Metastatic Colorectal Cancer - benign polyps

NOTE: PLEASE DISREGARD PREVIOUS POSTING
Short history: I'm a 57 year old female who was recently diagnosed with Stage 4 colorectal cancer (has metastasized in my lungs).  My CEA level was at a high of 372.5 at the 1st of September. After some initial chemo and radiation treatments, it has been reduced to its' current level of 113. My tumor, a mucinous adenocarcinoma, is pre-sacral and perirectal, measures approximately 11.5 x 11.5 cm; and constricts my rectum and bladder (urethra, esp). The first symptom I noticed was pain in my tailbone when sitting. (My other health history includes Barrett's Esophagus (from acid reflux) and Hypothyroid; also I had a complete hysterectomy due to endometriosis.)
According to the oncologist who performed my last colonoscopy (in July), the tumor did not originate in my colon or rectum. This colonoscopy, as well as two previous colonoscopies, produced a single benign polyp.

The following are findings about my primary tumor from three different CT scans:

Per CT scan of 5/21/09 (the first after discovering tumor):
"Large (11.4 x 11.3 cm) right perirectal mass seen. On this examination performed with rectal contrast, the intimate association of the mass with the lower rectum is more evident, and suspicion is for a rectal origin tumor. However, the differential diagnosis is rather broad, including: sarcoma, chordoma, and teratoma The patient gives a history of bilateral oophorectomy. Therefore, an ovarian origin is considered unlikely.
There is a prominent mass effect seen upon the rectum. No definite fat plane is seen between this mass and the rectum. The remainder of the visualized portions of the bowel are unremarkable, without definite wall thickening or caliber abnormality. No definitely enlarged lymph nodes are seen.
It is unclear if the biopsied mass connects with the rectum or other pelvis structures, or if it is freestanding."

Per CT scan of 7/01/09:
"Arising from the lower portion of the rectum there is a large complex cystic solid tumor displacing rectum to the left and the bladder anteriorly, measuring 11.4 x 11.3 cm. Some soft tissue compliance of this mass invades muscles of the pelvic floor. No definite vascular invasion is present. There is hyperdense sclerotic lesion in the right ischium measuring 4.3 cm in greatest size. This most likely represents metastatic lesion or changes of prior radiation therapy. No other definite bone metastasis is identified, however whole body bone scan may be performed to detect smaller lesions.
IMPRESSION:
1. Large rectal cancer lesion measuring up to 11.4 cm in greatest size.
2. Multiple lung metastases bilaterally.
3. Presumably metastatic lesion in the body of the right ischium. Bone scan is recommended for further evaluation."

Per CT scan of 9/15/09:
"A very large predominantly cystic rectal mass measures 11.5 cm in greatest axial dimension and appears similar to the previous study. In the rectosigmoid, there is new mural thickening and pericolonic stranding consistent with inflammation. Since the prior study, there has been an increased in the quantity of stool within the colon, indicating some degree of obstruction.
The spleen, pancreas, adrenal glands and kidneys have a normal enhanced appearance. There is a stable 5 mm low attenuation in the left hepatic lobe.
A small hiatal hernia is present. A lobulated mass in the posterior right lung base measuring 4.7 x 4.0 cm appears stable to the prior study.
Patchy areas of sclerosis involving the ischium on the right appears similar to the previous study, presumably representing metastatic lesions.
IMPRESSION:
1. Large rectal neoplasm appears stable in size, although there is some new wall thickening and pericolonic stranding at the rectosigmoid indicating inflammation with interval increase in quantity of stool indicating some degree of obstruction at this level.
2. Lobulated right lower lobe pulmonary mass appears stable. There is also a small nodule in the left lung base nearly one centimeter in size.
3. Right ischial lesion appears unchanged."

My questions:  How did I develop end stage colon cancer when at least two colonoscopies, performed within the last 5 years, tested clean?  How can they diagnose colorectal cancer when it didn't originate in the colon or rectum? As they have not determined exactly where my cancer originated, what difference, if any, does this make in my treatment? How important is it to know the exact origination point?  Could my Barrett's Esophagus, my thyroid condition, or my endometriosis have any bearing on my cancer?  Compared to other people's posts, my CEA levels appear to be through the roof.  Is this because my cancer is extremely advanced? Do you have any comments on anything else in the above? Thank you so much.

[Morecambe]

Reading through your posting I wonder whether the "science" is beyond the grasp of the mainly non-medical experts who participate in this particular forum.  Might it be an idea to re-post your question on Dr Pho's "ask a doctor" forum?

regards
Morecambe