Why Cetuximab Only Works in — RAS Wild-Type, Left-Sided Tumours

No. Being RAS mutant rules out cetuximab and the other anti-EGFR drugs, but several effective treatments remain. Chemotherapy regimens, bevacizumab-based combinations, and — where your tumour shows specific features such as microsatellite instability — immunotherapy or targeted agents may all be options. Your oncologist will explain which of these pathways fits your results. Being ineligible for one class of treatment is information that helps point toward what is most likely to work, not a closed door.

BRAF is another gene in the same growth-signalling chain as RAS. A specific change called the BRAF V600E mutation is found in a proportion of colorectal cancers. When present, it indicates the tumour is unlikely to respond well to anti-EGFR therapy even if RAS is wild-type. It also opens a different set of treatment options — specifically targeted BRAF and MEK inhibitor combinations that have shown benefit in BRAF-mutated colorectal cancer, according to NCCN guidance. Testing for BRAF alongside RAS gives your oncologist a fuller picture before recommending any regimen.

Yes, and this is a known limitation of testing done on the original biopsy. Tumours can develop new mutations as they evolve, and a small population of cells carrying a RAS mutation that was undetectable at first can become the dominant population after prolonged exposure to anti-EGFR therapy. This is sometimes called acquired resistance. If cetuximab stops working after a period of response, your oncologist may recommend repeat testing — sometimes on a liquid biopsy rather than a tissue sample — to look for mutations that may have emerged since treatment began.

The division is at the splenic flexure — the bend in the colon near the spleen. Tumours from there leftward, meaning the descending colon, sigmoid colon, and rectum, are considered left-sided. Tumours to the right of that point — the caecum, ascending colon, hepatic flexure, and most of the transverse colon — are considered right-sided. The original colonoscopy or surgical report describes the exact location. If you are not sure which side applies to you, ask your oncologist to confirm using the staging records.

Occasionally, in specific circumstances, and as part of a shared decision that weighs the very limited expected benefit against the side-effect profile. ESMO and NCCN guidance recommends against routine use in right-sided primary tumours based on the available evidence, but clinical situations are not always straightforward. If your oncologist is discussing cetuximab for a right-sided tumour, ask them to explain the specific reasoning for your case. A second opinion from another oncologist is a reasonable step when you are uncertain about the recommendation.

Usually one to two weeks from when the laboratory receives a usable tissue sample. If your original biopsy was taken some time ago, the stored block is generally still suitable for testing and a new biopsy is not automatically needed. Occasionally the sample is too small or degraded, in which case a repeat biopsy may be required before the question can be answered. Ask your oncologist when the sample was sent and when the result is expected so you have a realistic timeline rather than waiting without a clear date.