Impact of the risk-adapted Nordic anal cancer group consensus guidelines on the contouring of the elective clinical target volume in anal cancer.

Background and purpose: The recently published Nordic anal cancer group (NOAC) contouring guidelines aim for improved oncological outcomes and reduced toxicity in anal cancer patients treated with radiotherapy. The present work describes how the elective clinical target volume (CTVe) would change when applying the NOAC guidelines instead of the previous Australasian standard. According to the Australasian guidelines, the cranial border of the CTVe is at the common iliac bifurcation for all patients, and the external iliac region as well as the ischiorectal fossa are always included.

Materials and methods: Retrospectively, 166 anal cancer patients treated with curative radiotherapy according to Australasian guidelines between 2009 and 2017 were studied in a single-center analysis. Pretherapeutic scans, and clinical information were used to categorize patients according to the NOAC guidelines for a comparison with the Australasian guidelines.

Results: Applying the risk-adapted alternative of the NOAC guidelines had the external iliac region omitted in 41.0% of the patients. The cranial border was lowered from the common iliac bifurcation in 27.7% and elevated in 12.7% of the patients. Elderly patients (≥70 years) more often had the external iliac region omitted than younger patients (60.9% vs. 33.3%; p = 0.001). The entire ischiorectal fossa was included in 23.7% of the patients due to tumor extension beyond the levator ani muscles or external sphincter.

Interpretation: Contouring according to the NOAC risk-adapted guidelines changed, and mainly reduced, the CTVe in about half of all patients. Prospective follow-up is needed to determine if this is clinically beneficial.