Low rectal cancers represent a particular patient population that may behave differently from the remainder of patients with CRC. The low rectum has lymphatic drainage to inguinal nodes and the more common pelvic side wall and intra-abdominal nodes. The venous drainage of the low rectum may bypass the liver, and lead to increase in pulmonary metastases in this group. For these very reasons a recent investigation found that FDG PET-CT altered treatment plans in patients with untreated rectal cancers 38% of the time primarily by detecting disease in inguinal nodes  with most occult disease found in tumors within 6 cm of the anal verge. A case of rectal cancer with inguinal nodes is presented in Fig. 36.
Induction chemotherapy or induction chemora-diation therapy is often given to patients with advanced rectal cancer to downstage disease, allow sphincter-preservation, and decrease local recurrence rates. A number of groups have investigated the use of posttreatment, preoperative FDG PET in the prediction of local control and overall survival. It is hypothesized that a positive FDG scan posttreatment may portend a biologically more
aggressive or less chemosensitive-type tumor. In contradistinction, if complete pathologic responses to therapy can be predicted by PET, surgery might in theory be avoided. It seems that FDG PET is the current gold standard in determining the response to induction treatment  and prognosis .
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