Was there any history of intense pain ? Have you ever got that checked? How long the cavity was there? If answer to these questions are yes and long time All likelihood you need RCT Rct is a type of filling , only difference the filling is done till root end!

A tooth which has a cavity and causes facial swelling in all probability will have to go for a root canal and definitely cannot be treated with just a filling. If however the facial swelling is not related to a decayed tooth but an erupting third molar or any other reason only in that case there is a possibility of the tooth being spared of a root canal . Generally the swelling related to a decayed tooth is because the bacteria have now gone from the tooth to the bone .

First have to examine a full mouth x-ray(opg) To confirm whether the swelling is due to cavity or due to wisdom tooth eruption or due to anyother infection, so based on that the treatment plan should be decided....

After Clinical examination an X- ray is taken for the decayed tooth after reading the X-ray and establishing the diagnosis and prognosis( chances of saving the tooth) then the treatment planning for the tooth is made which can be either a Root Canal followed by a Crown or Extraction of the tooth followed by replacement of the tooth.