Post-treatment MRI Evaluation of Rectal Cancer

Post-treatment MRI Evaluation of Rectal Cancer

Author: Renata Rocha de Almeida, Shanna Matalon MD, Daniel Souza, Leslie Lee, Stuart Silverman

Date: December 2021

Post-treatment MRI Evaluation of Rectal Cancer

Indications and Types of Neoadjuvant Treatmen

Indication for neoadjuvant chemoradiation based on current NCCN guidelines:
       Stages II & III rectal cancer:

  • T3 (invades perirectal tissues) or T4 (invades adjacent organs), with any N disease,
  • N1 or N2 disease (at least one regional node/deposit), with any T disease,
  • Locally unresectable or medically inoperable tumors
  • Some low rectal tumors to allow for sphincter sparing resection

Current neoadjuvant chemotherapy options: 5-FU (infusional fluorouracil or oral capecitabine), oxaliplatin, +/- leucovorin, +/- irinotecan.

Neoadjuvant radiation:

  • Long course: ~50 Gy in 2528 fractions daily over 5-6 weeks + radiosensitizing chemotherapy (5-FU or capecitabine)
  • Short course: 25 Gy in 5 fractions over 1 week (without chemotherapy)

While highest radiation dose is aimed at the rectum, other organs also receive high doses of radiation, such as the bladder, sacrum, pre-sacral fat, and prostate. Lower radiation fields may be used to encompass inguinal lymphadenopathy in the setting of low rectal tumors.

Neoadjuvant radiation Neoadjuvant radiation

“Total neoadjuvant therapy” (TNT): All chemotherapy and radiation therapy given prior to surgery, with various permeations available. May be used for any of the following indications: any T4, any N2, tumors with threatened resection margin, and some low rectal tumors to allow for sphincter sparing resections.

MRI Technique - Post-treatment MRI

Sequence Area of interest

Coronal T2
Large FOV (include upper abdomen)

Screen for metastases

Sagittal T2
Large FOV (pelvis only)

Prescribe oblique planes
Residual tumor location/distance from anal verge
Relationship to peritoneal reflection and other pelvic structures (i.e. uterus, prostate)

Short axis oblique T2*
High-resolution, small FOV

Assess primary tumor and lymph nodes

Long axis oblique T2/coronal T2
High-resolution, small FOV

Relationship of primary tumor to sphincter complex (low cancers)

DWI & ADC*
Large FOV (pelvis only)

Assess for persistent diffusion restriction of residual tumor
Localize lymph nodes
Reassessment of extramural venous invasion

Post contrast**
**Not required per Society of Abdominal Radiology (SAR) guidelines but may add value in assessing treatment response in non-mucinous tumors

Assess for persistent enhancement of residual tumor and heterogeneity of lymph nodes

*Key sequence to assess residual tumor

Prescribing oblique planes for angled T2-weighted images:

  • Provides optimal anatomic information
  • Reduces volume averaging
Tumor may be difficult to identify after treatment Look at the pre-treatment MRI to identify tumor location Draw oblique lines in the form of a “fishbone” based on the long and short axis of tumor
Prescribing oblique planes for angled T2-weighted after treatment Prescribing oblique planes for angled T2-weighted pre-treatment Prescribing oblique planes for angled T2-weighted

T-Stage: Reassessment of The Tumor Bed

MRI Features of Treatment Response

Treatment response features on T2:

  • Decrease or resolution of T2-intermediate signal of viable tumor
  • Development of T2-hypointense fibrosis within the rectal wall or adjacent mesorectal fat (desmoplastic reaction)
  • Acellular mucin production: new or increased T2-hyperintense signal, similar to water

1. Fibrosis

  Initial Staging MRI – T2WI After Neoadjuvant Treatment MRI
– T2WI
Comment
Small / focal Initial Staging MRI – T2WI, small/focal After Neoadjuvant Treatment MRI – T2WI, Small / focal

Well-defined thin T2-hypointense signal (arrow)

Fibrosis confined to the rectal wall

Complete response

Semicircular/full-thickness 

Initial Staging MRI – T2WI, Semicircular/ full-thickness After Neoadjuvant Treatment MRI – T2WI, Semicircular/ full-thickness

Thick T2-hypointense signal (*)

Fibrosis involves the full thickness of rectal wall (*)

+/- Desmoplastic reaction (Arrow: T2-hypointense lines in mesorectal fat)

