The full effect of CMT is seen 6–8 weeks after the treatment has been completed. A significant proportion of local treatment failures most commonly occur within 18 months, and the majority occur within 36 months of starting treatment [14]. Locally recurrent disease is almost universally palpable in the lumen of the canal or at the anal margin [15] and may be detected before becoming symptomatic; regular outpatient follow-up with a high index of suspicion is recommended. The follow-up protocol from the ACT II trial consisted of clinical examination every 2 months for the first year, every 3 months during the second year, and every 6 months from 36 to 60 months.

Differentiation between radiation effect and early recurrence can be difficult, especially if there is residual ulceration at the site of the original cancer after CMT (Fig. 18.1). Particular care must be taken with patients receiving nicorandil for the management of cardiac disease because it is well recognized that this drug can cause iatrogenic perineal ulceration that looks very similar to the ulceration seen in patients with active anal cancer. If recurrent disease is ­suspected, examination under anesthesia and appropriate biopsy is mandatory. The assessments of such biopsies should be undertaken by a ­specialist histopathologist working within the context of the multidisciplinary team. In women, care must be taken during any biopsy of a lesion on the anterior wall of the anal canal or lower rectum to avoid creating a fistula into the vagina.

Although the identification of stranding of the ischiorectal or perirectal fat associated with a mass may be predictive of tumor relapse or persistence, a study performed by Cohan et al. [16] identified a false-positive rate in 4 of 19 patients. This small study reflects the current attitude in the authors’ our unit that identification of local disease is better performed by magnetic resonance imaging (MRI). Computed tomography (CT) of the thorax, abdomen, and pelvis remains a useful modality in identifying metastatic disease that may prevent salvage surgery being performed.

There is little in the published literature that provides strong objective evidence of the accuracy of pelvic MRI in the identification of local disease; however, it does provide much clearer imaging of the pelvis and anal canal than CT and is part of routine practice at both initial presentations and subsequent instances where treatment failure is suspected [17].

Preoperative MRI may aid in the assessment of the degree of radial invasion beyond the anal canal, but its accuracy is reduced after chemoradiotherapy. Although findings on MRI may not alter the initial treatment (although it can), it is extremely helpful to have pretreatment images as a comparator to subsequent studies. Similarly, high signal changes on T2-weighted images of lymph nodes of the groin or pelvis may identify nodal metastases, which then should lead to targeted biopsy to establish the presence of unresectable disease. Imaging of the perineum remains problematic.

Positron emission tomography/CT has been shown to be a potentially accurate imaging modality in anal cancer and was capable of detecting residual or recurrent disease with a sensitivity and specificity on a per-site basis of 86–97 %, respectively, in one study [18]. It is regularly used in staging of squamous cell carcinomas of other sites and it may be of use in detecting distant metastatic disease, but its value in anal cancer is not yet proven and is not standard practice in the UK.

There is no specific tumor marker that has shown any benefit in anal cancer and, certainly, there seems to be no value in measuring carcinoembryonic antigen levels in any anal malignancy with the exception of anal adenocarcinoma.

Patients should undergo protocol-driven follow-up in specialist units where there are experienced clinicians and diagnostic services familiar with the difficulties described and where the subsequent salvage treatments are available. Because of the relative rarity of this tumor and the problems it poses, there is no reason why follow-up of anal cancer should be outside high-volume specialist units that treat both primary and relapsed disease.