Precision Oncology: How Genetic Testing Is Changing Cancer Treatment

Targeted therapy's main advantage is that it is mostly oral medication without the classic side effects of chemotherapy, so it remains a reasonable option even in poor performance status, though this depends on the specific drug and its side-effect profile rather than being a blanket rule. Two cases illustrate this: a metastatic lung cancer patient with brain metastases who was bedridden with severe headache and vomiting, and a metastatic breast cancer patient with cerebellar metastasis causing incoordination, both showed marked improvement on targeted therapy despite their poor baseline state. Overall, more patients in poor performance status can be treated with targeted therapy than with chemotherapy, since chemotherapy is often not feasible in a very poor clinical state.

FNAC (fine needle aspiration cytology) involves inserting a needle into a tumour or fluid-filled cyst and aspirating fluid along with a few detached cells for slide evaluation under a microscope; sensitivity is low since so few cells come through, and a pathologist can typically only confirm abnormal cells are present, not the exact cancer subtype needed for hormone receptor and HER2 testing. Core biopsy uses a biopsy gun that removes a cylindrical piece of tissue roughly a centimetre long, providing enough tissue for full histopathological and genetic workup, which is why core biopsy, not FNAC, is preferred whenever cancer is suspected.

Breast cancer is broadly split into three types: hormone receptor positive, HER2 positive, and triple negative. The diagnostic procedure itself does not change based on subtype: a biopsy is still required, with mammography used only for initial evaluation. IHC testing on the biopsy sample determines ER, PR and HER2 status; if all three come back negative, the cancer is classed as triple negative, a more aggressive subtype that is harder to treat once it reaches stage four.

Neither is simply better in every case. MRI is the more definitive test to rule out cancer, but mammography, a form of X-ray, can miss a tumour hidden in fattier breast tissue, which is common in women under 30 to 35. In younger women, ultrasound or MRI is preferred; mammography becomes more effective after around 35 to 40, once fibrous tissue outweighs fat. MRI is more accurate but costlier and more involved for the patient, which is why mammography is offered first, with a PET-CT or a CT of the chest, abdomen and pelvis used for staging.

Immunotherapy is highly effective and carries far fewer side effects than standard chemotherapy. In cancers with a large initial tumour burden, such as lung cancer, it is usually combined with chemotherapy at first, since immunotherapy alone takes time to bring a heavy disease burden under control. Once the disease responds, treatment continues on immunotherapy alone to maintain that response.

Around 20 to 40 percent of lung cancer patients carry some form of targetable genetic abnormality, most commonly an EGFR mutation, seen in roughly 10 to 35 percent of cases.

Chemotherapy acts on any rapidly dividing cell in the body, including bone marrow and hair follicles, while a targeted drug acts only on cells carrying the specific mutation it is designed against.

A repeat MRI at two months showed all six to eight of her brain metastases had disappeared and her lung lesion had shrunk; three years later she remains on the same medication and is doing well.