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Rectal carcinoma

Epidemiology

  • Colorectal ca is 3rd leading cause of cancer death in each sex
  • Peak incidence – 70yrs

Etiology

  • Ageing – dominant risk factor
  • Hereditary conditions – FAP, HNPCC
  • Environmental and dietary – high fat diet, processed meats, smoking, alcohol
  • Inflammatory bowel disease – Crohn’s disease, ulcerative colitis
  • Other risk factors – male sex, history of other cancers

Pathogenesis

  • Results from the accumulation of multiple genetic mutations arising from 2 major pathways

Loss of Heterozygosity pathway (development of FAP)

  • Mutations
    • Mutations in APC gene and KRAS gene
    • Loss of DCC tumour suppressor gene
    • Mutation of p53
  • This pathway carries a poor prognosis

Replication Error Repair pathway (development of HNPCC)

  • Germline mutations in enzymes involved in repairing errors that occur normally during DNA replication
  • These genes are – hMSH2 + SH6; hMLH1 + LH3; hPMS1, + MS2
  • These replication errors accumulate and can be detected in microsatellites of repetitive DNA sequences
    • Leads to microsatellite instability
  • This pathway carries a better prognosis

Classification – WHO histological classification

  • Adenocarcinoma – 90%
  • Mucinous adenocarcinoma
  • Signet ring cell carcinoma
  • Small cell carcinoma – rare, very poor prognosis
  • Squamous cell carcinoma
  • Undifferentiated carcinoma

Staging – Duke’s Criteria

  • A – growth limited to rectal wall
  • B – invasion through rectal wall penetrating the muscular layer, but not involving lymph nodes
  • C – involvement of lymph nodes
  • D – widespread metastases

Clinical features

  • Can be asymptomatic
  • Change in bowel habits – diarrhoea, constipation, tenesmus
  • Rectal bleeding
  • Mucus in stools – spurious diarrhoea
  • Anal, perineal, sacral pain (invasion of sacral plexus)
  • Mass may be palpable on per rectal exam
  • Ascites, liver secondaries

Investigations

  • Digital rectal exam
  • Barium enema
  • Sigmoidoscopy and colonoscopy – biopsy can be made
  • Transrectal US – pre-operative staging of rectal carcinoma
  • MRI – imaging modality of choice to stage the disease
    • The mesorectal fascia is a layer of connective tissue enclosing the perirectal fat surrounding the rectum
      • It extends from the beginning of the rectum to levator ani
      • Contents – perirectal fat which contains the superior rectal artery and vein, lymph nodes and vessels
      • Important landmark in rectal cancer staging – forms the circumferential resection margin (CRM)
      • The distance between the tumour and the mesorectal fascia is predicative of whether complete resection will be possible
      • A margin of <1cm denotes that local recurrence is likely

Treatment

Surgery (see pic)

  • Abdomino-perineal resection (Miles operation)
    • For low rectal cancers
    • Removal of the anus, rectum and part of sigmoid colon
    • Remaining part of the sigmoid is brought out as end colostomy
  • Anterior resection
    • For growths in the mid and upper rectum
    • Followed by colorectal anastomosis
  • Hartmann’s operation
    • Used as a palliative procedure in elderly patients who are not fit for major resection
    • Resection of rectosigmoid colon with closure of the recto-anal stump, and formation of an end colostomy

Radiotherapy

  • Adenocarcinoma responds well to radiotherapy
  • Can be used postoperatively to downgrade the tumour

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