Hi! Had A Dental Xray 6/14- Showed A Black Shadow

Hi! Had a dental Xray 6/14- showed a black shadow on my right front tooth. Had a root canal with a follow up XR 12/14- getting better / dentist with another f/u in 6 months. 7/15- Had another XR & the shadow much bigger.
Dentist said it was a cyst & was removed & biopsy negative . Had a follow up XR & the shadow is still there in
Oct-,2015 much better as per dentist & I was d/c. My questions - how long will the shadow disappear? Do I go back to my regular dentist? It seems like I have frequent dental XR. What's the next step ? Txs.