Once the colon cancer is considered untreatable, it is usually categorized in the stage 4 category.
This is when the cancer has spread to adjacent lymph nodes and metastasized to the rest of the body.
The symptoms of colon cancer once it has reached the untreatable stage are similar to the ones one may have had during the "treatable" stage:
constipation
nausea/vomiting
cramping
thin stools
blood in stools
weight loss
And if the tumor is large enough, it will cause bowel obstruction - severe abdominal pain, and can lead to infarct (death) of the colon itself.
However, symptoms of the metastatic lesions are usually also apparent, and can affect liver function, lung function, the spinal cord, and the brain, depending on the metastatic lesions.
So the symptoms really depend on what part of the cancer is not-treatable.
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M.D. - Neurological Surgery