Thanks for providing more information.
As you describe possible causes may include Folliculitis(inflammation of hair follicle),Sebaceous cyst,Seborrheic dermatitis.
Usually cancer manifests with firm lump however as you have sebaceous carcinoma so this lump can be associated with it.
As you scratch it so it prevent it to heal properly.
So you should apply OTC Neosporin lotion or Betonil lotion on it.
Sometimes oral antibiotics can be advised.
Avoid scratching it.
If your symptom'll not respond to neosporin then a clinical examination by your regular doctor for further evaluation and management.
Further a referral to dermatologist and biopsy should be done.
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