NEW YORKNuclear medicine-based imaging techniques are now being
used to refine treatment strategies for cancer patients, with
positron emission tomography (PET) at the forefront. Patients with
complex cancers of the brain, head and neck, thyroid, and lung are
now able to receive more refined and accurate diagnoses through new
PET techniques, four speakers said at a nuclear medicine conference
sponsored by Memorial Sloan-Kettering Cancer Center and Johns Hopkins
University School of Medicine.
Fluorodeoxyglucose (FDG) is an agent taken up by tumor cells
exhibiting increased glycolysis. Because such increased
metabolism is often associated with malignant tumors, FDG-PET imaging
is widely used in studying brain tumors, said Ronald G.
Blasberg, MD, attending neurologist, Memorial Sloan-Kettering Cancer
Center, and professor of neurology, Weill Medical College of Cornell
However, there are problems associated with FDG-PET imaging of the
brain. Dr. Blasberg pointed out that while low-grade brain tumors
appear hypo-metabolic on an FDG-PET scan and high-grade tumors
usually have hypermetabolic foci (ie, 18F-FDG is taken up and appears
as a bright spot on the PET scan), there is often a heterogeneous
distribution of FDG signal within a given tumor.
Although more FDG uptake can usually equate with reduced
patient survival, the mixed uptake patterns complicate the diagnosis
and management of brain cancers, he explained.
To obviate some of these imaging challenges, Dr. Blasberg described a
computer-aided process called registration, whereby
FDG-PET and MR images are digitally superimposed upon each other.
By using various algorithms, the contrast-enhancing zone will
appear metabolically active, giving a more accurate assessment of
tumor physiology than the MR image alone, he said.
Dr. Blasberg also described using other iodine-labeled
radiopharmaceuticals like tyrosine, methionine, or thymidine to study
amino acid transport and DNA synthesis levels, for example.
Combinations of FDG and methionine scanning can demonstrate
changes in transport levels within tumor cells, and this is a key
feature of brain tumors that can be used to assess treatment
response, he noted. Since there is a functional relationship
between FDG uptake, rate of malignancy, and survival, such techniques
can also benefit other cancer types.
Head and Neck, Lung Cancer
Homer A. Macapinlac, MD, clinical director of the PET facility at
Sloan-Kettering, said that FDG-PET imaging is now being used in the
staging, post-treatment, and monitoring phases of head and neck
Although issues of instrument sensitivity still need to be
refined, various reports indicate that FDG-PET is more accurate than
CT or MRI in identifying both primary and recurrent head and neck
tumors, he said.
Importantly, Dr. Macapinlac cited a case in which FDG-PET scans
revealed a tumor hidden beneath healthy tissue in the posterior
nasopharynx. This resulted in surgical resection, avoiding the
extensive radiation treatment and chemotherapy that would have been
employed had the tumor remained undetected.
Similarly, FDG-PET is breaking new ground in distinguishing benign
from malignant solitary pulmonary nodules, said Henry Wagner, MD,
professor of medicine, radiology, and environmental health, Johns
Hopkins School of Hygiene and Public Health.
By using dual coincidence gamma cameras with FDG, a 97%
sensitivity and 80% specificity was shown in identifying proven
malignant lung lesions, he said. These data in 96 patients were
published in the April 1999 issue of the Journal of Nuclear
Medicine (40:574-578, 1999).
Steven M. Larson, MD, chief of nuclear medicine, Sloan-Kettering,
detailed how sequential use of FDG-PET and radiolabeled iodine scans
can affect the management of thyroid cancer patients.
Despite a low overall mortality rate, a high percentage of
fatalities are due to the less well-differentiated thyroid
cancers, he said. Since these tumors have little or no
iodine-concentrating activity, radiolabeled iodine scans can yield
false-negative results. Thus, FDG-PET scanning is also warranted.
Dr. Larson reported a study of 96 patients with thyroid cancer, 37 of
whom had negative whole-body iodine scans. Of these 37 patients, 22
had positive FDG- PET scans. Furthermore, 18 exhibited high
thyroglobulin (Tg) levels, and 19 had low Tg levels, both indicators
of residual tumor.
While FDG-PET scans in these patients positively identified
tumor, Tg levels themselves were a more sensitive detector of
residual cancer, Dr. Larson said. The key point was that
in 22 of the 37 patients, the FDG-PET scan results permitted correct
clinical changes in treatments, he said.
Dr. Larson also described separate data on the benefits of using the
positron-emitter iodine-124 (124I) in thyroid cancer scanning.
Preliminary data indicate that using this isotope can increase the
sensitivity and resolution of the PET scan. He said that 124I-PET
studies in five patients with thyroid cancer revealed tumors that
were otherwise occult.