Expansion
Testing that determines which targeted treatment will work
Expansion
| Marker | Tumour | Determines |
|---|---|---|
| HER2 | Breast, gastric | Trastuzumab and related agents |
| ER and PR | Breast | Endocrine therapy |
| EGFR mutation | Lung adenocarcinoma | Tyrosine kinase inhibitors |
| ALK and ROS1 rearrangement | Lung | Targeted inhibitors |
| KRAS or NRAS mutation | Colorectal | Predicts failure of cetuximab |
| BRAF V600E | Melanoma, colorectal | BRAF and MEK inhibitors |
| BRCA1 and BRCA2 | Ovarian, breast, prostate | PARP inhibitors, and family screening |
| MSI or mismatch repair loss | Colorectal, endometrial | Immunotherapy, and Lynch syndrome screening |
| PD-L1 expression | Lung, others | Checkpoint inhibitors |
| 1p/19q codeletion | Oligodendroglioma | Chemosensitivity and prognosis |
| Philadelphia chromosome (BCR-ABL) | CML, ALL | Imatinib and successors |
Predictive versus prognostic
- Predictive: tells you whether a particular treatment will work. HER2, EGFR and RAS are predictive
- Prognostic: tells you the likely outcome regardless of treatment. Stage and grade are prognostic
Some markers are both, which is a frequent source of confusion in reporting.
Universal testing now applies to some markers regardless of family history: mismatch repair testing in all colorectal and endometrial cancers, because it identifies Lynch syndrome, which has implications for surveillance and for the patient’s relatives.
Liquid biopsy, detecting circulating tumour DNA in plasma, is increasingly used where tissue is unobtainable, for resistance mutations at progression, and for monitoring minimal residual disease. A negative result does not exclude the mutation, since shedding varies, so tissue remains the reference.