N Staging

Nodal involvement can be locoregional or distant. The first lymph nodes to be involved by the tumour are perirectal lymph nodes. N1 indicates the presence of 1-3 pathologic perirectal lymph nodes, and N2 the presence of over 3 lymph nodes present in the same site (Fig. 6). Perirectal lymph nodes run along a central route that along the superior haemorrhoidal artery reaches the inferior mesenteric artery (IMA) to its opening into the abdominal aorta. Involvement of

IMA lymph nodes defines N3 (Fig. 7). Lateral outflow lymph nodes localised in the ischiorectal fossae through the inferior haemorrhoidal and pudenda reach the internal iliac chain. This is the usual route of neoplasms of the anal canal through which rectal tumours spread only in the presence of a lymphatic central obstruction. Involvement of lateral lymph nodes defines M1.

Nodal spread of metastases is detected on CT (74% sensitivity, 66% specificity, 71% accuracy) according to the criterion of the size: in the past only lymph nodes of over 1 cm in diameter were considered positive. In some cases, this finding resulted in understaging, thus with major prognostic and therapeutic failures [15,17, 20, 22, 25].

It is well known that the size of lymph nodes may have no relation to the neoplastic involvement, as proven by the frequent histologic finding of metastatic foci in lymph nodes of 1 cm or less in diameter [15,17,18, 20, 29].

However it should be underlined that CT does not always enable the differentiation of inflammatory and metastatic lymphadenopathy, because the den-sitometric pattern is almost identical. This could result in diagnostic failure and overstaging. Together with other Authors [15, 20] we consider pathological lymph nodes visualised at the perirectal level apart from their size while the latter is taken into account for iliac, mesenteric, inguinal and para-aortic lymph nodes. In our opinion the use of contrast helps in the differentiation of small vascular structures from lymph nodes.

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