Response Assessment with MRI after Chemoradiotherapy in Rectal Cancer: Current Evidences

Abstract

Baseline magnetic resonance imaging (MRI) has become the primary staging modality for surgical plans and stratification of patient populations for more efficient neoadjuvant treatment. Patients who exhibit a complete response to chemoradiotherapy (CRT) may achieve excellent local tumor control and better quality of life with organ-preserving treatments such as local excision or even watch-and-wait management. Therefore, the evaluation of tumor response is a key factor for determining the appropriate treatment following CRT. Although post-CRT MRI is generally accepted as the first-choice method for evaluating treatment response after CRT, its application in the clinical decision process is not fully validated. In this review, we will discuss various oncologic treatment options from radical surgical technique to organ-preservation strategies for achieving better cancer control and improved quality of life following CRT. In addition, the current status of post-CRT MRI in restaging rectal cancer as well as the main imaging features that should be evaluated for treatment planning will also be described for the tailored treatment.

Keywords: Magnetic resonance imaging; Neoadjuvant chemoradiotherapy; Pathological complete response; Rectal cancer; Tumor response.

Conflict of interest statement

The authors have no potential conflicts of interest to disclose.

Copyright © 2019 The Korean Society of Radiology.

Figures

Fig. 1. Traditional APE and extralevator APE.

A. Traditional APE: Dotted line indicates dissection plane. Because levator muscle is not included in surgical specimen, it is appropriate for tumors without threatening to levator muscle plane. B. Extralevator APE: Tumor infiltration extends to left levator ani muscle. Levators are removed in this surgical specimen. This type of surgery is mandatory for lower rectal cancer with levator involvement to achieve negative pCRM. APE = abdominoperineal excision, pCRM = pathologic circumferential resection margin

Fig. 2. Mesorectal fat bare area between distal end of mesorectum (black line) and level of anorectal junction (red line).

A. Schematic drawing depicting mesorectal fat bare area. Length of bi-directional arrow is approximately 1 cm. B. Cadaveric dissection image shows that mesorectal fat is absent along longitudinal extent corresponding to same bi-directional arrow. At this site, only rectal wall is present. (Adapted from Kim. Yonsei Med J 2005;46:737–749) (73). C. On T2-weighted oblique coronal image, imaginary black line is drawn at 1 cm (bi-directional arrow) above anorectal junction level (red line) which roughly corresponds to upper end of intersphincteric plane.

Fig. 3. Distal rectal cancer with left levator ani muscle involvement.

A. On baseline MRI, ulceroinfiltrative mass is observed in distal rectum with tumor infiltration into left levator ani muscle (arrow). B. On post-CRT MRI, signal drop is seen within entire tumor. However, shrunken tumor still invades levator muscle (arrow). Traditional APE was performed. Histopathology revealed tumors with pathologic tumor regression grade 3 and positive pCRM (not shown). C. Follow-up CT 6 months after operation shows recurrent mass at remnant left levator muscle (arrows). CRT = chemoradiotherapy, MRI = magnetic resonance imaging

Fig. 4. Progressive tumoricidal effect and fibrotic change in rectal cancer after routine timing window.

A. On baseline MRI, semi-circular mass with intermediate SI is noted at left lateral wall of rectum (curved line indicates circumferential tumor extent). B. Post-CRT MRI obtained at four weeks after completion of CRT shows increased proportion of fibrotic change as well as decreased proportion of tumor SI, indicating tumor regression. Patient refused surgical option at that time. C. On post-CRT MRI obtained at 22 weeks, proportion of fibrotic change is further increased with decreased circumferential tumoral extent. Histopathology after subsequent APE revealed ypT2 (not shown). SI= signal intensity

Fig. 5. Post-CRT mrTRG system.

A. ESGAR-based complete response: completely normalized rectal wall in initial tumor site (arrow). B. mrTRG 1: linear/crescentic scar in mucosa/submucosal layer (arrow) which corresponds to ESGAR-based fibrotic wall thickening without clear mass. C. mrTRG 2: dense fibrotic transmural mass without intermediate tumor SI (arrow) which also corresponds to ESGAR-based fibrotic wall thickening without clear mass. D. mrTRG 3: more than 50% of areas with fibrosis and visible intermediate tumor (arrows) which corresponds to residual mass on ESGAR-based grading. E. mrTRG 4: few areas with fibrosis and mostly intermediate tumor SI (arrows) which also corresponds to residual mass on ESGAR-based grading. ESGAR = European Society of Gastrointestinal and Abdominal Radiology, mrTRG = MRI-based tumor regression grade

Fig. 6. MRI-based T staging after CRT (ymrT) for rectal cancer.

