By Dr. Tarang Lad

Why Cancer Genetic Test Reports Differ - And Why That's Okay

Cancer is often thought of as a single disease, but in reality it's a collection of over 200 different conditions. What unites them is a problem in our genes. Genes normally control how cells grow, repair damage, or even die when necessary. But when harmful changes occur in these growth-controlling genes, cells can begin dividing out of control, leading to tumors. This is why genetic testing has become such an important tool. By analyzing tumor DNA or inherited variants, these tests help identify mutations linked to how a cancer begins, how it behaves, and how it might respond to treatment.

People are often surprised when they see their genetic test reports, because the reports don't always look the same. Sometimes they even list different genes for the same person or cancer type. This isn't an error—it's a reflection of cancer's complexity. Each cancer type has its own unique genetic fingerprint. For example, breast cancer often involves BRCA1, BRCA2, or TP53 mutations, while lung cancer frequently shows EGFR, KRAS, or ALK mutations, and colorectal cancer is often linked to APC, MLH1, or KRAS. Each organ's cells rely on different genes for their daily functioning, so when those genes break, cancer develops in that tissue. This is why breast cancer and lung cancer do not show the same set of genes, just like cars and trucks have different parts that can break down.

The type of test being done also affects which genes appear. Tumor sequencing looks at mutations inside cancer cells, germline testing looks for inherited mutations passed down in families, and liquid biopsies detect tumor DNA floating in the blood. Each approach sees a different slice of the picture. On top of that, testing panels vary—some labs check 50 genes, others check 500 or even more—so naturally the reports differ. The purpose of the test matters too. A diagnostic report will highlight genes that caused the cancer, a treatment report will focus on genes that can guide targeted therapies, and a risk report will look at inherited variants that increase the chance of future cancers.

Cancer also evolves over time, so the same person can receive different results at different stages. An early-stage lung tumor might only show an EGFR mutation, but after treatment, a new resistance mutation like T790M might appear. This explains why repeat testing is often recommended, because the genetic profile keeps shifting as the disease progresses or adapts to therapy.

Genetic differences between populations also shape the reports. Some mutations are more common in certain groups—EGFR is frequent in East Asians with lung cancer, BRCA1/2 mutations are more common in Ashkenazi Jewish women, and in India, TP53, PIK3CA, BRCA1/2, EGFR, KRAS, and HRAS are frequently reported because of a mix of genetic predispositions, lifestyle exposures like tobacco and betel nut, and infections such as HPV.

On top of all this, research and technology are constantly moving forward. New cancer genes are being discovered, advanced sequencing methods are catching mutations missed in the past, and clinical trials are adding more genes to the list of those that matter for treatment. Reports also separate driver mutations, which directly fuel cancer, from passenger mutations, which are more random and less important. Sometimes reports focus only on the drivers; sometimes they include passengers too, depending on the scope of the test.

So when you see differences in genetic test reports, it’s not a mistake. It’s a reflection of the complexity of cancer, the purpose of the test, the timing of the disease, and even your ancestry and environment. These reports are not just lists of genes—they are tools that help guide doctors in tailoring the best possible treatment, prevention, and risk prediction. As science advances, these tests will only become more accurate, giving patients more power in the fight against cancer.