MRI for Rectal Cancer: Updates and Controversies- AJR Expert Panel Narrative Review

Abstract

Rectal MRI is a critical tool in the care of patients with rectal cancer, having established roles for primary staging, restaging, and surveillance. The comprehensive diagnostic and prognostic information provided by MRI helps to optimize treatment decision-making. However, challenges persist in the standardization and interpretation of rectal MRI, particularly in the context of rapidly evolving treatment paradigms, including growing acceptance of nonoperative management. In this AJR Expert Panel Narrative Review, we address recent advances and key areas of contention related to the use of MRI for rectal cancer. Our objectives include discussing concepts regarding anatomic localization of rectal tumors; exploring the evolving rectal cancer treatment paradigm and implications for MRI assessment; reviewing updates and controversies regarding rectal MRI for locoregional staging, restaging, and surveillance; reviewing current rectal MRI acquisition protocols; and highlighting challenges in homogenizing and optimizing acquisition parameters.

Keywords: anatomic localization of rectal tumors; paradigm of rectal tumor; primary rectal tumor staging; rectal MRI; restaging rectal MRI; treatment.

Fig. 1—

Illustrations showing sigmoid take-off in sagittal (A) and axial (B) planes. (Figs. 1A and 1B © 2024 Memorial Sloan-Kettering Cancer Center, Memorial Hospital for Cancer and Allied Diseases, and Sloan-Kettering Institute for Cancer Research, each in New York, NY. All rights reserved. Republished with permission.) Examples on sagittal T2-weighted images of sigmoid tumor in 56-year-old patient (C), rectosigmoid tumor in 47-year-old patient (D), and upper rectal tumor in 62-year-old patient (E) (arrows), based on sigmoid take-off (STO) landmark (dashed line), which corresponds to mesorectum and mesocolon junction and is identified where colon curves anteriorly perpendicular to sacrum. Sigmoid tumors are situated completely above STO (C), rectosigmoid tumors straddles STO (B), and upper rectal tumors are below STO and above anterior peritoneal reflection (arrowhead).

Fig. 1—

Illustrations showing sigmoid take-off in sagittal (A) and axial (B) planes. (Figs. 1A and 1B © 2024 Memorial Sloan-Kettering Cancer Center, Memorial Hospital for Cancer and Allied Diseases, and Sloan-Kettering Institute for Cancer Research, each in New York, NY. All rights reserved. Republished with permission.) Examples on sagittal T2-weighted images of sigmoid tumor in 56-year-old patient (C), rectosigmoid tumor in 47-year-old patient (D), and upper rectal tumor in 62-year-old patient (E) (arrows), based on sigmoid take-off (STO) landmark (dashed line), which corresponds to mesorectum and mesocolon junction and is identified where colon curves anteriorly perpendicular to sacrum. Sigmoid tumors are situated completely above STO (C), rectosigmoid tumors straddles STO (B), and upper rectal tumors are below STO and above anterior peritoneal reflection (arrowhead).

Fig. 1—

Illustrations showing sigmoid take-off in sagittal (A) and axial (B) planes. (Figs. 1A and 1B © 2024 Memorial Sloan-Kettering Cancer Center, Memorial Hospital for Cancer and Allied Diseases, and Sloan-Kettering Institute for Cancer Research, each in New York, NY. All rights reserved. Republished with permission.) Examples on sagittal T2-weighted images of sigmoid tumor in 56-year-old patient (C), rectosigmoid tumor in 47-year-old patient (D), and upper rectal tumor in 62-year-old patient (E) (arrows), based on sigmoid take-off (STO) landmark (dashed line), which corresponds to mesorectum and mesocolon junction and is identified where colon curves anteriorly perpendicular to sacrum. Sigmoid tumors are situated completely above STO (C), rectosigmoid tumors straddles STO (B), and upper rectal tumors are below STO and above anterior peritoneal reflection (arrowhead).

Fig. 1—

Illustrations showing sigmoid take-off in sagittal (A) and axial (B) planes. (Figs. 1A and 1B © 2024 Memorial Sloan-Kettering Cancer Center, Memorial Hospital for Cancer and Allied Diseases, and Sloan-Kettering Institute for Cancer Research, each in New York, NY. All rights reserved. Republished with permission.) Examples on sagittal T2-weighted images of sigmoid tumor in 56-year-old patient (C), rectosigmoid tumor in 47-year-old patient (D), and upper rectal tumor in 62-year-old patient (E) (arrows), based on sigmoid take-off (STO) landmark (dashed line), which corresponds to mesorectum and mesocolon junction and is identified where colon curves anteriorly perpendicular to sacrum. Sigmoid tumors are situated completely above STO (C), rectosigmoid tumors straddles STO (B), and upper rectal tumors are below STO and above anterior peritoneal reflection (arrowhead).