A. Post-CRT MRI demonstrates localized transmural rectal wall thickening (ymrT1 or 2) with smooth outer margin (arrows). ypT2 was pathologically confirmed (not shown). B. Post-CRT MRI demonstrates circumferential mass (arrow) with irregular outer margin (ymrT3) within mesorectum. ypT3 was identified on histopathology (not shown). C. Post-CRT MRI reveals annular mass in lower rectum with posterior vagina infiltration (arrow) by tumor with intermediate SI (ymrT4b). After surgery, ypT4b was confirmed (not shown).

Fig. 7. Limitation of qualitative evaluation based on SI of anatomical T2-weighted imaging for tumor response evaluation.

A. Baseline T2-weighted image (left) shows bulky mass with MRF involvement at 8 o'clock direction (arrow). DWI (right) demonstrates strong diffusion restriction within whole tumor area (arrow). B. On post-CRT T2-weighted image (left), intermixed SI of tumor and fibrosis at initial tumor site (arrow) is noted. Therefore, mrTRG 3 was reported. CRM involvement by treated tumor is still suspected at same direction (arrow) because CRM threatening is determined by whole treated tumor including fibrotic component on post-CRT MRI interpretation. On DWI (right), however, no diffusion restriction is seen in entire tumor area (arrow). Pathologic complete response was confirmed after TME (not shown). CRM = circumferential resection margin, DWI = diffusion-weighted imaging, MRF = mesorectal fascia, TME = total mesorectal excision

Fig. 8. Tumor penetration through MRF with MRF thickening which suggests high risk for positive pCRM even after completion of CRT.

A. Post-CRT MRI shows circumferential mass with intermediate SI. Tumor penetration to MRF with MRF thickening (arrow) is noted. Pathologic result after TME was positive pCRM (not shown). B. In another patient, post-CRT MRI still demonstrates tumor infiltration anteriorly into prostatic gland (arrow). After APE with partial prostatectomy, pCRM was found to be positive. C. On post-CRT MRI in another patient, tumor with anterior MRF threatening by spicules of treated tumor (arrow) is noted. However, MRF thickening or penetration is not demonstrated (arrow). Pathologic result after TME was negative pCRM (not shown).

Fig. 9. Tumor response of lymph node after CRT.

A. On baseline MRI, suspicious metastatic node with short axis diameter of 9 mm is seen at right lateral pelvic side wall (arrow). B. Post-CRT MRI reveals interval decrease of node (arrow) and size is less than 5 mm. Lymph node dissection at right lateral pelvic side wall was performed. Viable tumor cells were not found in dissected lymph nodes (not shown). However, pathologic nodal status after CRT still remains uncertain because both normal and metastatic lymph nodes show interval decrease in size after CRT.

Fig. 10. Metastatic lymph nodes after CRT.

A. Post-CRT MRI reveals persistently enlarged lymph node (arrow) with short axis diameter of 16 mm in right obturator area. Selective lymph node dissection at right pelvic side wall is performed and lymph node metastasis is pathologically confirmed (not shown). B. In another patient, post-CRT MRI demonstrates persistently enlarged node (arrow) with short axis diameter of 7.5 mm within mesorectum. After TME, metastasis is also identified within lymph node (not shown).

Fig. 11. Positive EMVI on post-CRT MRI (ymrEMVI).

A. Baseline MRI shows positive mrEMVI of grade 4 which indicates irregular vessels replaced by soft tissue with intermediate SI (arrow). B. Post-CRT MRI still depicts positive ymrEMVI with minimal fibrotic change showing low SI (at least less than 25%) although prominent shrinkage is noted at primary rectal mass. EMVI = extramural venous invasion

Fig. 12. Typical MRI findings of pathologic complete response after CRT.

A. Baseline MRI (left) shows fungating tumor (arrow) with intermediate SI in lower rectum. DWI (right) demonstrates prominent diffusion restriction at tumor (arrow). B. Post-CRT MRI (left) reveals transmural fibrosis with dark SI in left lateral wall of rectum (arrow). There is no demonstrable residual tumor with intermediate SI. DWI (right) shows no evidence of diffusion restriction at corresponding tumor site.