# Cannot generate independent segments — Add always edits Segment\_1

**URL:** <https://discourse.slicer.org/t/cannot-generate-independent-segments-add-always-edits-segment-1/44823>\
**Category:** Support\
**Tags:** segmentation\
**Created:** [October 20, 2025, 4:16pm UTC](https://discourse.slicer.org/t/cannot-generate-independent-segments-add-always-edits-segment-1/44823 "2025-10-20T16:16:10Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Saliha\_Akcay](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/saliha_akcay/32/81282_2.png) [@Saliha\_Akcay](https://discourse.slicer.org/u/Saliha_Akcay)\
**Post date:** [October 20, 2025, 4:16pm UTC](https://discourse.slicer.org/t/cannot-generate-independent-segments-add-always-edits-segment-1/44823/1 "2025-10-20T16:16:10Z")

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Hello, I am working on lesion segmentation and need to extract a clean 2-mm peri-lesional bone shell for texture/fractal analysis.

The problem is: when I click “Add” to create a new segment (e.g. Segment\_2), Slicer still keeps modifying Segment\_1 only. Even if I manually select Segment\_2 and apply “Margin” (2 mm) or “Logical Operators → SUBTRACT”, Slicer always changes Segment\_1 — and Segment\_2 does not update at all.

What I have tried:  
• Set Application Settings → Segmentations → “Default overwrite mode” = Allow overlap  
• Manually selected the segment (Segment\_2) before applying Margin  
• Confirmed I am in the Segment Editor, not Segmentations  
• Tried both 2D slice views and 3D view  
• Restarted Slicer

Still, every operation affects only Segment\_1.

**My goal:**  
I want to duplicate the original lesion as Segment\_2, expand it by exactly +2 mm (Margin effect), then subtract the original lesion (Segment\_1) to isolate only the hollow peri-lesional shell.

What could still be forcing Slicer to always keep Segment\_1 active, even after adding and selecting a new segment?  
Is there any UI or masking setting I might still be missing?

If needed, I can provide a quick screen recording. Thank you in advance.

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**Author:** ![lassoan](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/lassoan/32/13_2.png) [@lassoan](https://discourse.slicer.org/u/lassoan)\
**Post date:** [October 20, 2025, 4:18pm UTC](https://discourse.slicer.org/t/cannot-generate-independent-segments-add-always-edits-segment-1/44823/2 "2025-10-20T16:18:52Z")

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Please provide more information. A few screen captures may be enough, but a screen recording video would be even better. You can also join the [weekly meeting](https://discourse.slicer.org/c/community/hangout/20) on one of the Tuesdays to share your screen, - we should be able to tell in a few minutes what is going on.