MRI is most accurate imaging modality for local staging. CT and PET/CT is more useful in detecting distant recurrence. Post-tx changes (surgery or CRT) may be difficult to differentiate from local recurrence as they can share similar imaging features and may also appear FDG avid. Increased size and early heterogeneous marked contrast enhancement invasive behavior and asymmetric appearance are suspicious for local recurrence. 

Before restaging, verify the neoadjuvant tx and results of previous examinations to understand primary tumors location and morphology. It is suggested to perform microenema before performing rectal MRI to reduce the amount of gas w/n the rectum to decrease artifacts at DWI.

Normal rectal wall adjacent to the tumor can manifest with post-CRT changes such as submucosal edema (thickened intermediate to high SI on T2WI). After tx the tumor may appear to be similar in appearance to the pretreatment tumor or may appear atrophic and fibrotic w/ low SI on T2WI. 

MUCIN RESPONSE: tumors can manifest with one of three different mucin responses:

• Mucin/colloid degeneration: can occur in nonmucinous tumros that become mucinous after CRT. It indicates a response to tx and better prognosis.

• Acellular mucin response: pathologic response of a mucinous tumor w no impact on recurrence free survival. 

• Mucinous tumor w/o response: no response to CRT and related to an increased risk of local recurrence and poor outcome. 

RESIDUAL TUMOR & FIBROSIS:

• Residual tumor has intermediate SI on T2WI whereas fibrosis and/or scaring has low SI. Differentiation is still challenging as residual tumor may occur w/n a scar. DWI can help; fibrosis has low SI on high B value while residual tumor shows high SI. 

ASSESSING OTHER POST-TX FEATURES

Regression grades range from 1 to 5; complete response to no response. Accuracy of rectal CA staging in post-CRT tumors is lower than primary staging. 

After CRT, many irradiated LNs will disappear and the majority of remaining nodes are sterilized. Evaluating nodal size in the short axis is more reliable than evaluating borders and shape to assess for residual malignancy. The absnece of LNs at DWI, decrease in size in at least 70% of LNS, and a nodal size < 2.5 mm in short axis is shown to be reliable predictors of negative node status following surgery.

Although 30% of patients can be asx, the majority of patients w/ local recurrence manifest sx and increased CEA levels.

LOCAL RECURRENCE LOCATIONS:

• Axial: recurrence in the anastomotic, residual mesorectum, or perirectal ST in center of the pelvis/perineum including pelvic floor.

• Anterior: recurrence in bladder, vagina, uterus, seminal vesicles, or prostate.

• Posterior: recurrence in presacral fascia, sacrum, coccyx, or sacral root sheaths.

• Lateral: recurrence in the pelvic ureters, iliac vessels, lateral LNs, pelvic nerves, sidewall mm, or lateral pelvic bones. 

Most local recurrences are anastomotic and easily identifie at clinical evaluation and/or endoscopy. 

CONSIDERATIONS AFTER NEOADJUVANT TX

Neoadjuvant tx regimens include short course of radiotherapy prior to TME for intermediate risk tumors and a long course of combined chemoradiotherapy for locally advanced tumors.  

In response to these treatments, rectal tumors typically decrease in size while undergoing a fibrotic transformation with their intermediate T2 signal dropping to markedly hypointense. This makes it difficult to discern on T2WI whether we are dealing with only fibrosis or fibrosis still containing nests of viable residual tumor. 

DWI highlights hypercellular tissues can detect areas of hypercellular residual tumor within fibrotically changed tumor bed. 

Tumor regression grade (mrTRG) is used to grade the degree of fibrosis vs residual tumor. patterns to help assess the risk of persistent MRF invasion in case of fibrosis after CRT.