usually the diagnosis of carcinoma requires a pathologic diagnosis. so, yes, you may find a subclinical tumor on ct/mri/xray/pet/etc, but to determine which treatment (surg/chemo/rt) is best for the patient, you need pathology.
one use of imaging is to depict the extent of cancer (loco-regional vs metastatic) which helps us stage patients. based on their stage, we have optimal treatment regimens for pts. this may include surgery, chemotherapy, radiation (either solo or a combination).
in radiotherapy, we use imaging to help us design our treatment fields. for exampe, for lung cancer, some people advocate the fusion of pet and ct scans to help us better delineate tumor. for gbm's, we fuse mri and ct scans to better delineate the tumor, edema and its volumes.
