Why is biopsy needed for discolouration and swelling of lower leg that has nodules under the skin? how much chance that it might be cancer?

Rule out...: My first concern is leukocytoclastic vasculitis. I trust this is not kaposi's, but there are many other concerning entities that a biopsy is required to diagnose. Talk about the results with your physician.

See your derm: Any new lesions involving your skin should be brought to the attention of your friendly dermatologist.

To determine its cau: There could be many causes of these lesions. A biopsy can define what it is.