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This is just an educated guess, however I found some backing on the Internet and in my Glossary.
Balg in a medical context means "skin". In terms of a cyst it means the tissue around the cyst cavity. I found references for both possibilities. And I suppose you can use them depending on the context. "Cyst cavity" literally means "Zystenhohlraum". It can often be used as a synonym for Zystenbalk (see first link).
The second link provides good examples for cyst cavity and cyst wall.
If both cavity and wall are not possible soultions, I would go for follicle: