RECTUM/ANUS
RECTAL CA STAGING
POST TREATMENT RECTAL CANCER
RESTAGING
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.
SURGICAL MANAGEMENT
Total mesorectal excision (TME): standard transabdominal surgery w/ complete resection of the mesorectum along the MRF plane.
Low anterior resection (LAR): indicated for middle or upper rectal tumors, TME + partial or total sigmoid resection; anastomosis b/n rectum and sigmoid colon. must have sufficient margin b/n lower tumor border and anal canal. Most commonly side-to-side anastomosis.
Standard abdominoperineal resection (APR): indicated for tumors that infiltrate the anal canal or levator ani and/or external sphincter, located < 1 cm from the anal verge. Resection includes the sphincter complex resulting in incontinence and permanent colostomy.
Intersphincteric APR: sphincter sparing surgery considered when the intersphincteric plane is not involved allowing the external sphincter to be preserved.
Transanal endoscopic microsurgery: focal endoscopic resection of tumor for the following indications: well/moderately differentiated rectal CA categorized as cT1 or cN0, size <3 cm, w/n 8 cm of anal verge, or involve <30% of wall circumference.
Ultra-low anterior resection: sphincter sparing surgery for low rectal CA above the ARJ. Coloanal anastomosis is created 1 cm distal to lower edge of tumor.
Extralevator APR: tumors that infiltrate the intersphincteric plane and external sphincter and/or levator ani. A broader dissection of the sphincter complex and includes levator ani muscles.
RSNA EVALUATING TX RESPONSE
RSNA RECTAL CA TX RESPONSE.pdfCONSIDERATIONS 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.
ANUS
Anal canal: extraperitoneal
Anatomic: extends from dentate line to anal verge
Surgical: extends from ARJ to anal verge
Anal sphincter complex:
EAS: outermost striated external anal sphincter; voluntary control. 15-20% of resting anal tone.
Intersphincteric space: thin fat containing space that contains longitudinal smm layer
IAS: smooth muscle layer; maintains anal sphincter resting tone (85% of maximal anal resting tone)
Puborectalis mm
Provides constant tone causing anterior displacement of anal canal resulting in acute anorectal angle.
Acute angle: resists fecal outflow, essential for maintaining rectal continence.
To defecate, puborectalis relaxis and brief valsalva augments pelvic floor descent.
To defer defecation, puborectalis contracts, causing rectum to become more perpendicular to anal canal which elevates pelvic floor and lengthens anal canal.
Arterial supply: inferior rectal artery
Venous drainage: pudendal vein -> internal iliac vein
Lymphatic drainage: superficial inguinal nodes
Nerve supply: pudendal nerves and inferior rectal nerves
PERIANAL FISTULA/ABSCESS
Perianal fistula: abnormal communication b/n anal canal and perianal/perineal skin; chronic condition.
Simple: minimal involvement of external sphincter.
Superficial fistula: no relation to the sphincter or perianal glands
Sinus tract refers to a primary tract that terminates blindly in SQ fat tissue.
Primary cause of PAF (cryptoglandular theory): obstruction of anal gland -> secretion accumulation -> infection -> abscess -> sinus/fistulous tracts in fatty tissue.
Secondary cause of PAF: Crohn, TB, infection, diverticulitis, trauma, malignancy, radiotherapy, surgery
Parks classification system uses the external anal sphincter as landmark to assess the relationship of primary tract to anal sphincter.
Treatment of AF is dictated by classification and amount of sphincter complex that is involved. All types can be complicated by abscess/tracts
Four different types: Intersphincteric (45%), transphincteric (30%), suprasphincteric (20%), extrasphincteric (5%)
REPORTING
Internal opening: Report mucosal opening on axial images using anal clock positioning.. Most are located at level of dentate line and at posterior midline.
Course:
Course of primary fistulous tract and its relationship w IAS and EAS as well as secondary associated tracts.
Describe extension into ischioanal, perineal, gluteal or supralevator regions.
External opening: can be multiple.
Report distance of mucosal defect to the perianal skin on coronal images.
Treatment focuses on the elimination of primary and secondary tracts, prevention of recurrence and to retain continence.
Most fistulas are simple w/ minimal involvement of the EAS. They can be treated easily w fistulotomy w minimal risk to continence.
Treatment considerations:
Fistula is 2/2 cryptoglandular disease vs Crohn's disease
Crohn's dz requires a combined medical (steroids, immunomodulators) and surgical approach (seton placement)
Complexity and location of fistula (low vs high)
If fistula represents a recurrent process.
Definite surgical tx includes: fistulotomy or cutting setons for low, simple fistulae vs mucosal advancement flaps and surgical variants for more complex fistulae.
Fistulotomy (AKA laying open technique)
MC surgical procedure; Involves creating an incision in fistula tract to open it and merge it with the anal canal to allow healing.
Seton fistulotomy: rubber ligature/vessel loop pulled through fistula then tightened every 2 wks to cause pressure necrosis so mm is slowly cut and fibrosed in order to cause as little damage as possible to sphincteric complex. The mucosal defect is then surgically closed. Marsupialization of fistulotomy wounds has been reported to improve healing.
Indications:
Intersphinctric and low transsphincteric fistula: Low risk of fecal incontinence.
Complex perianal fistulaes: >50% fecal incontinence
Fistulectomy
Excision of the complete fistula tract. Creates a larger wound than fistulotomy.
Indication: intersphincteric or low transsphincteric types.
Similar rate of recurrence compared to fistulotomy.
Noncutting seton: preferred as an alternative to fistulotomy/ectomy which can both result in nonhealing wounds. Setons prevent the closure of the external opening of the fistula which allows drainage and thereby helps to prevent abscess recurrence. Removal of seton has recurrence rate of 39%. Rate of fecal incontinence is 5%.
Mucosal advnacement flap repair: preferred tx for complex perianal fistula. Excision of flap of mucosal tissue around internal opening of fistua. flap is used to close the fistula tract via sutering.
Proctectomy: tx extrasphincteric fistula in Crohns. required in 10-15% and up to 4% require proctocolectomy w creation of permanent ileostomy.
