The rise of the PET scan
Tzen Kai-yuan, director of NTUH's Department of Nuclear Medicine, says that in order to rapidly grow and reproduce, cancer cells absorb and metabolize two to 30 times more nutrients than ordinary cells. PET scans take advantage of this fact. Prior to being scanned, patients are injected with a tiny amount of a safe, short-lived radioactive isotope: fluorine-18 fluorodeoxyglucose. This enables the scan to identify points of abnormally high concentration or absorption of glucose, which are likely locations of cancerous cells.
Tzen says that PET scans are 90-95% accurate in diagnosing lung cancer, colon cancer, lymphoma, melanoma, esophageal cancer, head and neck cancer, breast cancer, thyroid cancer, and cervical cancer.
"In addition to identifying the point at which a tumor has arisen, the PET scan can also detect whether a lymphoma has spread to the surrounding tissue and whether a cancer has metastasized," says Tzen, who supports the use of PET scans as a diagnostic tool.
In July 2004, the Bureau of National Health Insurance agreed to cover PET scans for eight of the above cancers (all but cervical), though limiting lung- and breast-cancer scans to small-cell cancers and women, respectively. Moreover, it only covers scans for patients whose cancer has already been confirmed through pathological examination. That is, BNHI covers the use of PET scans only in determining the extent of a cancer, not for general screening.
How effectively do PET scans identify cancers not already known to exist? Hung says that the Shin Kong Medical Club has carried out roughly 25,000 scans over the last eight years, discovering cancers in 1.39% of them, most of which were lung, colorectal, or breast cancers.
The Tokyo-area Yamanakako Clinic, the first in Japan to offer PET screenings, carried out 21,804 scans on 8,615 healthy people from 1994 to 2004 and found cancers in 1.48% of them, a rate 0.09 percentage points higher than that at Shin Kong.
NTUH's Wu Ming-shian says the fact that PET scans are expensive (NT$30-50,000 each) and that Taiwan and Japan are the only two nations in the world to use them as screening devices reflects the Taiwanese and Japanese public's willingness to spare no expense when it comes to cancer detection.
But Wu's take on PET scans is much more conservative than that of experts in nuclear medicine and imaging examinations.
He says that the scans are undeniably the most expensive and best cancer detection tools we have, but points out that they have a sensitivity of only about 50% for the early-stage liver and digestive-tract cancers that are most common among Taiwanese.
"The fact is that for many common cancers, the most effective detection tools (with a sensitivity of 80-90%) are traditional endoscopy, ultrasound, and mammograms, exams that cost only a few hundred NT dollars," says Wu, exposing the examination market's fascination with expensive procedures.
Chen Ming-fong, superintendent of NTUH, similarly notes that aggressive promotions and advertising contribute to the myth that the newer and more expensive a test is, the better it is. The public never compares the efficacy and expense of these tests. "The emphasis on new devices and their blind use without thoroughly evaluating them first or having experts interpret their results after the fact leads to even more examination-related disputes such as allegations of misdiagnosis," says Chen.
Cancer cells produce unique proteins that can serve as tumor markers. For example, blood samples that test positive for alpha-fetoprotein indicate liver cancer, while those containing CA-125 (a cancer antigen) point to breast cancer. But such tests are insufficiently accurate. For example, only about 30% of patients with abnormal levels of PSA are ultimately diagnosed with prostate cancer.