Fig. 1—

Illustrations showing sigmoid take-off in sagittal (A) and axial (B) planes. (Figs. 1A and 1B © 2024 Memorial Sloan-Kettering Cancer Center, Memorial Hospital for Cancer and Allied Diseases, and Sloan-Kettering Institute for Cancer Research, each in New York, NY. All rights reserved. Republished with permission.) Examples on sagittal T2-weighted images of sigmoid tumor in 56-year-old patient (C), rectosigmoid tumor in 47-year-old patient (D), and upper rectal tumor in 62-year-old patient (E) (arrows), based on sigmoid take-off (STO) landmark (dashed line), which corresponds to mesorectum and mesocolon junction and is identified where colon curves anteriorly perpendicular to sacrum. Sigmoid tumors are situated completely above STO (C), rectosigmoid tumors straddles STO (B), and upper rectal tumors are below STO and above anterior peritoneal reflection (arrowhead).

Fig. 2—

Anal canal anatomy. Coronal T2-weighted images in 70-year-old patient (A) and 66-year-old patient (B), illustration (C), and sagittal T2-weighted images in 65-year-old patient (D), 62-year-old patient (E), and 43-year-old patient (F). Anatomic anal canal starts from dentate line (C, arrowhead), which is not characterized on MRI. Surgical anal canal starts from anorectal junction (ARJ, dashed lines) and extends to anal verge (white arrows). ARJ corresponds to upper margin of puborectalis muscle (A, C, D, purple delineation) and can be estimated as imaginary line extending from lower edge of pubic bone to lower margin of coccyx (F, dashed line). Additional components of levator ani muscle superior to puborectalis are pubococcygeus and iliococcygenus muscles (A–C, dark arrows). Anal verge can be estimated by intersphincteric groove, which is defined as fat plane between external anal sphincter (A, C, D, green delineation) and internal anal sphincter (A, C, D, blue delineation) at lower aspect of intersphincteric space (A, C, D, yellow line). (Fig. 2C © 2024 Memorial Sloan-Kettering Cancer Center, Memorial Hospital for Cancer and Allied Diseases, and Sloan-Kettering Institute for Cancer Research, each in New York, NY. All rights reserved. Republished with permission.)

Fig. 2—

Anal canal anatomy. Coronal T2-weighted images in 70-year-old patient (A) and 66-year-old patient (B), illustration (C), and sagittal T2-weighted images in 65-year-old patient (D), 62-year-old patient (E), and 43-year-old patient (F). Anatomic anal canal starts from dentate line (C, arrowhead), which is not characterized on MRI. Surgical anal canal starts from anorectal junction (ARJ, dashed lines) and extends to anal verge (white arrows). ARJ corresponds to upper margin of puborectalis muscle (A, C, D, purple delineation) and can be estimated as imaginary line extending from lower edge of pubic bone to lower margin of coccyx (F, dashed line). Additional components of levator ani muscle superior to puborectalis are pubococcygeus and iliococcygenus muscles (A–C, dark arrows). Anal verge can be estimated by intersphincteric groove, which is defined as fat plane between external anal sphincter (A, C, D, green delineation) and internal anal sphincter (A, C, D, blue delineation) at lower aspect of intersphincteric space (A, C, D, yellow line). (Fig. 2C © 2024 Memorial Sloan-Kettering Cancer Center, Memorial Hospital for Cancer and Allied Diseases, and Sloan-Kettering Institute for Cancer Research, each in New York, NY. All rights reserved. Republished with permission.)

Fig. 2—

Anal canal anatomy. Coronal T2-weighted images in 70-year-old patient (A) and 66-year-old patient (B), illustration (C), and sagittal T2-weighted images in 65-year-old patient (D), 62-year-old patient (E), and 43-year-old patient (F). Anatomic anal canal starts from dentate line (C, arrowhead), which is not characterized on MRI. Surgical anal canal starts from anorectal junction (ARJ, dashed lines) and extends to anal verge (white arrows). ARJ corresponds to upper margin of puborectalis muscle (A, C, D, purple delineation) and can be estimated as imaginary line extending from lower edge of pubic bone to lower margin of coccyx (F, dashed line). Additional components of levator ani muscle superior to puborectalis are pubococcygeus and iliococcygenus muscles (A–C, dark arrows). Anal verge can be estimated by intersphincteric groove, which is defined as fat plane between external anal sphincter (A, C, D, green delineation) and internal anal sphincter (A, C, D, blue delineation) at lower aspect of intersphincteric space (A, C, D, yellow line). (Fig. 2C © 2024 Memorial Sloan-Kettering Cancer Center, Memorial Hospital for Cancer and Allied Diseases, and Sloan-Kettering Institute for Cancer Research, each in New York, NY. All rights reserved. Republished with permission.)

