Rigid sigmoidoscopy and MRI are not interchangeable in determining the position of rectal cancers

Purpose1) To analyse for interchangeability of rigid sigmoidoscopy and MRI in determining the distance from anus to tumour, and to determine if anterior/posterior location influences this difference. 2) To analyse the effect of preoperative chemo-radiotherapy on the distance from anus to tumour.MethodsRetrospective investigation of endoscopy reports and MRI series of 144 consecutive patients operated for rectal cancer.ResultsThe mean distance from the anal verge to the tumour measured by sigmoidoscopy was 82 mm and by MRI 61 mm (p < 0.01). For tumours in the anterior quadrant this difference was 30 mm and for tumours located in the posterior quadrant only 12 mm. The distributions of the cancers as low, middle and high differ by more than 10% between the two methods. The coefficient of correlation between measurements was 0.9 but the variation was not acceptable. The length of the tumours decreased by 16 mm after neoadjuvant treatment, but the distance from tumour to anus increased by only 4 mm.Conclusion1) MRI and sigmoidoscopy are not interchangeable in determining the distance from anus to tumour simply by correcting for the length of the anal canal. It has not been determined if measurements from MRI or sigmoidoscopy are more accurate, but current evidence concerning the effect of neoadjuvant irradiation at different positions in the rectum is based upon rigid sigmoidoscopy. 2) The gain in tumour free distance above the anus induced by neoadjuvant treatment is small. Facilitation of sphincter-saving surgery should not be an argument for neoadjuvant treatment.