International journal of radiation oncology, biology, physics 2001-11-01
Elective groin irradiation is not indicated for patients with adenocarcinoma of the rectum extending to the anal canal.   
To evaluate the inguinal nodal failure rate in patients with locally advanced rectal cancer with anal canal involvement (ACI) treated with pelvic chemoradiation without elective inguinal irradiation.
From 1990 and 1998, 536 patients received preoperative or postoperative chemoradiation for rectal cancer with curative intent; 186 patients had ACI (<4 cm from the anal verge on rigid proctoscopy). Two patients had positive inguinal nodes at presentation. Chemoradiation was delivered preoperatively (45 Gy in 25 fraction) or postoperatively (53 Gy in 29 fractions) with concurrent continuous infusion of 5-fluorouracil (300 mg/m2/d). The inguinal region was specifically irradiated in only 2 patients who had documented inguinal nodal disease.
The median follow-up was 50 months. Only 6 of 184 ACI patients who had clinically negative inguinal nodes at presentation developed inguinal nodal recurrence (5-year actuarial rate 4%); 4 of the 6 cases were isolated. Two patients underwent successful salvage. Only 1 died of uncontrolled groin disease. Local control was achieved in both patients with inguinal nodal disease at presentation, but both died of metastatic disease. Only 3 patients with tumors >4 cm from the verge developed inguinal recurrence (5-year actuarial rate <1%).
Inguinal nodal failure in rectal cancer patients with ACI treated with neoadjuvant or adjuvant chemoradiation is not high enough to justify routine elective groin irradiation.

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The RTOG contouring atlas doesn't give a consensus on this issue.