Usually complete response
Spiculated Initial Staging MRI – T2WI, Spiculated
Irregular spiculated tumor (*)
After Neoadjuvant Treatment MRI – T2WI, Spiculated

Spiculated T2-hypointense signal (arrow)

Desmoplastic reaction (arrowhead)

More frequent residual tumor

2. Mucinous degeneration in non-mucinous adenocarcinoma

Initial MRI – T2WI During Neoadjuvant Treatment – T2WI After Neoadjuvant Treatment – T2WI
Initial MRI – T2WI, Nodular T2-intermediate tumor signal During Neoadjuvant Treatment – T2WI, Replaced by T2-hyperintense signal of mucin After Neoadjuvant Treatment – T2WI, May be replaced by T2-hypointense scar
Nodular T2-intermediate tumor signal (arrow) Replaced by T2-hyperintense signal of mucin (arrow) May be replaced by T2-hypointense scar (arrow)

3. Increased mucin in mucinous/signet ring cell cancers

Subtype Initial MRI After Neoadjuvant Treatment
Mucinous adenocarcinoma

T2-hyperintense signal reflects acellular /nonviable tumor
T2-intermediate signal reflects cellular/viable tumor
(Not shown)

Increased T2-hyperintense signal
Decreased/Resolved T2-intermediate signal
(Not shown)

Signet-ring cell adenocarcinoma T2-intermediate thickening of submucosa Increased T2-hyperintense signal of submucosa

Treatment response features on DWI & ADC:

  • Resolved or decreased extent of restricted diffusion in tumor bed
  • Increased ADC signal and decreased DWI signal in tumor bed
  • Signal on DWI is similar to ADC (high-DWI & high-ADC OR low-ADC & low-DWI)

Non-mucinous Adenocarcinoma

Sequence Initial MRI After Neoadjuvant Treatment Comment
T2 Non-mucinous Adenocarcinoma, T2-intermediate tumor
T2-intermediate tumor (*)
Non-mucinous Adenocarcinoma, T2 After Neoadjuvant Treatment Increased signal in tumor bed (*) due to acellular mucin production
DWI Non-mucinous Adenocarcinoma, DWI
DWI hyperintense tumor (*)
Non-mucinous Adenocarcinoma, DWI, After Neoadjuvant Treatment Decreased signal in tumor bed (*) due to loss of cellularity
ADC Non-mucinous Adenocarcinoma, ADC
ADC hypointense tumor (*)
Non-mucinous Adenocarcinoma, ADC, After Neoadjuvant Treatment Increased signal in tumor bed (*) due to loss of cellularity

MRI Features of Residual Tumor

Residual tumor features on T2 and T1FSC+:

  • Persistent nodular-shaped T2-intermediate signal in tumor bed (in non-mucinous tumors)
  • Persistent T2-intermediate signal in submucosa (signet-ring cell subtype)
  • Early enhancement tumor bed ≥ than initial MRI
  • Unchanged or minimally decreased cellular mucin (in mucinous tumors)

1. Residual disease in a non-mucinous adenocarcinoma

Initial MRI After Neoadjuvant Treatment Comment
Residual disease, T2-intermediate tumor
T2-intermediate tumor (arrow)
Residual disease, After Neoadjuvant Treatment

Persistent nodular-shaped T2-intermediate signal in tumor bed (arrow)

Residual disease, Hyperenhancing tumor
Hyperenhancing tumor (*)
Residual disease, After Neoadjuvant Treatment

Early enhancement tumor bed (arrow) > than initial MRI

2. Residual disease in a mucinous asdenocarcinoma

Initial MRI After Neoadjuvant Treatment Comment
Residual disease in a signet ring cell adenocarcinoma, Initial MRI
T2-hyperintense tumor (arrows)
Internal intermediate signal (*)
Residual disease in a signet ring cell adenocarcinoma, After Neoadjuvant Treatment

Decreased T2-hyperintense mucinous tumor (arrows)

Persistent T2-intermediate signal (*) of cellular mucin

3. Residual disease in a signet ring cell adenocarcinoma

Initial MRI After Neoadjuvant Treatment Comment
Residual disease in a signet ring cell adenocarcinoma, Initial MRI
T2-intermediate tumor in submucosa (*)
Residual disease in a signet ring cell adenocarcinoma, After Neoadjuvant Treatment Improved but persistent T2-intermediate signal of cellular mucin in mucinous tumor