Fig. 2—

Anal canal anatomy. Coronal T2-weighted images in 70-year-old patient (A) and 66-year-old patient (B), illustration (C), and sagittal T2-weighted images in 65-year-old patient (D), 62-year-old patient (E), and 43-year-old patient (F). Anatomic anal canal starts from dentate line (C, arrowhead), which is not characterized on MRI. Surgical anal canal starts from anorectal junction (ARJ, dashed lines) and extends to anal verge (white arrows). ARJ corresponds to upper margin of puborectalis muscle (A, C, D, purple delineation) and can be estimated as imaginary line extending from lower edge of pubic bone to lower margin of coccyx (F, dashed line). Additional components of levator ani muscle superior to puborectalis are pubococcygeus and iliococcygenus muscles (A–C, dark arrows). Anal verge can be estimated by intersphincteric groove, which is defined as fat plane between external anal sphincter (A, C, D, green delineation) and internal anal sphincter (A, C, D, blue delineation) at lower aspect of intersphincteric space (A, C, D, yellow line). (Fig. 2C © 2024 Memorial Sloan-Kettering Cancer Center, Memorial Hospital for Cancer and Allied Diseases, and Sloan-Kettering Institute for Cancer Research, each in New York, NY. All rights reserved. Republished with permission.)

Fig. 2—

Anal canal anatomy. Coronal T2-weighted images in 70-year-old patient (A) and 66-year-old patient (B), illustration (C), and sagittal T2-weighted images in 65-year-old patient (D), 62-year-old patient (E), and 43-year-old patient (F). Anatomic anal canal starts from dentate line (C, arrowhead), which is not characterized on MRI. Surgical anal canal starts from anorectal junction (ARJ, dashed lines) and extends to anal verge (white arrows). ARJ corresponds to upper margin of puborectalis muscle (A, C, D, purple delineation) and can be estimated as imaginary line extending from lower edge of pubic bone to lower margin of coccyx (F, dashed line). Additional components of levator ani muscle superior to puborectalis are pubococcygeus and iliococcygenus muscles (A–C, dark arrows). Anal verge can be estimated by intersphincteric groove, which is defined as fat plane between external anal sphincter (A, C, D, green delineation) and internal anal sphincter (A, C, D, blue delineation) at lower aspect of intersphincteric space (A, C, D, yellow line). (Fig. 2C © 2024 Memorial Sloan-Kettering Cancer Center, Memorial Hospital for Cancer and Allied Diseases, and Sloan-Kettering Institute for Cancer Research, each in New York, NY. All rights reserved. Republished with permission.)

Fig. 2—

Anal canal anatomy. Coronal T2-weighted images in 70-year-old patient (A) and 66-year-old patient (B), illustration (C), and sagittal T2-weighted images in 65-year-old patient (D), 62-year-old patient (E), and 43-year-old patient (F). Anatomic anal canal starts from dentate line (C, arrowhead), which is not characterized on MRI. Surgical anal canal starts from anorectal junction (ARJ, dashed lines) and extends to anal verge (white arrows). ARJ corresponds to upper margin of puborectalis muscle (A, C, D, purple delineation) and can be estimated as imaginary line extending from lower edge of pubic bone to lower margin of coccyx (F, dashed line). Additional components of levator ani muscle superior to puborectalis are pubococcygeus and iliococcygenus muscles (A–C, dark arrows). Anal verge can be estimated by intersphincteric groove, which is defined as fat plane between external anal sphincter (A, C, D, green delineation) and internal anal sphincter (A, C, D, blue delineation) at lower aspect of intersphincteric space (A, C, D, yellow line). (Fig. 2C © 2024 Memorial Sloan-Kettering Cancer Center, Memorial Hospital for Cancer and Allied Diseases, and Sloan-Kettering Institute for Cancer Research, each in New York, NY. All rights reserved. Republished with permission.)

