Cutting/Trimming a 3D surface builts on BpreFile Function

Hello,
I have 3D cloud points that are annotated from computed tomography images.
In OpenCasCade, I want to do the following:

1. Make a 3D parametric surface that passes through all points with a predefined threshold.

2: After having the 3D surface, I want to trim in two steps. Initially, I want to remove some part of the surface using two curves on the produced surface.
I followed the instructions. However, for cutting, the surface got changed (II guess some of the control points are omitted). 

Are there any easy solutions?