$

but what should one do about the radius range?

Thank you.