 |
DISCUSSION
F-18
FDG is being used increasingly to assess patients with cancer. PET scanning
can be of help in the differential diagnosis of malignant from benign
conditions. It has been demonstrated that it could play an important role
to screen regional and distant metastasis in a variety of primary tumors.
In particular PET is useful in lung, lymphomas and other thoracic tumors.
Likewise, PET can also be used after therapy as follow up. In colorectal
carcinomas, it can differentiate between recurrences from residual areas
of fibrosis after surgery (1,2).
When
exploring the thorax, it is desirable that normal organs do not interfere
with the interpretation of images. Normal areas in the lungs and mediastinum
are not a problem, but the heart may present relative important uptake
of F-18 FDG, especially if they have had meals.
F-18
FDG uptake in the heart parallels that of glucose, being taken up and
phosphorylated but FDG is not metabolized. Myocardial glucose kinetics
is related with feeding state, insulin and free fatty acid levels and
heart work (3,4).
To
achieve high quality images, patient preparation is important prior to
inject F-18 FDG. Maintaining a fasted state helps to decrease glucose
heart uptake and makes it easier the interpretation of lesions in the
thorax. Related to this, mediastinum and left lung can be explored better
without significant organ superimposition. In fasting state, heart metabolism
is mainly oriented to fatty acids and less to myocardial glucose utilization.
Further, to confirm the effectiveness of fasting on glucose metabolism,
some researchers recommend to check blood glucose level, prior to F-18
FDG injection. If the BGL is over 140-160 mg/dl, it would indicate more
glucose availability for metabolism and possible increased heart uptake
in normal insulin level (6,8,9)
Our
results show that a significant number of patients 42% category Grade
3 and 16% category Grade 2, that is to say 58%, show important heart uptake,
in spite the indication of fasting and that BGL around or below 140 mg/dl
was achieved. We are not sure if fasting was long enough in all patients,
but nonetheless, at least 4 h was accepted as a minimum. So it appears
that a short period of fasting, as it is usually recommended, is not a
guarantee for a low heart uptake.
These
results showed no significant difference in the group with low uptake
and those with the most intense activity. So a relatively low BGL was
not able to predict diminished glucose heart uptake.
It
is not easy to find a reasonable explanation for our findings. Heart metabolism
is complex and fasting state is probably not the only factor playing a
role. Maybe other conditions in the patient such as stress, cathecolamine
and insulin and even glucagon levels and fat metabolism would need to
be assessed simultaneously to have a complete view of myocardium and glucose
relationship (3-5). The single diabetic patient in our group had no heart
uptake and BGL of 173 mg/dl, the highest we noted. Likewise, the effect
of some drugs, could be another variable to be considered. Acipimox, a
nicotinic acid derivative blocks lipolysis, facilitating glucose uptake
in the heart (10). It is also important to know about the myocardium status
itself. F-18 FDG may vary if there are ischemic territories, it was not
the case in this group of patients, clinically free of coronary artery
disease (4).
Further
investigations are warranted in this field in non-diabetic and diabetic
groups. A simple test and/or simple instruction would be ideal to assure
heart uptake will be low, and so to improve and facilitate F-18 FDG interpretation
in the thoracic region.
Empirically,
we would recommend a fasting period longer than 4 h in every patient.
A decision would have to be made, patient to patient, whether to inject
the F-18 FDG or not if the BGL is high and to decide about the use of
insulin.
|
