Lump Between Ribs

My daughter is 6 Yrs old. Recently we found lump on her body. 

We went for frontal and lateral radiorgaphs of the chest. it is showing report as.. 

There is a soft tissue lobbulated mass in the left mid zone with splaying and thinning of the left 5th and 6th ribs. Extrinsic impresssion on the ribs is also noted. Lateral view shows the mass lying posteriorly with displacement of the major fissuer as well. the heart and mediastium show no abnormality. the remaining lungs fields and costophrenic sinuses are clear. normal aorta. 

Comments :mass in the left hemithorax lateral and posterior in location with splaying and displacement of the ribs. This is most likely a neurofibroma. 

CT Scan Report Impression : Large, lobulated well defined, fat density lesion occupying both intra & extra thoracic compartments in left lateral chest wall, in inter costal location displacing & splaying adjacent left 5 ^th & 6 ^Th ribs. No evidence of calcification / nodular component / erosion of ribs. Benign lesion appears likely, most likely lipomatous lesion in left lateral chest wall. 

Doctor suggested biopsy... 

Please let us know like What to do ? And is it serious ? What are the treatments available?