Fig. 3—

Examples of mid rectal tumor in 53-year-old patient (A, B) and low rectal tumor in 49-year-old patient (C, D), based on following landmarks: origin of levator ani at pelvic sidewall (dashed lines), lower edge of tumor (white arrowheads), and anterior peritoneal reflection (dark arrowheads). Sagittal (A) and coronal (B) T2-weighted images of mid-rectal tumor show that lower edge of tumor (arrowhead) is situated below anterior peritoneal reflection (dark arrowhead) and above origin of levator ani at pelvic side wall (dashed lines). Sagittal (C) and coronal (D) T2-weighted images of low rectal tumor show that lower edge of tumor (white arrow) is below origin of levator ani at pelvic side wall (dashed line).

Fig. 3—

Examples of mid rectal tumor in 53-year-old patient (A, B) and low rectal tumor in 49-year-old patient (C, D), based on following landmarks: origin of levator ani at pelvic sidewall (dashed lines), lower edge of tumor (white arrowheads), and anterior peritoneal reflection (dark arrowheads). Sagittal (A) and coronal (B) T2-weighted images of mid-rectal tumor show that lower edge of tumor (arrowhead) is situated below anterior peritoneal reflection (dark arrowhead) and above origin of levator ani at pelvic side wall (dashed lines). Sagittal (C) and coronal (D) T2-weighted images of low rectal tumor show that lower edge of tumor (white arrow) is below origin of levator ani at pelvic side wall (dashed line).

Fig. 3—

Examples of mid rectal tumor in 53-year-old patient (A, B) and low rectal tumor in 49-year-old patient (C, D), based on following landmarks: origin of levator ani at pelvic sidewall (dashed lines), lower edge of tumor (white arrowheads), and anterior peritoneal reflection (dark arrowheads). Sagittal (A) and coronal (B) T2-weighted images of mid-rectal tumor show that lower edge of tumor (arrowhead) is situated below anterior peritoneal reflection (dark arrowhead) and above origin of levator ani at pelvic side wall (dashed lines). Sagittal (C) and coronal (D) T2-weighted images of low rectal tumor show that lower edge of tumor (white arrow) is below origin of levator ani at pelvic side wall (dashed line).

Fig. 3—

Examples of mid rectal tumor in 53-year-old patient (A, B) and low rectal tumor in 49-year-old patient (C, D), based on following landmarks: origin of levator ani at pelvic sidewall (dashed lines), lower edge of tumor (white arrowheads), and anterior peritoneal reflection (dark arrowheads). Sagittal (A) and coronal (B) T2-weighted images of mid-rectal tumor show that lower edge of tumor (arrowhead) is situated below anterior peritoneal reflection (dark arrowhead) and above origin of levator ani at pelvic side wall (dashed lines). Sagittal (C) and coronal (D) T2-weighted images of low rectal tumor show that lower edge of tumor (white arrow) is below origin of levator ani at pelvic side wall (dashed line).

Fig. 4 —

Examples of different T categories of rectal cancer. (A) Axial oblique T2-weighted image (T2WI) in 56-year-old patient shows rectal tumor with T category of T3b, extending beyond muscularis propria and abutting anterior peritoneal reflection (arrow), without thickening of peritoneal reflection. (B) Axial oblique T2WI in 52-year-old patient shows rectal tumor with T category of T4a, with nodular thickening and infiltration of anterior peritoneal reflection (arrow). (C, D) Rectal cancer with bladder involvement, consistent with T category of T4b, in 45-year-old patient. Axial oblique (C) and sagittal (D) T2WI show mucinous rectal tumor with high signal intensity, extending anteriorly with involvement of posterior urinary bladder wall (arrow). (E, F) Rectal cancer with extramesorectal vessel involvement, consistent with T category of T4b, in 59-year-old patient. Axial oblique (E) and sagittal (F) T2WI show rectal tumor extending through extramesoretal vein (arrow), which according to expert opinion warrants classification as T4b. (G, H) Rectal cancer with pelvic floor muscle involvement, consistent with T category of T4b, in 42-year-old patient. Axial oblique (G) and coronal oblique (H) T2WI show mucinous tumor invading anal sphincter complex and extending laterally to involve right puborectalis muscle and at least abut right ilio-pubococcygeus muscle (blue arrow). On left, mucinous component of lesion abuts left iliococcygeus muscle posteriorly (green arrow). Accurate delineation of pelvic floor muscle tumor involvement is important to guide treatment and surgical planning. (I) Rectal cancer with T with external anal sphincter involvement, consistent with T category of T4b, in 47-year-old patient. Axial oblique T2WI shows low rectal mucinous tumor involving external anal sphincter (arrow).

