Watch-and-Wait Approach to Rectal Cancer: The Role of Imaging

Abstract

The diagnosis and treatment of rectal cancer have evolved dramatically over the past several decades. At the same time, its incidence has increased in younger populations. This review will inform the reader of advances in both diagnosis and treatment. These advances have led to the watch-and-wait approach, otherwise known as nonsurgical management. This review briefly outlines changes in medical and surgical treatment, advances in MRI technology and interpretation, and landmark studies or trials that have led to this exciting juncture. Herein, the authors delve into current state-of-the-art methods to assess response to treatment with MRI and endoscopy. Currently, these methods for avoiding surgery can be used to detect a complete clinical response in as many as 50% of patients with rectal cancer. Finally, the limitations of imaging and endoscopy and future challenges will be discussed.

Graphical abstract

Figure 1:

Timeline of development of various surgical techniques and some of the important trials and studies influencing the management of rectal cancer in the past century. cCR = clinical complete response, LARC = locally advanced rectal cancer, nCR = near complete response, nCRT = neoadjuvant chemoradiotherapy, OPRA = Organ Preservation of Rectal Adenocarcinoma, TME = total mesorectal excision, TNT = total neoadjuvant therapy, XRT = radiation therapy.

Figure 2:

(A) Complete, near complete, and incomplete response within the primary tumor (arrows) on axial T2-weighted (T2W) MRI scans at restaging performed after completion of total neoadjuvant therapy. (B) Complete, near complete, and incomplete response (arrows) within the primary tumor on axial diffusion-weighted images at restaging performed after completion of total neoadjuvant therapy (TNT). (C) Complete, near complete, and incomplete response (arrows) within the lymph nodes on axial T2-weighted MRI scans at restaging performed after completion of total neoadjuvant therapy (TNT). (D) Endoscopy images depicting complete, near complete, and incomplete response (arrows) within the primary tumor obtained after completion of TNT.

Figure 3:

Images in a 53-year-old man with bulky middle to upper rectal adenocarcinoma involving the mesorectal fascia. (A, E, I) Axial and (B, F, J) coronal oblique T2-weighted MRI scans, (C, G, K) axial diffusion-weighted images (b value = 800 sec/mm²), and (D, H, L) apparent diffusion coefficient maps through the mid rectum at baseline (A–D), 12 weeks after total neoadjuvant therapy (E–H), and 14 months surveillance after total neoadjuvant therapy while the patient was on a watch-andwait (W&W) strategy (I–L). Baseline images show the primary rectal tumor with multifocal involvement of the mesorectal fascia (red arrows). Post-total neoadjuvant therapy images at 12 weeks show some T2-weighted mixed dark and intermediate signal intensity within the tumor and desmoplastic reactions extending up to the mesorectal fascia (blue arrows). Endoscopy images show intense inflammation (images not shown). Surveillance images at 14 months while the patient was on the W&W strategy show darker T2-weighted dark signal intensity in the scar, no tumor regrowth, clear mesorectal fascia (green arrows), and continued absence of restricted diffusion.

Figure 4:

Images in a 65-year-old man with a low rectal tumor with anterior perforation extending to the anterior mesorectal fascia (red arrow). (A, D) Axial T2-weighted MRI scans, (B, E) axial diffusion-weighted images (b value, 800 sec/mm²), and (C, F) apparent diffusion coefficient maps at baseline (A–C) and 12 weeks after completion of total neoadjuvant therapy (D–F). Post-total neoadjuvant therapy images show decreased size of the tumor with new scar (blue arrow) and some residual restricted diffusion (yellow arrow), consistent with near-complete response. The patient opted for nonsurgical management and remains free of tumor regrowth at 3.5 years of surveillance.

Figure 5:

Images in a 58-year-old man with rectal adenocarcinoma. Baseline (A) axial T2-weighted MRI scan, (B) axial diffusion-weighted image (b value, 800 sec/mm²), and (C) apparent diffusion coefficient map show a large rectal mass with an extraluminal component on the right side contacting the mesorectal fascia (red arrow) with restricted diffusion. Axial (D) T2-weighted MRI scan, (E) axial diffusion-weighted image (b value, 800 sec/mm²), and (F) apparent diffusion coefficient image at 4 weeks after completion of total neoadjuvant therapy show a scar in the tumor bed extending to the mesorectal fascia and minimal residual restricted diffusion (blue arrow), consistent with near-complete response. The patient underwent low anterior resection. Final histology revealed a few foci of residual cancer and no involvement of the mesorectal fascia.

