Is the omission of the external iliac or upper pelvic lymph nodes from elective irradiation safe in selected anal canal squamous cell cancers?

Background: Guidelines recommend elective irradiation of the external iliac and upper pelvic lymph nodes (LNs) regardless of clinical stage, but the supporting evidence for this recommendation is limited. Methods: We conducted a retrospective analysis of 68 consecutive patients with squamous cell carcinoma of the anal canal who underwent volumetric modulated arc therapy chemoradiation, excluding external iliac LNs from elective irradiation. In patients with negative bilateral inguinal LNs, both external iliac regions were omitted, while in those with unilateral positive inguinal LNs, only the ipsilateral external iliac region was included and the contralateral side was omitted. For patients with early-stage tumours, the cranial border of elective irradiation was located in the inferior aspect of the sacroiliac joints. Results: The median follow-up was 4.0 years. Six patients (9 %) experienced recurrence of the primary tumour and three (4 %) developed distant metastases. No isolated nodal recurrences were seen in the LNs that were positive at baseline, or in or outside electively irradiated regions. Notably, no recurrences were seen in any of the 124 external iliac regions omitted from elective irradiation across the 68 patients, or in the upper pelvic region among the subgroup of 33 patients where the cranial irradiation border was at the bottom of the sacroiliac joint. Reducing the elective irradiation volume significantly decreased the dose-volume parameters for organs-at-risk. Conclusions: The present findings suggest that omitting the external iliac and upper pelvic LNs from elective irradiation is safe for selected patients and allows dose reduction in organs-at-risk.