Fig. 4 —

Examples of different T categories of rectal cancer. (A) Axial oblique T2-weighted image (T2WI) in 56-year-old patient shows rectal tumor with T category of T3b, extending beyond muscularis propria and abutting anterior peritoneal reflection (arrow), without thickening of peritoneal reflection. (B) Axial oblique T2WI in 52-year-old patient shows rectal tumor with T category of T4a, with nodular thickening and infiltration of anterior peritoneal reflection (arrow). (C, D) Rectal cancer with bladder involvement, consistent with T category of T4b, in 45-year-old patient. Axial oblique (C) and sagittal (D) T2WI show mucinous rectal tumor with high signal intensity, extending anteriorly with involvement of posterior urinary bladder wall (arrow). (E, F) Rectal cancer with extramesorectal vessel involvement, consistent with T category of T4b, in 59-year-old patient. Axial oblique (E) and sagittal (F) T2WI show rectal tumor extending through extramesoretal vein (arrow), which according to expert opinion warrants classification as T4b. (G, H) Rectal cancer with pelvic floor muscle involvement, consistent with T category of T4b, in 42-year-old patient. Axial oblique (G) and coronal oblique (H) T2WI show mucinous tumor invading anal sphincter complex and extending laterally to involve right puborectalis muscle and at least abut right ilio-pubococcygeus muscle (blue arrow). On left, mucinous component of lesion abuts left iliococcygeus muscle posteriorly (green arrow). Accurate delineation of pelvic floor muscle tumor involvement is important to guide treatment and surgical planning. (I) Rectal cancer with T with external anal sphincter involvement, consistent with T category of T4b, in 47-year-old patient. Axial oblique T2WI shows low rectal mucinous tumor involving external anal sphincter (arrow).

Fig. 4 —

Examples of different T categories of rectal cancer. (A) Axial oblique T2-weighted image (T2WI) in 56-year-old patient shows rectal tumor with T category of T3b, extending beyond muscularis propria and abutting anterior peritoneal reflection (arrow), without thickening of peritoneal reflection. (B) Axial oblique T2WI in 52-year-old patient shows rectal tumor with T category of T4a, with nodular thickening and infiltration of anterior peritoneal reflection (arrow). (C, D) Rectal cancer with bladder involvement, consistent with T category of T4b, in 45-year-old patient. Axial oblique (C) and sagittal (D) T2WI show mucinous rectal tumor with high signal intensity, extending anteriorly with involvement of posterior urinary bladder wall (arrow). (E, F) Rectal cancer with extramesorectal vessel involvement, consistent with T category of T4b, in 59-year-old patient. Axial oblique (E) and sagittal (F) T2WI show rectal tumor extending through extramesoretal vein (arrow), which according to expert opinion warrants classification as T4b. (G, H) Rectal cancer with pelvic floor muscle involvement, consistent with T category of T4b, in 42-year-old patient. Axial oblique (G) and coronal oblique (H) T2WI show mucinous tumor invading anal sphincter complex and extending laterally to involve right puborectalis muscle and at least abut right ilio-pubococcygeus muscle (blue arrow). On left, mucinous component of lesion abuts left iliococcygeus muscle posteriorly (green arrow). Accurate delineation of pelvic floor muscle tumor involvement is important to guide treatment and surgical planning. (I) Rectal cancer with T with external anal sphincter involvement, consistent with T category of T4b, in 47-year-old patient. Axial oblique T2WI shows low rectal mucinous tumor involving external anal sphincter (arrow).