Figure 6:

Images in a 53-year-old woman with locally advanced upper rectal tumor. Axial T2-weighted MRI scans (A) at baseline, (B) 12 weeks after total neoadjuvant therapy, and (C) 12 months after total neoadjuvant therapy (surveillance). (D) Subsequent fused axial fluorine 18 (¹⁸F) fluorodeoxyglucose (FDG) PET/CT image. In A, intermediate T2-weighted signal intensity left lateral extramural vascular invasion (arrow) is seen. In B, partial regression of extramural vascular invasion (arrow) is shown. No residual tumor was seen at endoscopy. In C, tumor regrowth within the extramural vascular invasion site (arrow) is visible. Moderately intense ¹⁸F-FDG uptake (maximum standard uptake value, 4.1) is seen within the extramural vascular invasion (arrow) on D.

Figure 7:

Images in a 58-year-old man with rectal adenocarcinoma. (A, C) Axial T2-weighted MRI scans and (B, D) axial diffusion-weighted (b value, 800 sec/mm²) images at baseline (A, B) and 4 weeks after completion of total neoadjuvant therapy (C, D). Baseline images shows several superior rectal lymph nodes with intermediate signal intensity measuring up to 1.0 cm in short-axis dimension (arrow in A). Post-total neoadjuvant therapy image shows decreased size of the lymph nodes with uniform signal intensity measuring up to 0.5 cm in the short-axis dimension (arrow in B). Nodes are well seen on diffusion-weighted images, which helps in detection of the nodes (arrows in B and D). The patient underwent low anterior resection. Final histologic examination showed no evidence of metastasis in the lymph nodes.

Figure 8:

Images in a 56-year-old woman with locally advanced rectal cancer with enlarged 1.3-cm left internal iliac lymph nodes containing heterogenous signal intensity. Axial T2-weighted MR images at (A, B) baseline, (C, D) 8 weeks after total neoadjuvant therapy, and (E, F) 3.5 years after total neoadjuvant therapy (surveillance). Post-total neoadjuvant therapy images show near-complete response within the primary tumor (arrow in C) and only slight regression of the pelvic node measuring 1.3 cm (arrow in D). Endoscopic biopsy of the tumor bed at this assessment timepoint was negative for malignancy. The patient opted for nonsurgical management and remains free of tumor regrowth at 3.5-year follow-up (arrow in E). The left internal iliac lymph node shows regression with decreased size and mixed T2-weighted signal changes measuring 0.8 cm (arrow in F).

Figure 9:

Images in a 46-year-old man with locally advanced rectal cancer. Axial (A) T2-weighted MRI scan, (B) axial diffusion-weighted image, and (C) apparent diffusion coefficient map at baseline show a tumor with intermediate T2-weighted signal intensity (arrow in A), concordant with (D) the endoscopic findings of a fungating and ulcerating rectal mass (arrow). Note the rectal gel in the lumen masking the high signal intensity on B. (E) Posttotal neoadjuvant therapy image obtained 8 weeks after completion of therapy shows T2-weighted dark signal intensity in the tumor bed (arrow) and (F, G) minimal restricted diffusion. (H) No residual tumor was seen at endoscopy, consistent with complete response.

Figure 10:

Images in a 68-year-old man with locally advanced rectal cancer. Axial (A) T2-weighted MRI scan and (B) diffusion-weighted image obtained 12 weeks after total neoadjuvant therapy show a T2-weighted dark scar (arrow). (C) Endoscopy 8 months after total neoadjuvant therapy shows radiation-related telangiectatic changes (arrow), with no evidence of tumor. (D) Surveillance scans obtained 12 months after total neoadjuvant therapy show higher intermediate signal intensity tumor regrowth (arrow in D), with restricted diffusion (arrow in E). (F) Endoscopy at 12-month follow-up shows a 0.5-cm ulcerated nodule in the tumor bed, consistent with tumor regrowth.