Reassessment of Circumferential Resection Margin (CRM)

Reassessment of mesorectal fascia (MRF) involvement

  • Resolved MRF involvement: Development of clear fat plane (≥ 2mm) ± desmoplastic reaction between tumor and MRF
  • Persistent MRF involvement: Persistent T2-intermediate or T2-hyperintense signal infiltration of MRF
  • Indeterminate MRF involvement: Diffuse fibrotic (T2-hypointense) infiltration of MRF
Initial MRI After Neoadjuvant Treatment Comment
Reassessment of Circumferential Resection Margin (CRM), Initial MRI
T2-intermediate tumor involves MRF (arrows)
Reassessment of Circumferential Resection Margin (CRM), After Neoadjuvant Treatment

New fat plane (arrow) with desmoplastic reaction (arrowhead) between tumor bed and MRF

Resolved MRF Involvement
Reassessment of Circumferential Resection Margin (CRM), Initial MRI
T2-intermediate tumor involves MRF (arrow)
Reassessment of Circumferential Resection Margin (CRM), After Neoadjuvant Treatment

Persistent T2-intermediate signal of tumor along MRF (arrow)

Persistent MRF Involvement
Reassessment of Circumferential Resection Margin (CRM), Initial MRI
T2-intermediate tumor involves MRF (arrow)
Reassessment of Circumferential Resection Margin (CRM), After Neoadjuvant Treatment

Thick T2-hypointense signal of fibrosis along MRF (arrow)

Indeterminate MRF Involvement

Reassessment of sphincter complex in low rectal cancers

  • Allow sphincter-sparing surgery: Tumor involves up to the intersphincteric fat but does not involve external sphincter; > 2 cm from anal verge
  • Does not allow sphincter-sparing surgery: Tumor invades external sphincter; < 1-2 cm from anal verge

Reassessment of extramural vascular invasion

Initial MRI After Neoadjuvant Treatment Comment
Reassessment of extramural vascular invasion, Initial MRI
T2-intermediate signal along vessels (arrow)
Reassessment of extramural vascular invasion, After Neoadjuvant Treatment

Resolved T2-intermediate signal along mesorectal vessels

Replacement by linear T2-hypointense fibrosis (arrow)

Resolved extramural vascular invasion (EMVI)

Reassessment of extramural vascular invasion, Initial MRI
T2-intermediate signal along vessels (arrow)
Reassessment of extramural vascular invasion, After Neoadjuvant Treatment

Improved but persistent nodular T2-intermediate signal along mesorectal vessels (arrow)

Persistent extramural vascular invasion (EMVI)

N-Stage: Nodal Reassessment

Locoregional nodes (N1/2): Mesorectal, superior rectal, obturator (posterior to external iliac vessels), and internal iliac stations (and inguinal nodes, sometimes considered locoregional if a low rectal cancer involving anal canal).

  • “Lateral lymph nodes”: Refers to obturator and internal iliac stations, which are not removed with standard surgical resection; it is important to report these separately as at some institutions, a lateral lymph node dissection will also be performed.

Non-locoregional nodes (M1): external iliac (anterior or lateral to external iliac vessels), common iliac, and paraaortic, and inguinal stations (if mid or high rectal cancer)

MRI Features of Nodal Treatment Response

  • Disappearance or decreased size to < 5mm short axis
  • Replacement by T2-hypointense fibrosis
  • New/Increased T2-hyperintense signal of acellular mucin
Initial MRI After Neoadjuvant Treatment Comment
MRI Features of Nodal Treatment Response, Initial MRI
Enlarged and heterogeneous mesorectal (white arrow) and internal iliac nodes (black arrow)
MRI Features of Nodal Treatment Response, After Neoadjuvant Treatment

Complete disappearance of mesorectal and internal iliac nodes  

Replacement by T2-hypointense fibrosis (arrow)

MRI Features of Nodal Treatment Response, Initial MRI
Enlarged & T2 hyperintense superior rectal nodes (arrows) with internal T2-intermediate signal (*)
MRI Features of Nodal Treatment Response, After Neoadjuvant Treatment

New/Increased T2-hyperintense signal of acellular mucin (arrow)

Decreased size (arrowhead)

