(Concern) Dense breast tissue found during health screening

The results from my health checkup indicated dense breasts.

In the biennial check-up, it sometimes appears in certain years and does not appear in others.

A few years ago, I had a breast ultrasound at a university hospital's obstetrics and gynecology department due to dense breast tissue.

I was relieved because I was told I was normal, but last year during a health checkup, I was told I have dense breast tissue again.

I'm curious and worried about why it changes every year, disappearing and reappearing.

I'm wondering how often I should get a breast ultrasound.