• Colorectal cancer. The five-year survival rate for colorectal cancer is 50%. Risk factors include age, diet, colonic polyps, chronic ulcerative colitis, and familial polyposis.

FDG-PET/CT can be used as a primary staging tool and in assessing recurrent disease. It is also performed prior to metastectomy, when assessing tumor response to chemo- or radiotherapy, and on masses that are difficult to biopsy. Patients with an unexplained rise in serum carcinoembryonic antigen and a history of colorectal cancer may also be imaged with FDG-PET/CT.

One study found that the sensitivity and specificity of PET/CT to colon cancer recurrence in patients after resection were 96.5% and 82.1%, respectively. Using serum CEA levels led to a sensitivity of 62% and a specificity of 87.5%.⁵

False-positive diagnoses may be caused by physiological uptake of FDG, inflammation (e.g., diverticulitis or colitis), polyps, and uptake caused by postoperative changes at the stoma site.

Small-volume disease, some mucinsecreting tumors, and carcinoid tumors can lead to false-negative calls. Peritoneal metastases can go undiagnosed on PET, though they may be picked up on CT.

• Head/neck cancer. Most head and neck cancers occur in the tongue base or tonsillar fossa. Occurrence is linked strongly to alcohol and tobacco use. Treatment may involve surgery, radiotherapy, and/or chemotherapy.

FDG-PET/CT can be used for staging, identifying sites of recurrence (Figure 2), distinguishing postoperative changes from residual disease, finding the site of an unknown primary tumor, and assessing response to therapy. It may also be used as a prognostic tool. PET/CT has been shown to be more accurate than PET alone for the depiction of head and neck cancers (96% versus 90%; p = 0.03)^.6

• Skin cancer. Approximately 75% of all skin cancers are basal cell carcinoma, while the remainder are squamous cell carcinoma and malignant melanoma. Almost all melanoma metastases greater than 1 cm in diameter are detected by FDG-PET/CT. Combined imaging is twice as accurate as CT alone for the detection of distant disease. PET/CT avoids unnecessary surgery and results in significant management change.

Patients with malignant melanoma who have suspected nodal or distant metastatic involvement or a high risk of such spread may undergo an FDGPET/ CT scan for staging purposes. The combined imaging modality may also be used to restage melanoma patients prior to surgical metastectomy and to confirm suspected recurrence.

FDG-PET/CT has a possible, but as yet unproven, role in monitoring response to therapy and disease surveillance. Most melanoma metastases are highly FDG-avid. Ocular melanoma is the main exception. Radiology researchers have shown the sensitivity, specificity, and accuracy of PET/CT for the depiction of melanoma metastases to be 98%, 94%, and 96%, respectively.⁷

• Gynecological malignancies. Ovarian cancer is associated with late presentation and poor prognosis. The role of FDG-PET has largely been limited to the assessment of recurrent disease. Moving to PET/CT is likely to make this investigation even more successful and reduce the number of false-positive reports.

Uterine carcinomas, such as cancers of the endometrium and cervix, tend to present earlier, and patients have much better survival rates. Although studies have shown that FDG accumulates in endometrial cancers, PET/CT is not used for the routine management of these tumors. There is, however, growing evidence that FDG-PET and PET/CT can contribute to the management of patients with cervical cancer.⁸