RESUMEN
AIM: The aim of this study was to identify criteria for three-dimensional anorectal ultrasonography (3D-AUS) to assess the response of rectal cancer to chemoradiotherapy; the 3D-AUS results were compared with the histopathological findings of the resected specimen. METHOD: Thirty-five patients underwent 3D-AUS and were grouped according to the presence (GI; n = 19) or absence (GII; n = 16) of anal canal invasion. All patients received chemoradiotherapy, then underwent a second 3D-AUS. The response (complete, partial or insignificant and lymph node metastasis) was evaluated. Tumour length (cm) and volume (cm(3) ), length and volume regression percentage (%), distal length regression, and distance between the distal tumour edge and the proximal border of the internal anal sphincter were measured before and after chemoradiotherapy. All patients underwent surgery, and the 3D-AUS image was compared with the histopathological findings. RESULTS: Before chemoradiotherapy, the average tumour length was similar in G1 and GII, but the volume differed significantly (P = 0.0408). The response was insignificant in seven (37%) patients, partial in 10 (53%) patients and complete in two (10%) patients in GI. The corresponding figures for GII were one (6%) patient, 12 (75%) patients and three (19%) patients (P = 0.0318). The agreement between pathological and post-chemoratherapy 3D-AUS findings was almost identical for the identification of residual tumour or complete response (κ = 1.0) and substantial for lymph node metastases (κ = 0.74). The mean distance to the internal anal sphincter was greater in GII. A sphincter-saving resection was performed in 2/19 patients in GI and in 14/16 patients in GII (P < 0.0001). The histopathological examination revealed a free distal margin. CONCLUSION: 3D-AUS was shown to evaluate accurately the response to chemoradiotherapy, helping in the selection of patients for a sphincter-saving resection. The distance between the tumour and the internal anal sphincter was the most important parameter in this respect.