LEARNING OBJECTIVES:

1) To propose a step-by-step approach for standardized MRI staging of pre-treatment rectal carcinoma using the mnemonic "DISTANCE".

2) To become familiar with the different treatment strategies and organ preservation in rectal cancer.

3) To know the performance of modern MRI for the prediction of treatment response.

4) To learn how to evaluate response after chemoradiotherapy and understand how MRI findings may alter surgical approach and affect the likelihood of local and distant recurrence.

ABSTRACT:

Recent changes in the management of patients with locally advanced rectal cancer highlight the need for accurate assessment of tumor response to chemoradiation (CRT). In the past, CRT was followed by surgical resection in nearly all patients, irrespective of response to CRT. However, new data suggest that surgery may not be necessary in patients with complete response. MR imaging has become an essential tool to enable the oncology team to make appropriate treatment decisions. MRI has so far relied on changes in morphology as a measurement for response. However, this evaluation is hampered by the difficulties in differentiating residual tumor from radiation-induced fibrosis. Recent studies have suggested that adding diffusion-weighted imaging (DWI) to conventional MRI can aid this differentiation and thus improve the prediction of response after neoadjuvant therapy. The learning objectives of this lecture are first to know the performance of modern MR imaging for the diagnosis and assessment of response in rectal cancer and to understand how MRI influences treatment. Second, we will use the mnemonic “DISTANCE” ((DIS, for Distance from the Inferior part of the tumor to the transitional Skin; T, for T staging; A, for Anal complex; N, for Nodal staging; C, for Circumferential resection margin; and E, for Extramural vascular invasion) to emphasize the diagnostic clues to interpret rectal MR images before and after CRT.

MAIN DIAGNOSIS TIPS AND TAKE HOME MESSAGES

Clues at the Workstation for T staging (disTance):

-T stage must be assessed on planes strictly perpendicular to the tumor. Incorrect prescription of the acquisition plane leads to blurring of the muscularis propria and may lead to overstaging.

-In differentiating between stage T2 and T3 tumors, the crucial criterion is involvement of the perirectal fat. In stage T3, the muscularis propria is completely disrupted and cannot be clearly distinguished from the perirectal fat: The tumor spreads beyond the muscularis propria into the perirectal fat with a broad-based bulge or nodular appearance.

-Outer longitudinal layer of the muscularis propria can be focally disrupted by small vessels penetrating the wall; this does not necessarily indicate tumor invasion.

-Peritoneal reflection must be assessed in upper rectal tumors. It may be identified on sagittal T2W images as a low-signal linear structure that can be seen extending from the posterior aspect of the dome of the bladder to the ventral aspect of the rectum. On axial, the point of attachment has a V-shaped configuration

Clues at the Workstation for staging low lying tumors (distAnce):

-High-spatial-resolution T2W FSE coronal imaging must be added to optimally depict the tumor relationship with the levator and puborectal muscles, sphincter complex, and intersphincteric plane.

-On coronal T2W images, the commencement of the puborectalis sling marks the start of the narrowest part of the mesorectum; below lies the anal canal. The first question to answer in low lying tumors is where the lower edge of the tumor is located in relation to the puborectalis sling: if the tumor is located above the puborectalis sling, sphincter involvement can be easily excluded.

-When the tumor extends below the puborectalis sling: 3 areas have to be evaluated and reported on: Muscularis propria: Does the tumor invade partially or the full thickness of the muscularis propria (Stage T1 versus T2)

Is there an extension into the intersphincteric plane (Stage T3)?

Is there an extension into the external sphincter (Stage T4)?

-Levator, puborectalis muscles or external sphincter involvement are considered as Stage T4.

Clues at the Workstation for Nodal staging (distaNce):

-Uniform nodes with homogeneous signal intensity are not suspicious.

-Nodes with irregular borders, mixed signal intensity, or both are considered to be suspicious.

-One to three suspicious nodes is stage N1 and four or more is stage N2.

-Any lymph node lying within 1 mm of the circumferential resection margin must be reported because it is highly suspicious of CRM involvement.

-Recording the location and size of any suspicious pelvic sidewall lymph nodes is critical. This will inform the radiotherapy team to change and adjust the radiotherapy field. Secondly, the surgeon will need to perform an extended lymph node resection with additional removal of the internal iliac nodes. This lymph node group is not removed when a regular TME is performed.

Clues at the workstation for a Positive CRM (circumferential resection margin) (distanCe):

-The mesorectal fascia represents the potential CRM in patients undergoing total mesorectal excision surgery.

-A positive margin is defined as tumor lying within 1mm of the mesorectal fascia.

-Positive margins can be due to tumor deposits, main tumor extension, extramural vascular invasion, or suspicious lymph nodes

-Anteriorly the mesorectal fat can be thin, and the rectum can be close to the CRM. In cases where the rectum abuts the mesorectal fascia anteriorly; the tumor must be at least a Stage T3 before discussing CRM involvement.

Clues at the workstation for EVMI (distancE; extramural vascular invasion):

-By definition, EMVI must be associated with tumors that are at least category T3.

-Signs suggestive for EMVI: Presence of tumor signal intensity within a vascular structure. Expanded vessels. Tumoral expansion through and beyond the vessel wall, disrupting the vessel border.

-Finally, If extramural venous invasion is present, considerations of whether the involved veins threaten the mesorectal fascia (i.e., whether they are within 1 mm of the fascia) have to be made.