Suggest Treatment For A Purple Patch On The Scalp

my mother has a dark purple patch of skin on the occipital region of the scalp. This was noticed 6 weeks ago, and she knows this is an acute problem. She has no PMH such as illness, No known allergies and is on no Meds. The site on palpation does not change in colour, does not cause pain.
The mark is aprox. 5cm wide,

Do you have any idea what this could be

Thanks

Gary Earl