PET scans better than CT in detection of vaginal cancer
20 July 2005
St Louis, USA. A study conducted by researchers at the School of
Medicine, Washington University in St Louis has found that positron emission
tomography (PET) scans detect twice as many primary tumours and cancerous
lymph nodes as computed tomography (CT) scans in patients with vaginal
cancer. The research was published in the 1 July issue of the International
Journal Radiation Oncology.(1)
Medicaid, Medicare and many private insurers in the USA, however, specify
CT (computed tomography) for diagnosing and monitoring this cancer.
Like cervical cancer, vaginal cancer advances predictably, spreading to
lymph nodes increasingly higher up in the body as the disease progresses.
Doctors use information about the size of the tumour and the involvement of
lymph nodes to determine treatment, such as where to target radiation and
whether to use surgery or chemotherapy.
The results of this study suggest that the use of PET, or positron
emission tomography, would make diagnosis of vaginal cancer much more
accurate and allow better selection of treatment, according to study author
Perry W. Grigsby, M.D., professor of radiation oncology and radiology.
However, until the procedure is reviewed and approved by the Centers for
Medicaid and Medicare Services (CMS), vaginal cancer patients will most
likely not be evaluated using PET scans.
CMS policies set standards often followed by private health insurance
companies and therefore strongly determine what procedures physicians use.
Studies such as this one play an essential role in CMS acceptance of new
"In 1999, we began publishing papers showing that PET scans picked up
more cancerous lymph nodes in patients with cervical cancer," Grigsby says.
"Armed with data from these kinds of studies, I went to Washington in 2003
to petition CMS to approve the use of PET scans for diagnosis of cervical
cancer. In January of 2005, the procedure was approved for cervical cancer."
Several other cancers, in addition to cervical cancer, can be diagnosed
and monitored using PET scans as the result of CMS acceptance. Grigsby is
now working to persuade CMS that PET scans will improve the diagnosis and
treatment of vaginal cancer.
"CT scans are useful in many cases, but they have a limit to their
resolution," says Grigsby, who sees patients at the Siteman Cancer Center
and is affiliated with Barnes-Jewish and St. Louis Children's Hospitals.
"When you're evaluating lymph nodes for cancer using CT, the node has to be
at least a centimetre for it to be considered abnormal. But PET scans can
detect much smaller nodes that have cancerous cells."
PET scans are effective for this purpose because they use a different
detection method than CT scans. CT scans obtain cross-sectional views of the
body by detecting the amount of X-rays that pass through the body's tissues.
Small tumours can easily escape detection.
On the other hand, PET scans detect radioactivity that emanates directly
from a tumour after a patient has received a dose of radioactive glucose,
which accumulates in tumours. Even tiny tumours will collect enough "hot"
glucose to show up on the PET scan.
Vaginal cancer is very similar to cervical cancer; both are linked to the
presence of human papillomavirus. About 1 percent of gynaecological
malignancies are vaginal. The rates of survival with vaginal cancer are
considered to be similar to those of cervical cancer.
According to Grigsby, if cervical cancer has not spread beyond its
primary site, about 90 percent of patients will survive. The rate of
survival drops to 70 percent if the cancer has spread to lymph nodes in the
pelvis. The next stage of progression, in which the cancer has spread to
nodes near the heart, has a survival rate of 30 to 40 percent. After that,
untreated cervical cancer will move to nodes near the collar bone and will
not be survivable.
"It is very important to know at the time of diagnosis, for both cervical
and vaginal cancer, not only what the patient has in the pelvis, but where
the tumor has spread," Grigsby says. "That will absolutely determine the
kind of treatment."
1. Lamoreaux WT, Grigsby PW, Dehdashti F, Zoberi I,
Powell MA, Gibb RK, Rader JS, Mutch DG, Siegel BA. FDG-PET evaluation of
vaginal carcinoma. International Journal of Radiation Oncology, Biology,
Physics. July 1, 2005; 62(3): 733–737.