MRI Features of Residual Tumor in Nodes

  • Size ≥ 5 mm in short axis (**only feature considered by SAR/ESGAR panels)
  • Persistent T2-intermediate signal, irregular contour, heterogeneous signal
Initial MRI After Neoadjuvant Treatment
MRI Features of Residual Tumor in Nodes, Initial MRI

Enlarged lymph nodes with T2-intermediate signal and irregular contour in the obturator (white arrow) and internal iliac (black arrow) stations. O=Obturator muscle

MRI Features of Residual Tumor in Nodes, After Neoadjuvant Treatment
Most nodes have decreased size but there is a persistent node with ≥ 5 mm in short axis and irregular contour (white arrow). O=Obturator muscle

Pitfalls on Post-Treatment MRI

Pseudotumor

  • Pseudotumor is a mucosal edema after radiation involving the rectal wall not previously involved by tumor, appearing as T2-hyperintense signal
Initial MRI – T2WI After Neoadjuvant Treatment – T2WI
Pitfalls on Post-Treatment MRI, Initial MRI - T2WI
T2-intermediate tumor invading the mesorectal fat at 12 o’clock (arrow) with heterogeneous tumor deposits (arrowheads)
Pitfalls on Post-Treatment MRI, After Neoadjuvant Treatment – T2WI
T2-hypointense fibrosis in rectal wall at 12 o’clock (arrow). New adjacent T2-hyperintense masslike thickening posteriorly (*) (Pseudotumor)

Persistent restricted diffusion versus T2-shine-through OR T2-dark-through

  • T2-shine-through and T2-dark-through are seen in tumor response
    • T2-shine-through (hyperintense signal on DWI and ADC) happens due to decreased cellularity at the tumor bed
    • T2-dark-through (hypointense signal on DWI and ADC) happens due to increased cellularity of dense fibrosis replacing the tumor bed
  • Tip: Equal (low-low or high-high) signal in DWI and ADC means response rather than viable tumor

T2-shine-through in a patient with a complete response

Sequence Initial MRI After Neoadjuvant Treatment
T2WI T2-shine-through in a patient with a complete response, Initial MRI, T2WI
Circumferential T2-intermediate tumor (*)
T2-shine-through in a patient with a complete response, After Neoadjuvant Treatment, T2WI
Persistent T2-intermediate signal in tumor bed (*) raises question of residual tumor

DWI

T2-shine-through in a patient with a complete response, Initial MRI, DWI
DWI hyperintense tumor (*)
T2-shine-through in a patient with a complete response, After Neoadjuvant Treatment, DWI
Persistent DWI hyperintense signal (*) in tumor bed also raises question of residual tumor
ADC T2-shine-through in a patient with a complete response, Initial MRI, ADC
ADC hypointense tumor (*)
T2-shine-through in a patient with a complete response, After Neoadjuvant Treatment, ADC
New ADC hyperintense signal in tumor bed due to T2-shine-through and treatment response

T2-dark-through in a patient with complete response

Sequence Initial MRI After Neoadjuvant Treatment
T2WI T2-dark-through in a patient with complete response, Initial MRI, T2WI
T2-intermediate tumor (*)
T2-dark-through in a patient with complete response, After Neoadjuvant Treatment, T2WI
New T2-hypointense signal in tumor bed (*) due to fibrosis

ADC

T2-dark-through in a patient with complete response, Initial MRI, ADC
ADC hypointense tumor (*)
T2-dark-through in a patient with complete response, After Neoadjuvant Treatment, ADC
Persistent ADC hypointense signal in tumor bed (*) raises question of residual tumor

DWI

T2-dark-through in a patient with complete response, Initial MRI, DWI
DWI hyperintense tumor (*)
T2-dark-through in a patient with complete response, After Neoadjuvant Treatment, DWI
New DWI hypointense signal in tumor bed (*) due to T2-dark-through

Complications of Neoadjuvant Treatment

Fistulas:
Tumor-to-bladder (arrow)
Complications of Neoadjuvant Treatment, Fistulas

Proctitis, Enteritis, Colitis:
Enterocolitis (arrows)
Complications of Neoadjuvant Treatment, Proctitis, Enteritis, Colitis

Insufficiency Fracture:
Sacrum (arrows)
Complications of Neoadjuvant Treatment, Insufficiency Fracture

Pre-sacral and mesorectal edema:
Complications of Neoadjuvant Treatment, Pre-sacral and mesorectal edema

Other (not shown): Avascular necrosis of femoral heads, Cystitis

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