On p. Col-9,  2012 NCCN recommends the use of CT scans of chest and abdomen in the situation of serial CEA elevations but also says “Consider PET/CT scan”.

CMS guidelines state that PET would rarely be used in the diagnosis of colorectal cancer. However, starting July 1, 2001, HCFA, now called the Centers for Medicare and Medicaid Services (CMS), is covering FDG-PET imaging for diagnosis, staging, and restaging of colorectal cancer.

Gade M, Kubik M, Fisker RV, Thorlacius-Ussing O, Petersen LJ. Diagnostic value of (18)F-FDG PET/CT as first choice in the detection of recurrent colorectal cancer due to rising CEA. Cancer Imaging. 2015;15(1):11. Published 2015 Aug 13. doi:10.1186/s40644-015-0048-y

Abdel-Nabi H, Doerr RJ, Lamonica DM, et al. Staging of primary colorectal carcinomas with fluorine-18 fluorodeoxyglucose whole-body PET: correlation with histopathologic and CT findings. Radiology, 1998;206:755-760.

Podoloff DA, Advani RH, Allred C, Benson AB 3rd, Brown E, Burstein HJ, Carlson RW, Coleman RE, Czuczman MS, Delbeke D, Edge SB, Ettinger DS, Grannis FW Jr, Hillner BE, Hoffman JM, Kiel K, Komaki R, Larson SM, Mankoff DA, Rosenzweig KE, Skibber JM, Yahalom J, Yu JM, Zelenetz AD.
NCCN task force report: positron emission tomography (PET)/computed tomography (CT) scanning in cancer.
J Natl Compr Canc Netw. 2007 May;5 Suppl 1:S1-S22; quiz S23-2.

Erratum in:
J Natl Compr Canc Netw. 2007 Aug;5(7)

Sobhani I, Tiret E, Lebtahi R, Aparicio T, Itti E, Montravers F, Vaylet C, Rougier P, AndréT, Gornet JM,
Cherqui D, Delbaldo C, Panis Y, Talbot JN, Meignan M, Le Guludec D, Early detection of recurrence by 18FDG-PET in the follow-up of patients with colorectal cancer.
Br J Cancer. 2008;98(5): 875.

For Lay version see here

The impact of CT-based follow-up for the detection of resectable disease recurrence was addressed in a review of 705 patients who underwent resection of isolated colorectal cancer liver metastases at a single institution over a 14-year period. All were followed with a similar surveillance protocol, which included outpatient clinical examinations at 3, 6, 12, 18, and 24 months, and annually thereafter, with measurement of CEA and CA 19-9 levels at each visit. In addition, all patients had CT of the thorax, abdomen and pelvis every three months for the first two years, at six monthly intervals for three more years, then annually from year six to ten.

Of the 444 patients with a recurrence diagnosed on a surveillance CT, 404 were detected within two years. The site of recurrent disease was liver only in 36 percent, extrahepatic only in 38 percent, and both hepatic and extrahepatic sites in 26 percent. The authors did not report how many recurrences were detected by serum tumor markers versus CT scans.

In total, recurrent disease was treated surgically in 124 patients. At every time point (within one year of original surgery, one to two years, beyond two years), those patients treated by liver and/or lung resection had significantly better median survival than did those who received palliative chemotherapy alone. The mean number of scans performed per resectable recurrence was 35.3, and the cost per life-year gained was £2883, a value that compares favorably to other cost-effectiveness ratios that are considered acceptable in the US and elsewhere. UPTODate recommends the following surveillance strategy for patients with stage IV disease who are rendered surgically NED (no evidence of disease):

CEA every three months for two years, then every six months for three to five years

• CT of the chest/abdomen and pelvis every three to six months for two years, then every 6 to 12 months up to a total of five years
• Colonoscopy in one year; if no advanced adenoma repeat in three years, then every five years; if advanced adenoma is found, repeat in one year

Gomez D, Sangha VK, Morris-Stiff G, Malik HZ, Guthrie AJ, Toogood GJ, Lodge JP, Prasad KR

SO Outcomes of intensive surveillance after resection of hepatic colorectal metastases. Br J Surg. 2010;97(10):1552.

Yan TD, Sim J, Black D, Niu R, Morris DL Systematic review on safety and efficacy of repeat hepatectomy for recurrent liver metastases from colorectal carcinoma. Ann Surg Oncol. 2007;14(7):2069.

http://www.uptodate.com/contents/management-of-potentially-resectable-colorectal-cancer-liver-metastases#H650939148

Read the Layperson version here.