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One Tumor Biopsy May Be Insufficient to Reveal Genetic Landscape

Posted Mar 09 2012 12:00am

We are about to undergo a major change in tumor genotyping for cancer patients, possibly even before the basic protocol is broadly adopted. Not only will the genotype of a tumor be required to initially determine appropriate chemotherapy (see:  Identifying Therapeutically Relevant Genetic Abnormalities in Cancer Patients ) but the process may need to be repeated multiple times subsequently (see: One Biopsy Not Enough to Reveal Genetic Landscape of a Tumor ). Below is an excerpt an article that addresses this topic:

A small study that shows a surprising complexity of genetic changes within a single tumor has far-reaching implications for the march toward personalized cancer therapy....A single biopsy from a tumor might not be sufficient to give a full picture of its genetic landscape [according to a recent study]. When the researchers examined 10 biopsies taken from a single kidney cancer, they found "an extraordinary amount of diversity" in the genetic changes that had taken place in different parts of the tumor....The [research] team also found differences in genetic changes between the primary tumor and metastases that developed both locally and at a distant site (in the chest wall). Similar findings have been documented by other research groups, but it is the extent of intratumoral heterogeneity that is surprising....The findings have far-reaching implications for the efforts currently being directed toward personalized cancer therapy, in which therapy is targeted at genetic changes identified in tumor tissue. An example of this is the SnaPshot broad genetic screen program being used in routine clinical practice at the Massachusetts General Hospital in Boston. 

Here's more about the SnaPshot program (see:  Screening: Smile, they're taking a SNaPshot of your cancer-causing genes ):

Personalized cancer treatment is just around the corner, suggests a new study. Using SNaPshot, a “multiplexed, robust, highly sensitive and quick clinical test,” [researchers] at Massachusetts General Hospital have screened patients with non-small-cell lung cancer (NSCLC) for multiple cancer-causing gene mutations and demonstrated that implementing broad genotyping in routine clinical practice can identify the right type of treatment for the right patients.

In this emerging era of personalized medicine and the need to match patient chemotherapy to the genomic profile of a tumor, we are now coming to better understand the genetic intratumoral heterogeneity, to quote a phrase from this article. Cellular genetic characteristics appear to change over time and a single, initial biopsy of a tumor may provide insufficient information to guide treatment long-term because of this heterogeneity. 

My first thought about how to address this problem was to examine circulating tumor cells (CTCs) (see: Circulating Tumor Cells for Assessing Survival in Pancreatic Cancer Patients ). However, this approach will probably not suffice, even if this technology were sufficiently mature. I suspect that the metastasizing, circulating cells are genetically different than their parent cells. In fact, it may be these genetic differences that determine these cells' ability to circulate and implant in distant organs. The only "practical" solution that I can conjure up in the long run is for continuous genetic monitoring of a tumor using molecular imaging. I say "practical" here because this technology is still largely confined to research labs.

In the short-term, we will need to be satisfied with periodic sampling of tumor tissue using standard biopsy techniques combined with genetic analysis to determine which chemotherapeutic agents will be most effective. Recall, however, that there is some morbidity associated with obtaining a tumor biopsy, even with fine needle aspiration (FNA). However, it must also be stated that two of the most common types of tumors (GI and breast) are accessible via endoscope or percutaneously (see:  Assessment of morbidity and mortality associated with EUS-guided FNA: a systematic review ).

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