The various possibilities of a round itchy lesion is varicose dermatitis (occurs over a varicose vein),
discoid eczema (this may be both the dry or wet type), asteatotic
eczema or a variant of discoid eczema over dry skin, gravitational eczema, atopic dermatitis, contact dermatitis (if you work outdoors or somewhere where some chemical or plant or any thing else comes in contact with your arm), or even psoriasis and tinea corporis (a fungal infection).