After the crocodile bit off Captain Hook’s hand in Peter Pan, the telltale tick-tock of the pirate’s swallowed wristwatch served as a dead giveaway to the reptile’s presence.
Tumors, likewise, shed telltale substances into the blood, alerting medical practitioners of their presence. But like the croc’s internalized accoutrement, a tumor is a ticking clock. Too many ticks and tocks, and it eventually becomes invincible.
A great hope of the medical profession is to be able to detect various blood-borne substances, or biomarkers, shed by tumors early enough to hunt for, find, and eliminate the cancerous growths while they’re still small. But a new study by Stanford radiologist Sam Gambhir, MD, PhD, dashes some cold water on that ambition.
Borrowing from mathematical models used to quantify the flow of drugs from a pill or needle into the blood and, eventually, to the target organ, Gambhir and his post-doc, Sharon Hori, PhD, generated their own model in which the pill or needle was replaced by the tumor, and the drug by the secreted biomarker. This imaginative adaptation let them estimate the blood concentrations of a tumor biomarker based on how fast a particular tumor type’s cells divide, the known rate at which that kind of tumor sheds a particular biomarker, the ability of current assays to detect that marker, and so forth.
Applying their equations to a well-studied protein that is shed by ovarian tumors (and in smaller amounts by healthy cells), Gambhir and Hori showed that such a tumor would need to reach a size of about 1.7 billion cells, or the volume of a cube 2 cm on a side, before it would show up on the best available test for the biomarker protein. That translates to about 10 to 12-1/2 years from the time that tumor began life as a single cancer cell.
As I explained in a news release on the study, this result can be viewed as both bad news and good news:
The bad news, Gambhir said, is that by the time a tumor reaches a detectable size using today’s available blood tests, it is likely to have metastasized to other areas of the body, making it much more deadly than if it had been caught early on. “The good news is that we have, potentially, 10 years or even [longer] to find the tumor before it reaches this size, if only we can improve our blood-based methods of detecting tumors,” he said. “We think our mathematical model will help guide attempts to do that.”