An mri pathologist's report shows the presence of a liver tumor. a followup mri to be done in 2 months. would it be better to get a pet to compare?

Benign vs malignant?: Benign tumors /neoplasms of the liver are not cancerous; malignant tumors are. See prior answer regarding differential for benign ones. Malignant tumors include: fibrolamellar carcinoma, hepatoblastoma, cholangiocarcinoma, cystadenocarcinoma, angiosarcoma, undifferentiated sarcomas, epitheliod endothelioma. Metastatic liver tumors originate elsewhere (frequently from stomach, breast, lung, colon)

Liver mass: Liver "masses" are actually not uncommon and most of the time benign. There are certain characteristics of the lesion seen on MRI which give clues to what type of lesion is present. A PET scan would not help further characterize the lesion. It may help identify metastatic disease in a patient with known liver cancer.