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INTRODUCTION

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Staging of rectal cancer is the process of determining the extent of tumor spread, and serves two related goals: to provide information on the prognosis and to guide treatment choices. With an increasing number of treatment options and the current trend for further personalizing treatment, accurate staging of rectal cancer is becoming highly relevant in making therapeutic decisions. Traditionally imaging was mainly used for assessing distant spread of rectal cancer to the liver or lungs at primary presentation (cM-staging) while locoregional extent of tumor was assessed through histological examination of the resection specimen (pTN-staging). This histological staging was then used to guide decisions on adjuvant systemic therapy to prevent systemic recurrence, and on adjuvant locoregional (chemo)radiation to prevent local recurrence. When (chemo)radiation was shown to be more effective in a neoadjuvant setting,1 the selection process for (chemo)radiation changed from a histology-based to an imaging-based risk assessment. In addition, the neoadjuvant therapy altered the histological postoperative assessment, as well as the selection process for adjuvant systemic therapy—which is increasingly predicated on the primary imaging-based risk assessment. The histological risk factors associated with local recurrence and distal recurrence are generally similar: T stage, N stage, circumferential resection margin (CRM), extramural venous invasion (EMVI), perineural invasion, lymph and blood vessel invasion, and histological grade.2,3 Although modern CT techniques are improving and are, to some extent, able to provide information for locoregional staging, endorectal ultrasonography (ERUS) and magnetic resonance imaging (MRI) are considered the two best locoregional staging methods, providing information on the rectal tumor and surrounding mesorectum (T and N stage, distance to the mesorectal fascia, EMVI). The histological cutoff points of the TNM staging system, such as the distinction between a T2 and T3 tumor, do not always easily transfer to staging through imaging, because of the lack of histological precision. The importance of this difference for a tumor that is on the borderline is, however, questionable, and imaging can provide other prognostic factors such as depth of ingrowth into the mesorectum.

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After neoadjuvant chemoradiotherapy (CRT) there is often considerable downsizing and downstaging of the tumor, with 8% to 24% of patients obtaining a pathological complete response (pCR).4 Traditionally surgeons have based the extent of the resection on the pre-CRT imaging, mainly because of the fear of leaving behind small islands of viable tumor in the treatment-induced radiofibrosis. There is, however, an increasing awareness that the response to neoadjuvant CRT can be better exploited in order to perform a less extensive operation with less morbidity and a better functional outcome. In locally advanced tumors that respond very well, it may no longer be necessary to perform an extramesorectal resection, and in tumors that are very close to the sphincter it may become possible to perform a sphincter-saving total mesorectal excision (TME). A number of reports even suggest the options of local treatment, that is, transanal excision (TAE) of the small tumor remnant or scar, or a ...

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