Fig. 4 —

Examples of different T categories of rectal cancer. (A) Axial oblique T2-weighted image (T2WI) in 56-year-old patient shows rectal tumor with T category of T3b, extending beyond muscularis propria and abutting anterior peritoneal reflection (arrow), without thickening of peritoneal reflection. (B) Axial oblique T2WI in 52-year-old patient shows rectal tumor with T category of T4a, with nodular thickening and infiltration of anterior peritoneal reflection (arrow). (C, D) Rectal cancer with bladder involvement, consistent with T category of T4b, in 45-year-old patient. Axial oblique (C) and sagittal (D) T2WI show mucinous rectal tumor with high signal intensity, extending anteriorly with involvement of posterior urinary bladder wall (arrow). (E, F) Rectal cancer with extramesorectal vessel involvement, consistent with T category of T4b, in 59-year-old patient. Axial oblique (E) and sagittal (F) T2WI show rectal tumor extending through extramesoretal vein (arrow), which according to expert opinion warrants classification as T4b. (G, H) Rectal cancer with pelvic floor muscle involvement, consistent with T category of T4b, in 42-year-old patient. Axial oblique (G) and coronal oblique (H) T2WI show mucinous tumor invading anal sphincter complex and extending laterally to involve right puborectalis muscle and at least abut right ilio-pubococcygeus muscle (blue arrow). On left, mucinous component of lesion abuts left iliococcygeus muscle posteriorly (green arrow). Accurate delineation of pelvic floor muscle tumor involvement is important to guide treatment and surgical planning. (I) Rectal cancer with T with external anal sphincter involvement, consistent with T category of T4b, in 47-year-old patient. Axial oblique T2WI shows low rectal mucinous tumor involving external anal sphincter (arrow).

Fig. 4 —

Examples of different T categories of rectal cancer. (A) Axial oblique T2-weighted image (T2WI) in 56-year-old patient shows rectal tumor with T category of T3b, extending beyond muscularis propria and abutting anterior peritoneal reflection (arrow), without thickening of peritoneal reflection. (B) Axial oblique T2WI in 52-year-old patient shows rectal tumor with T category of T4a, with nodular thickening and infiltration of anterior peritoneal reflection (arrow). (C, D) Rectal cancer with bladder involvement, consistent with T category of T4b, in 45-year-old patient. Axial oblique (C) and sagittal (D) T2WI show mucinous rectal tumor with high signal intensity, extending anteriorly with involvement of posterior urinary bladder wall (arrow). (E, F) Rectal cancer with extramesorectal vessel involvement, consistent with T category of T4b, in 59-year-old patient. Axial oblique (E) and sagittal (F) T2WI show rectal tumor extending through extramesoretal vein (arrow), which according to expert opinion warrants classification as T4b. (G, H) Rectal cancer with pelvic floor muscle involvement, consistent with T category of T4b, in 42-year-old patient. Axial oblique (G) and coronal oblique (H) T2WI show mucinous tumor invading anal sphincter complex and extending laterally to involve right puborectalis muscle and at least abut right ilio-pubococcygeus muscle (blue arrow). On left, mucinous component of lesion abuts left iliococcygeus muscle posteriorly (green arrow). Accurate delineation of pelvic floor muscle tumor involvement is important to guide treatment and surgical planning. (I) Rectal cancer with T with external anal sphincter involvement, consistent with T category of T4b, in 47-year-old patient. Axial oblique T2WI shows low rectal mucinous tumor involving external anal sphincter (arrow).

Fig. 4 —

Examples of different T categories of rectal cancer. (A) Axial oblique T2-weighted image (T2WI) in 56-year-old patient shows rectal tumor with T category of T3b, extending beyond muscularis propria and abutting anterior peritoneal reflection (arrow), without thickening of peritoneal reflection. (B) Axial oblique T2WI in 52-year-old patient shows rectal tumor with T category of T4a, with nodular thickening and infiltration of anterior peritoneal reflection (arrow). (C, D) Rectal cancer with bladder involvement, consistent with T category of T4b, in 45-year-old patient. Axial oblique (C) and sagittal (D) T2WI show mucinous rectal tumor with high signal intensity, extending anteriorly with involvement of posterior urinary bladder wall (arrow). (E, F) Rectal cancer with extramesorectal vessel involvement, consistent with T category of T4b, in 59-year-old patient. Axial oblique (E) and sagittal (F) T2WI show rectal tumor extending through extramesoretal vein (arrow), which according to expert opinion warrants classification as T4b. (G, H) Rectal cancer with pelvic floor muscle involvement, consistent with T category of T4b, in 42-year-old patient. Axial oblique (G) and coronal oblique (H) T2WI show mucinous tumor invading anal sphincter complex and extending laterally to involve right puborectalis muscle and at least abut right ilio-pubococcygeus muscle (blue arrow). On left, mucinous component of lesion abuts left iliococcygeus muscle posteriorly (green arrow). Accurate delineation of pelvic floor muscle tumor involvement is important to guide treatment and surgical planning. (I) Rectal cancer with T with external anal sphincter involvement, consistent with T category of T4b, in 47-year-old patient. Axial oblique T2WI shows low rectal mucinous tumor involving external anal sphincter (arrow).

