Cancers generally exhibit abnormal control of growth and replication. Cancers are often associated with such varied clinical and biochemical manifestations that no single biochemical test can alert the oncologist to detect the presence of tumor, assess its
.extent (or stage), estimate the prognosis, and monitor response to therapy.
Tumor size or volume has a major impact on both the treatment decision process and patient outcome. The detection of a cancer at a smaller size or volume generally results in a more favorable outcome. The introduction of newer and more sensitive imaging
?technology has improved the detection of small tunors. Suspicious lesions before and following therapy may be detected by CT or MRI which were previous
never , never appreciated by more conventional radiographic techniques. However, this increased precision may sacrifice its specificity. This could present a ,: oblem to the clinician in the evaluation of patients with tumors responsive to chemotherapy or
diotherapy with a relativeiy high expectation of cure.
t has been suggested that neither the CT appear´,ance of abdominal masses nor the magnitude of.
.Fy.
hange during therapy could exclude the possibillty
´of residual carcinoma. Therefore, the presence of
residual radiographic abnormalities may be an inac- curate measure of the efficacy of a particular treat" ment. It is frequently unclear whether anatomic
anges at the site of the tumor represent recrudesk nce of tumor or a benign process to the therapy.
The development of positron emission tomography
(PET) and nuclear resonance imaging-guided spec~troscopy (MRS) has moved us into the era of bio~ .chemical imaging. It is a picture which represents the NI1 numerical value of a measured bichemical or ;,x"physiological parameter. To carryout a PET scan, a
positron-emitting radionuclide tagged to a suitable
metabolic substrate must be administered. The simul
taneous detection of two 511-keV gamma photons at 180 degrees to each other by paired detectors forms the basis of PET, PET has been applied to measure blood flow and volume, oxygen utilization, glucose and amino acid metabolism, protein synthesis, pH, and drug effect. In oncology PET has been utilized to detect tumor, assess its extent, determine its metabolism, estimate its prognosis and monitor its response to therapy.
CT or MRI is the primary modality for the diagnosis and staging of cancer but is nonspecific in the differential diagnosis of tumor and treatment changes.
Pathologic specimens can be difficult to interpret. Metabolic changes seen on PET often precede clinical or anatomic findings. PET using ´ "F - fluorodeoxyglucose (FDG) has been mostly useful in differentiating residual or recurrent various tumors from posttreatment changes with putative 80% sensitivity and 94% specificity. It is well known that an increase in glycolysis is characteristic of malignancy.
Glucose use correlates ex vivo with DNA synthesis rate, but FDG uptake is also related to neoplastic as well as inflammatory cells. Treated or low-grade tumors seem to be better demonstrated with "C-methionine, Radiolabeled amino acid tracers or thymidine concentrate in areas of cell proliferation, and thus may prove to be more accurate indicators of tumor growth. There have been observations of significant changes in the binding of estrogen receptor with "Festradiol. Results from patient studies indicated that "F-estradiol has provided useful diagnostic information on both primary and metastatic breast tumors. Tracers that characterize tumors by blood flow and permeability may aid in therapy of planning. Preoperative location of the functional somatosensory cortex in patients with brain tumors is very helpful to neurosurgeons.
Both MRI and MRS are based on the fact that certain nucleil exhibit a magnetic moment. To obtain the
MR signals, an oscillating magnetic field is applied as a pulse at the Larmor frequency. At resonance, the nuclei absorb energy and are thus perturbed from their state of equilibrium. After the pulse, the nuclei release this absorbed energy and produce signals by inducing a current in a coil. Mappng the spatial distribution of nuclei associated with a particular chemical shift is called chemical shift imaging. 31P MRS shows information of bioenergics and membrane metabolites, reflecting cell viability and proliferation. ´H MRS demonstrates N - acetylaspartate, phosphocreatine, choline and lactate. There appears a potential for MRS to provide increased specificity in differentiating malignant tumors from normal tissue, and also to monitor therapeutic response. The usefulness of MR spectroscopic imaging for discriminating between lipid and water was applied to the differentiation of adenomas from carcinomas.
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