Many people say that when they were diagnosed with colorectal cancer and heard talk of chemotherapy, they used to think everyone received similar drugs. But these days it is common to take a piece of tumor tissue and run a gene test first, before starting treatment. Even for the same colorectal cancer, the genetic characteristics the tumor carries are each different, so depending on those results, the drugs that work well and the drugs that have no effect diverge. The term targeted therapy sounds difficult, but put simply, it is treatment that picks out and blocks the specific switch the cancer cells use to grow.
The most frequently mentioned is the RAS gene, and among them KRAS and NRAS. If there is no mutation in this gene — so-called 'wild-type' — you can use a targeted drug that blocks a signal called EGFR. Conversely, if there is a mutation, this class of drug barely works, so it is not used at all. A single test ends up completely changing the drug choice. One patient who first heard this explanation said "the few days waiting for the test result were the longest," which shows how much this result governs the direction of future treatment.
The BRAF mutation also matters. It used to be known that having this mutation tends to make the course difficult, but these days, as combinations that aim at this mutation by pairing the drug with other drugs have come out, the options have increased. One more: there is also the angiogenesis inhibitor class, which blocks the path that builds new blood vessels to supply nutrition to the tumor. This can be used regardless of whether there is a RAS mutation, so it remains a treatment card whichever way the gene result goes.
What has drawn particular attention recently is MSI-High, that is, the type with high microsatellite instability. Such tumors tend to respond well to immunotherapy, so a path different again from targeted therapy opens up. Because of this, more hospitals now check this part together at the first test. When HER2 is highly expressed, a drug that aims at it separately may also be considered. In the end, the picture is to first look at 'what hand my tumor is holding' and choose the weapon to match it.
What actually confuses patients is that targeted therapy is not a magic drug without side effects. The EGFR class commonly causes skin rash and hand-foot trouble, and with angiogenesis inhibitors you have to watch blood pressure and bleeding. Still, because the side-effect pattern differs from ordinary cytotoxic chemotherapy, quite a few people receive treatment while maintaining daily life if they know and prepare in advance. If the tissue is insufficient, there is also a method of testing with blood, so there is no need to hesitate because the test feels burdensome.
What is written here is put together to help set the big framework, and the drug that suits your own tumor's gene result and body condition should be decided in direct consultation with your attending physician.