I found some suggestions on Tumblr, so I´m just going to comment on my own post (hope that´s ok)

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Someone had the same question and got this answer:
- Gynaecomastia surgery
- Lung surgery
- Pectus excavatum or pectus carinatum
- Removal of harmless lumps/tumours
- Being vague, for example corrective surgery, or saying that you're okay now and prefer not to talk about it.
I think I will just tell them something about my sternum growing slightly outward and needing a correction for it. Hopefully I won´t disrespect anybody who actually has this (does not seem fun).