Colorectal Cancer & CT Colonography
- Colorectal cancer is, in the imaging sense, an apple core: a tumor that grows around and through the bowel wall, narrowing the lumen.
- CT colonography (CTC) is a CT scan of an air-inflated, cleaned-out colon — a "virtual" walk-through used to find polyps and cancers, not to stage them.
- Staging (the T/N/M of how deep, how far, and where it's spread) is a job for the regular contrast CT chest/abdomen/pelvis and, for rectal tumors, pelvic MRI.
- The whole point of screening is to catch a polyp before it ever becomes the apple core.
Here is the slightly grim but genuinely hopeful thing about colorectal cancer: most of it starts as a harmless little lump — a polyp — that sits around for years quietly deciding whether to misbehave. That long fuse is exactly why we screen. Find the lump, snip the lump, and the cancer that would have happened simply never does. Imaging gets to be the hero here, which is a nice change from the parts of radiology where we mostly confirm that something has already gone wrong.
The apple core, and why the bowel makes one
A colorectal cancer typically starts in the lining and then grows in two directions at once: around the inside of the tube and outward through the wall. As it circles the lumen, it builds a tight, irregular ring. On a barium study or a CT you get the classic "apple core" lesion — a short segment where the channel is squeezed down to a ragged core with abruptly overhanging edges (radiologists call them "shouldered" margins), like someone took a bite out of the bowel from all sides.
That narrowing is also why the disease announces itself the way it does: change in stool caliber, obstruction, or bleeding. A right-sided tumor sits in a wide, watery part of the colon and tends to bleed quietly (anemia) before it ever blocks anything; a left-sided one is in a narrower tube with firmer stool, so it strangles the lumen and obstructs sooner.
CT colonography: a guided tour of a very clean colon
CT colonography — also called virtual colonoscopy — is what happens when you take a normal screening idea and hand it to a CT scanner. The patient does a bowel prep (the part nobody enjoys), the colon is gently inflated with gas through a small rectal tube so the walls spread apart, and then we scan, usually in two positions (back and belly-down) so anything that's just fluid or stool sloshes to a new spot while a real polyp stays put.
The radiologist reads it two ways: a 2D scroll through the slices and a 3D fly-through, where software builds a little endoscope's-eye view and you cruise down the lumen like a tiny submarine looking for bumps on the wall. It is exactly as fun as it sounds, and also genuinely useful.
CTC is a detection tool. It finds polyps and masses; it cannot biopsy them and it does not see the depth of invasion or distant spread. A clinically significant lesion on CTC sends the patient onward — typically to optical colonoscopy for tissue, or to staging imaging if it already looks like cancer.
Retained stool is the great impersonator on CTC. The two tricks that unmask it: it usually moves between the supine and prone scans (real polyps are anchored), and "tagged" stool — left deliberately dense by an oral contrast prep — lights up bright white, while a true polyp stays soft-tissue gray. If a "polyp" is mobile or glowing, be suspicious.
Staging: depth, nodes, and the far-away stuff
Once it's cancer, the question shifts from is it there to how far has it gone. We answer that in the language of TNM staging: T for how deeply the tumor invades the wall and beyond, N for lymph nodes, M for distant metastases. The liver is the favorite landing spot for colorectal spread, followed by the lungs, so a staging workup is a contrast-enhanced CT of the chest, abdomen, and pelvis.
Rectal cancer gets a special guest: high-resolution pelvic MRI. The rectum lives in a tight neighborhood with a thin fatty envelope called the mesorectum, and the surgeon desperately wants to know whether the tumor reaches the edge of that envelope — the circumferential resection margin. MRI shows those millimeters in a way CT simply can't, so it drives whether the patient gets chemoradiation before surgery.
| Question | Best test | What it answers |
|---|---|---|
| Is there a polyp/cancer? | CT colonography or colonoscopy | Detection, screening |
| How deep / any spread? | Contrast CT chest/abdomen/pelvis | T, N, and M (especially liver, lung) |
| Rectal tumor margins? | High-resolution pelvic MRI | Depth, mesorectal margin, local nodes |
A tumor that obstructs the colon can hide a second one upstream that the scope (and a partly distended CT) never reaches. After an obstructing cancer is resolved, the colon proximal to it still needs a full look — synchronous lesions are a known trap.
The one thing to carry out
Colorectal cancer is a slow story with an early, beatable chapter. Imaging's job splits cleanly in two: find it (CT colonography or colonoscopy, hunting that apple core or the polyp that precedes it), then map it (CT for the body, MRI for the rectum). Get the order straight and the rest of the workup almost writes itself. And remember the lesion that obstructs can mask one hiding further up the tube — the same shoulder-margined trick that fooled us once can fool us twice. If you want the broader picture of how a colon strangles itself, the bowel obstruction page is next door.