Fig. 4 —

Examples of different T categories of rectal cancer. (A) Axial oblique T2-weighted image (T2WI) in 56-year-old patient shows rectal tumor with T category of T3b, extending beyond muscularis propria and abutting anterior peritoneal reflection (arrow), without thickening of peritoneal reflection. (B) Axial oblique T2WI in 52-year-old patient shows rectal tumor with T category of T4a, with nodular thickening and infiltration of anterior peritoneal reflection (arrow). (C, D) Rectal cancer with bladder involvement, consistent with T category of T4b, in 45-year-old patient. Axial oblique (C) and sagittal (D) T2WI show mucinous rectal tumor with high signal intensity, extending anteriorly with involvement of posterior urinary bladder wall (arrow). (E, F) Rectal cancer with extramesorectal vessel involvement, consistent with T category of T4b, in 59-year-old patient. Axial oblique (E) and sagittal (F) T2WI show rectal tumor extending through extramesoretal vein (arrow), which according to expert opinion warrants classification as T4b. (G, H) Rectal cancer with pelvic floor muscle involvement, consistent with T category of T4b, in 42-year-old patient. Axial oblique (G) and coronal oblique (H) T2WI show mucinous tumor invading anal sphincter complex and extending laterally to involve right puborectalis muscle and at least abut right ilio-pubococcygeus muscle (blue arrow). On left, mucinous component of lesion abuts left iliococcygeus muscle posteriorly (green arrow). Accurate delineation of pelvic floor muscle tumor involvement is important to guide treatment and surgical planning. (I) Rectal cancer with T with external anal sphincter involvement, consistent with T category of T4b, in 47-year-old patient. Axial oblique T2WI shows low rectal mucinous tumor involving external anal sphincter (arrow).

Fig. 4 —

Examples of different T categories of rectal cancer. (A) Axial oblique T2-weighted image (T2WI) in 56-year-old patient shows rectal tumor with T category of T3b, extending beyond muscularis propria and abutting anterior peritoneal reflection (arrow), without thickening of peritoneal reflection. (B) Axial oblique T2WI in 52-year-old patient shows rectal tumor with T category of T4a, with nodular thickening and infiltration of anterior peritoneal reflection (arrow). (C, D) Rectal cancer with bladder involvement, consistent with T category of T4b, in 45-year-old patient. Axial oblique (C) and sagittal (D) T2WI show mucinous rectal tumor with high signal intensity, extending anteriorly with involvement of posterior urinary bladder wall (arrow). (E, F) Rectal cancer with extramesorectal vessel involvement, consistent with T category of T4b, in 59-year-old patient. Axial oblique (E) and sagittal (F) T2WI show rectal tumor extending through extramesoretal vein (arrow), which according to expert opinion warrants classification as T4b. (G, H) Rectal cancer with pelvic floor muscle involvement, consistent with T category of T4b, in 42-year-old patient. Axial oblique (G) and coronal oblique (H) T2WI show mucinous tumor invading anal sphincter complex and extending laterally to involve right puborectalis muscle and at least abut right ilio-pubococcygeus muscle (blue arrow). On left, mucinous component of lesion abuts left iliococcygeus muscle posteriorly (green arrow). Accurate delineation of pelvic floor muscle tumor involvement is important to guide treatment and surgical planning. (I) Rectal cancer with T with external anal sphincter involvement, consistent with T category of T4b, in 47-year-old patient. Axial oblique T2WI shows low rectal mucinous tumor involving external anal sphincter (arrow).

Fig. 4 —

Examples of different T categories of rectal cancer. (A) Axial oblique T2-weighted image (T2WI) in 56-year-old patient shows rectal tumor with T category of T3b, extending beyond muscularis propria and abutting anterior peritoneal reflection (arrow), without thickening of peritoneal reflection. (B) Axial oblique T2WI in 52-year-old patient shows rectal tumor with T category of T4a, with nodular thickening and infiltration of anterior peritoneal reflection (arrow). (C, D) Rectal cancer with bladder involvement, consistent with T category of T4b, in 45-year-old patient. Axial oblique (C) and sagittal (D) T2WI show mucinous rectal tumor with high signal intensity, extending anteriorly with involvement of posterior urinary bladder wall (arrow). (E, F) Rectal cancer with extramesorectal vessel involvement, consistent with T category of T4b, in 59-year-old patient. Axial oblique (E) and sagittal (F) T2WI show rectal tumor extending through extramesoretal vein (arrow), which according to expert opinion warrants classification as T4b. (G, H) Rectal cancer with pelvic floor muscle involvement, consistent with T category of T4b, in 42-year-old patient. Axial oblique (G) and coronal oblique (H) T2WI show mucinous tumor invading anal sphincter complex and extending laterally to involve right puborectalis muscle and at least abut right ilio-pubococcygeus muscle (blue arrow). On left, mucinous component of lesion abuts left iliococcygeus muscle posteriorly (green arrow). Accurate delineation of pelvic floor muscle tumor involvement is important to guide treatment and surgical planning. (I) Rectal cancer with T with external anal sphincter involvement, consistent with T category of T4b, in 47-year-old patient. Axial oblique T2WI shows low rectal mucinous tumor involving external anal sphincter (arrow).

