Anal canal cancer metastasize more to the iliac nodes and of anal margin go more to the inguinal nodes. A recent review found that the accuracy of PET scan was poor for anal cancer even with palpable inguinal nodes. PET scan sensitivity was 50%, specificity = 72%, predictive value positive (PVP) = 50%, predictive value negative = 80%. 64% of scans were performed within 6 months of treatment; in these, PVP = 33% and PVN = 66%. PET scanning for anal SCC provides accurate staging of disease at presentation and may alter treatment planning by identifying inguinal node involvement not apparent on clinical examination. In this series, PET scan results did not change post-chemoradiation management in any case. Importantly, resolution of primary tumor defined by PET scan was accurate only 80% of the time. PE and Bx within 3 months of treatment were more accurate than PET scan in assessing disease. This review concludes that : "Post-treatment evaluation of anal SCC should continue to include careful PE, CT scan and Bx when appropriate". however, this review did not distinguish between anal canal and anal margin cancers. Most appears to have been anal margin.
NCCN lists PET on p.4 of its anal cancer guidelines for the initial staging of anal canal but not for anal margin cancer prior to 5FU/Mitomycin and radiaiton. A guideline says of anal canal cancer: "Positron emission tomography (PET) scanning may have a role as an adjunct to CT of the pelvis, abdomen, and chest, identifying sites of metastasis not observed on CT in 25 percent of cases. Almost 20 percent of patients with inguinal nodes that are negative by both physical examination and CT scan are positive on PET. This has prognostic significance and may influence the radiation treatment plan."
Fleshner PR, Chalasani S, Chang GJ, Levien DH, Hyman NH, Buie WD, Standards Practice Task Force of the American Society of Colon and Rectal Surgeons. Practice parameters for anal squamous neoplasms. Dis Colon Rectum 2008 Jan;51(1):2-9.
Nagle, D., Henry, D., Iagaru, A., Mastoris, J., Chmielewski, L., Rosenstock, J. The utility of PET scanning in the management of squamous cell carcinoma of the anus J Clin Oncol (Meeting Abstracts) 2006 24: 4152