Complex sentence: Extramammary Paget's disease is usually seen in isolation and is not usually associated with an underlying invasive malignancy, unlike Paget's disease of the breast, which is almost always associated with an underlying invasive malignancy, i.e. breast cancer (e.g. mammary ductal carcinoma).
Split into simpler sentences: Extramammary Paget's disease is usually seen in isolation and is "not" usually associated with an underlying invasive malignancy. This is unlike Paget's disease of the breast, which is almost always associated with an underlying invasive malignancy, i.e. breast cancer (e.g. mammary ductal carcinoma).