Carcinoma Rectum :MRI

Case Report : 53 year old female, known CA rectum with no prior details for MRI shows grossly irregular rectum with circumscribed mural lesion from approx. 9 O’ clock to 2 O’clock positions with perirectal fat ill definition, maintained mesorectal fascia, no neighboring organ invasion or significant lymphadenopathy ( closest distance between tumor margin & mesorectal fascia of approx. 10mm – likely represent CA rectum of T3 variety in view of history.

 Discussion by Dr MGK Murthy, Dr GA Prasad

MR-  useful to assist staging  & identify­ patients who may benefit from preoperative chemotherapy–radiation therapy, and in surgical planning.

Currently, surgical resection with stage-appropriate neoadjuvant combined-modality therapy is the mainstay in the treatment of rectal cancer.

Total mesorectal excision (TME) has reduced  the prevalence of local recurrence from 38% to less than 10%. TME is  surgical en bloc resection of the primary tumor and the mesorec­tum by means of dissection along the mesorectal fascial plane or the circumferential resection margin (CRM) . Even with TME, however, the presence of a tumor or malignant node within 1 mm of the CRM remains an important pre­disposing factor for local recurrence.

Randomized trials have shown that combined preoperative radiation therapy–TME reduces the prevalence of local recurrence from 8% to 2% and is superior to postoperative radia­tion therapy alone. Also lone  radiation therapy yields little survival benefit and results in significant morbidity when used to treat stage T1–T2 or favorable-risk early stage T3 tumors (<5 advanced="" contrast="" in="" invasion="" mm="" more="" muscularis="" outside="" propria="" stage="" t3="" the="" to="" tumors="">5 mm invasion outside the muscularis propria).

Key sequences -

Orthogonal,  sagittal and coronal high-resolution T2-weighted images of the primary tumor; the mesorectal fascia, peri­toneal reflection, other pelvic viscera; and superior rectal and pel­vic sidewall lymph nodes.  Diffusion-weighted imaging – helpful for identifying nodes and also primary tumor.

Rectal gel may be used for the staging of polypoid tumors, previously treated lesions, small rectal tumors,howevert should not be used to stage large or low rectal tu­mors.

Assessment of the Primary Tumor - 

(a) stage; (b) depth of invasion outside the muscularis propria ( <5mm amp="" early="">5mm - advaced; and (c) relationship to the mesorectal fascia, anal sphincter, and pelvic sidewall.

On T2-weighted images, stage T1 tumors are confined to the submucosa, which manifests as a hy­perintense layer; stage T2 tumors extend into, but not beyond, the muscularis propria, which manifests as a hypointense layer; and stage T3 tumors extend beyond the muscularis propria into the mesorectal fat.

Distance of 1 mm or less on high-resolution T2-weighted images to be indicative of CRM involvement. Measured distance is the distance to the me­sorectal fascia from either (a) the tumor margin, (b) a tumor deposit in the mesorectum, (c) tu­mor thrombus within a vessel, or (d) a malignant node.

Limitations - difficulty in differentiating fibrosis from tumor infiltration i.e. ability to distinguish early stage T3 tumors from stage T2 tumors. Though MR imaging is accurate in advanced stage T3 tumors, considerable experience and good-quality images are required to assess the subtle findings that help distinguish early stage T3 tumors from stage T2 tumors.

Nodal size criteria is less accurate.  30%–50% of metastases in rectal cancer occur in nodes that are less than 5 mm. Nodal margins and internal nodal characteristics are the most reliable indicators of malignancy.