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Uterus has three layers- endometrium, myometrium and perimetrium.
A mass in the middle layer is what you are mentioning.
It could be a fibroid. It can cause pain , bleeding and infertility. It has to be removed if symptomatic.
MRi will show whether it is fibroid.
The one in the myometrium has got more chances of becoming malignant.
malignancy may be als be diagnsoed with MRI.
Since MRI does not show any malignant changes, no need to worry.
But since malignant transformation and other complications can occur, it has to be removed.