Can you tell me about hickman line in there chest the same time as having a mastectomy?

It is for IV access. Hickman or other central line catheters are used for venous access when intravenous medication is recommended and peripheral line is not adequate. Central line is placed via the subclavian or Jugular vein and the catheter tip is advanced to superior Vena Cava which is one of the two large veins returning blood to the heart. In your case I suspect you are a candidate for chemotherapy.
Same side? The Hickman is in a vein, frequently the subclavian, and the surgical field can likely be adapted to avoid that area. Local infection can be a concern.