Key Concept: Radiological imaging should be used to confirm clinical findings. In the absence of complete clinical response, do not look for radiological evidence to support nonoperative management. It is better to fail identification of complete response in favor of radical surgery and end up with a pCR than to miss residual cancer and end up with an early local recurrence or tumor regrowth.

Radiological assessment of response is of paramount importance to appropriately select patients for an alternative treatment strategy such as the “watch and wait” approach following a complete clinical response. As a matter of fact, the developments in radiological imaging, including both PET/CT and MR, have been quite significant. Proper magnetic resonance imaging with the use of diffusion-weighted techniques is now used routinely for the assessment of response in these patients. Currently, we would only consider a true complete responder (1) in a patient showing low signal intensity area replacing the area of the previous tumor or (2) in a patient with no detectable abnormalities in standard MR associated with no evidence of disease on clinical and endoscopic examination (Fig. 16.3). A recent publication has reported three different patterns of low signal intensity that are compatible with a complete clinical response: minimal fibrosis, transmural fibrosis, and irregular fibrosis [38]. Others have attempted to estimate tumor regression grades (as described for pathological assessment) [39] by standard MR imaging. [40] In addition, diffusion-weighted MR series should provide evidence of absence of restriction to diffusion to fulfill the criteria for a radiological complete response [41]. In our previously reported experiences with this “watch and wait” treatment strategy, MR imaging was not available to a significant proportion of patients [42]. Therefore, there is a hope that incorporation of these findings for the selection of patients with complete clinical response will significantly impact the outcomes of the watch and wait strategy. The presence of mixed signal intensity (Fig. 16.4) within the area of the previous cancer should raise a suspicion of an incomplete clinical response (Fig. 16.5). In addition to the assessment of the rectal wall, the mesorectum is also at risk for the presence of residual cancer despite complete primary regression (ypT0N1). Therefore, MR imaging should also provide the colorectal surgeon with information regarding possible mesorectal (or even lateral node) involvement regardless of primary tumor response (Fig. 16.6).

Molecular imaging may also play a role in the assessment of tumor response. PET/CT imaging offers information on tumor metabolism in addition to standard radiological anatomical features. In this setting, PET/CT has been used for the assessment of tumor response to neoadjuvant chemoradiation therapy [12, 43]. In addition to the visual identification of FDG uptake within the area of the rectal wall harboring the tumor or within the mesorectum, PET/CT allows the estimation of the metabolism profile. Standard uptake values are direct estimations of tissue metabolism and may be used for the distinction of residual inflammatory changes and residual cancer. Measurement of SUV in two different intervals from FDG injection is routinely performed (at 1 and 3 h) and allows two distinct patterns (dual time) of metabolism. Increases in SUVs (between 1 and 3 h) suggest the presence of residual cancer whereas decreases suggest inflammatory or fibrotic changes [28]. Even though we have used PET/CT to distinguish between complete and incomplete responses in the setting of a prospective study with acceptable overall accuracy (85 %), PET/CT may not be appropriate for routine use for this purpose, mainly due to increased cost and need for multiple studies with significant radiation exposure [12]. Instead, the use of this molecular imaging modality should perhaps be considered for patients with significant discordant results between studies, particularly between clinical and radiological findings.