Evaluation of the extent of the tumor infiltration into the mesorectum (differentiating minimal from advanced T3 tumors and minimal T3 from T2 tumors) is of clinical interest in determining the need for neoadjuvant treatment but remains a challenge for TRUS [59,60]. Although TRUS performs better than MRI for T1 tumors, similar for T2 and T3, it may be less accurate in characterizing locally advanced T4 tumors with a tendency to understage [36]. The use of TRUS in assigning patients to transanal endoscopic microsurgery versus traditional surgery remains controversial. Despite some authors reporting good accuracy for some T stages, a retrospective evaluation of the use of TRUS in patients selected to undergo transanal endoscopic microsurgery for presumed early-stage disease showed disappointing results with inaccurate staging seen in 44.8% of the 165 patients who underwent TRUS preoperatively (32.7% were understaged and 12.1% were overstaged) [61]. In a 2019 study of 500 patients, neither TRUS or MRI distinguished between T1 and T2 disease [62]. A significant limitation of TRUS is the limited field of view that compromises assessment of the relationship of the tumor, mesorectal tumor implants, tumor invasion in extramural vessels, and malignant nodes to the mesorectal fascia. MRI may better evaluate these findings because it offers a larger field of view. TRUS is also limited in its assessment of high rectal tumors. CT Abdomen and Pelvis Much of the literature on CT restaging was generated more than 5 years ago, demonstrating low accuracy for T- stage re-evaluation or assessment of complete response. CT may remain helpful in limited situations to assess for resection margin, overall decrease in tumor, or interval change in node size, and may be of benefit to assess for overall tumor susceptibility to CRT, or in rare cases to detect distant metastatic disease that has developed during the course of neoadjuvant CRT.
Staging of Colorectal Cancer. Evaluation of the extent of the tumor infiltration into the mesorectum (differentiating minimal from advanced T3 tumors and minimal T3 from T2 tumors) is of clinical interest in determining the need for neoadjuvant treatment but remains a challenge for TRUS [59,60]. Although TRUS performs better than MRI for T1 tumors, similar for T2 and T3, it may be less accurate in characterizing locally advanced T4 tumors with a tendency to understage [36]. The use of TRUS in assigning patients to transanal endoscopic microsurgery versus traditional surgery remains controversial. Despite some authors reporting good accuracy for some T stages, a retrospective evaluation of the use of TRUS in patients selected to undergo transanal endoscopic microsurgery for presumed early-stage disease showed disappointing results with inaccurate staging seen in 44.8% of the 165 patients who underwent TRUS preoperatively (32.7% were understaged and 12.1% were overstaged) [61]. In a 2019 study of 500 patients, neither TRUS or MRI distinguished between T1 and T2 disease [62]. A significant limitation of TRUS is the limited field of view that compromises assessment of the relationship of the tumor, mesorectal tumor implants, tumor invasion in extramural vessels, and malignant nodes to the mesorectal fascia. MRI may better evaluate these findings because it offers a larger field of view. TRUS is also limited in its assessment of high rectal tumors. CT Abdomen and Pelvis Much of the literature on CT restaging was generated more than 5 years ago, demonstrating low accuracy for T- stage re-evaluation or assessment of complete response. CT may remain helpful in limited situations to assess for resection margin, overall decrease in tumor, or interval change in node size, and may be of benefit to assess for overall tumor susceptibility to CRT, or in rare cases to detect distant metastatic disease that has developed during the course of neoadjuvant CRT.