You need to subtract the sinus (to go from the center "upwards" on the UI canvas):
endPoint.X = center.X - (radius * Math.Sin(radians));
Origin 0,0 is Top Left corner, not bottom left.
[Edit] Oh and you are confusding x and y: think x is horizontal coordinated and y is vertical, so this is wrong:
center.X = top + (height / 2);
center.Y = left + (width / 2);
and this is wrong:
endPoint.X = center.X + (radius * Math.Sin(radians));
endPoint.Y = center.Y + (radius * Math.Cos(radians));
Corrected:
center.Y = top + (height / 2);
center.X = left + (width / 2);
and (with the subtraction fix I mentioned)
endPoint.Y = center.Y - (radius * Math.Sin(radians));
endPoint.X = center.X + (radius * Math.Cos(radians));