PET Imaging Can Change
Management of Cancer Patients

by Mohammed Moinuddin, M.D., F.A.C.P.

Very few tests have had such a profound impact on patient care as CT scanning since the 1970s. MRI, a decade later, was responsible for further improvements in imaging particularly in CNS, Cardiovascular and Musculoskeletal Systems. Newly arrived is Position Emission Tomography (PET), referred to as metabolic or molecular imaging because of the use of Fluorine 18 Deoxy glucose (FDG) which localizes in metabolically active cells such as cancer cells. According to the recent data, PET is found to be more accurate than CT or MRI, in most tumors.

The Heath Care Foundation Association (HCFA) has approved the following indications for the use of FDG-PET for reimbursement by Medicare:

1. Lung Cancer (non small cell) for Diagnosis, Staging and Restaging (DSR)
2. Esophageal Cancer (DSR)
3. Colorectal Cancer (DSR)
4. Melanoma (DSR)
5. Lymphoma (DSR)
6. Head and Neck Tumors (Except Brain tumors and Thyroid tumors)
7. Myocardial viability only if SPECT was unsatisfactory.
8. Brain imaging to identify/localize ictal (seizure) foci for Surgical Treatment.

Lung Cancer
Realizing that 130,000 solitary pulmonary modules are detected every year in the USA and one-third to one-half of these are malignant, it is a formidable problem to sort out which ones are cancerous. PET scan, though not 100% accurate, has a higher accuracy than CT Scan. Therefore, it is important to know the relative accuracies of imaging tests, which are sometimes used either as complementary or as competitive tests. On the basis of meta-analyses, the comparative sensitivity and specificity are:

PET CT
Sensitivity 92 90
Specificity 90 65

Inflammatory lung modules are often negative or mildly positive with FDG. Mediastinal metastases or distant metastases are also better seen on PET. PET causes changes in management in 37% of the patients. Whether PET will play an important role in early diagnosis of lung cancer in nodules, thereby improving the morbidity and mortality, remains to be seen.

Colorectal Cancer
These cancers are the third most common cancers after gender related and lung cancers. The incidence of colorectal cancer, which is 10-11% of all cancers, ranks as third in cancer deaths. PET carries 92% sensitivity and 99% specificity, according to a meta-analysis study. Liver metastases are readily seen on PET scans. PET has been found to cause a 36% change in staging and a 32% change in management for recurrence of colorectal cancers.

Melanoma
The incidence of melanoma is increasing at the rate of 5% per year. FDG has profound affinity for melanomas. The sensitivity and specificity of PET in melanoma are 77% and 90% respectively and responsible for changes in management of 26% of patients. However, for regional metastases, sentinel lymph node mapping by isotopes or blue dye remains the test of choice and PET is therefore not indicated for regional metastases. PET has been approved by HCFA for detection of metastases elsewhere.

Head and Neck Cancers
The sensitivity and specificity of PET compared with CT are as follows:
Sensitivity Specificity
CT 73 84
PET 83 93

Because of higher accuracy of PET compared with conventional imaging, HCFA has approved PET for head and neck tumors. Currently, thyroid cancer and brain tumors are excluded from approval, but likely to be approved soon because PET is found to be very helpful, particularly in patients with Thyroid cancer with abnormal Serum Thyroglobulin and a negative I131 scan.

Lymphoma
The comparative accuracy of CT and PET are as follows:
Sensitivity Specificity
PET 91 94
CT 68 75

PET is beginning to replace gallium scanning in staging of lymphomas and is also used in evaluating treatment response. PET caused changes in the management of 10-21% of patients with lymphoma.
Esophageal Cancer: Although endoscopy and conventional imaging are as accurate as PET for the diagnosis of esophageal cancer, PET is more sensitive than conventional testing for regional and distant metastases, thus responsible for changes in the management in 14 – 20% of patients.

Although HCFA has approved only the above cancers, PET has been found to be useful in diagnosis and staging of other tumors involving the musculoskeletal system, pancreas, ovary, cervix, bladder, breast, thyroid, etc. Of these, breast cancer is expected to be approved by HCFA by December 2001. Preliminary reports on the use of PET in benign conditions such as infections (osteomyelitis) or inflammations are also promising. The response to chemotherapy and radiation therapy can also be assessed on PET imaging and is covered under restaging discussed earlier. A common scenario is a persistent mass seen on CT or MRI post chemo or radiotherapy. Is it healing granulation tissue or persistent cancer? PET can differentiate between them – if it is positive, it is a tumor and if it is negative, it indicates successful treatment.

Conclusion
Pet scintigraphy represents the latest advancement in imaging. It is a very useful but expensive modality. An intelligent use of our resources requires its judicious use with a thorough understanding of its strengths and limitations. If you have any questions or are unsure of whether Pet is likely to help in certain clinical situations, please give us a call at (901) 226-5258 (Nuclear Medicine Department) or call:
M. Moinuddin, M.D. beeper (800)-255-3538
H. Lynn Magill, M.D. beeper (901) 418-9702
John Buchignani, M.D. beeper (901) 418-9700