Fig. 5—

Illustration shows axial view of rectum with mesorectum (yellow) and mesorectal fascia (MRF) (green), depicting four different situations per quadrant that would change status of MRF. MRF is considered involved when distance between extramural vascular extension (A), primary tumor extension (B), irregular node completely replaced by tumor (C), or tumor deposit (D) and MRF is less than 1 mm. However, smoothly outlined lymph nodes close to MRF do not change status of MRF, as their presence has not been associated with increased local recurrence. Illustration. (© 2024 Memorial Sloan-Kettering Cancer Center, Memorial Hospital for Cancer and Allied Diseases, and Sloan-Kettering Institute for Cancer Research, each in New York, NY. All rights reserved. Republished with permission.)

Fig. 6—

Illustration shows sagittal and axial oblique views of rectum, highlighting variations based on tumor location and relationship with anterior peritoneal reflection (APR), peritoneum (blue), and mesorectal fascia (MRF) (green). Below APR, MRF is circumferential, while above APR, MRF may be present laterally and posteriorly. In upper rectum, anterior tumor above level of APR will not involve MRF (MRF−); if there is nodular thickening of peritoneum, tumor will be classified as T4a. However, lateral or posterior tumor extension may result in both T4a and MRF involvement (MRF+). Tumors below APR with locoregional involvement of organs are classified as T4b with involved MRF (MRF+). (© 2024 Memorial Sloan-Kettering Cancer Center, Memorial Hospital for Cancer and Allied Diseases, and Sloan-Kettering Institute for Cancer Research, each in New York, NY. All rights reserved. Republished with permission.)

Fig. 7—

Sagittal (A) and axial oblique (B) T2-weighted images from baseline MRI in 62-year-old patient show large mid rectal tumor broadly involving mesorectal fascia (MRF) posterolaterally. Axial oblique T2-weighted image (C) post chemoradiotherapy shows marked decrease in tumor bulk with extensive scaring and fibrosis in right side of MRF, with no restricted diffusion on DWI (D). On restaging MRI, it is important to report that MRF is involved based on bulky fibrotic tissue given that MRI cannot exclude residual microscopic tumor within fibrotic tissue.

Fig. 7—

Sagittal (A) and axial oblique (B) T2-weighted images from baseline MRI in 62-year-old patient show large mid rectal tumor broadly involving mesorectal fascia (MRF) posterolaterally. Axial oblique T2-weighted image (C) post chemoradiotherapy shows marked decrease in tumor bulk with extensive scaring and fibrosis in right side of MRF, with no restricted diffusion on DWI (D). On restaging MRI, it is important to report that MRF is involved based on bulky fibrotic tissue given that MRI cannot exclude residual microscopic tumor within fibrotic tissue.

Fig. 7—

Sagittal (A) and axial oblique (B) T2-weighted images from baseline MRI in 62-year-old patient show large mid rectal tumor broadly involving mesorectal fascia (MRF) posterolaterally. Axial oblique T2-weighted image (C) post chemoradiotherapy shows marked decrease in tumor bulk with extensive scaring and fibrosis in right side of MRF, with no restricted diffusion on DWI (D). On restaging MRI, it is important to report that MRF is involved based on bulky fibrotic tissue given that MRI cannot exclude residual microscopic tumor within fibrotic tissue.

Fig. 7—

Sagittal (A) and axial oblique (B) T2-weighted images from baseline MRI in 62-year-old patient show large mid rectal tumor broadly involving mesorectal fascia (MRF) posterolaterally. Axial oblique T2-weighted image (C) post chemoradiotherapy shows marked decrease in tumor bulk with extensive scaring and fibrosis in right side of MRF, with no restricted diffusion on DWI (D). On restaging MRI, it is important to report that MRF is involved based on bulky fibrotic tissue given that MRI cannot exclude residual microscopic tumor within fibrotic tissue.