prostatic manual segmentation and mask extraction

Operating system: macOS Big Sur 11.2.3
Slicer version: 4.11.20200930

Hallo folks! Need your help!
I am working with a prostatic MRI data set, my boss is suggesting to extract a binary mask and to manually segment the prostate.
He affirms I can outline 5/6 slices and then extract a mask without filling them and merge them. Is this true? Is there any way on 3D slicer?
Thanks for the help!