Reply to Ichimasa et al.

We thank Ichimasa et al. for their thoughtful comments on our recent article describing magnetic resonance imaging (MRI)-based identification of rectal cancers suitable for endoscopic intermuscular dissection (EID) [1], and are pleased to respond to the two points raised.

First, regarding the 1-mm cutoff for preserved muscularis propria, we acknowledge the potential variability in rectal wall thickness related to tumor location, patient factors, and rectal distension. However, there are currently no published studies providing detailed histological measurements of the individual layers – circular and longitudinal – of the muscularis propria in the rectum. The limited data available, such as those by Violi et al., describe overall muscularis propria thickness (2.2–2.9 mm) in the rectosigmoid region but do not distinguish between layers or focus on the rectum itself [2]. Importantly, the MERCURY trial demonstrated excellent correlation between in vivo MRI measurements and histopathology, particularly for tumor spread ≤1 mm [3]. Additional data from Professor Brown’s thesis confirm strong agreement between MRI and histological assessment of rectal wall layers [4]. Although further histopathological evaluation of depth of each of the muscularis propria layers may prove interesting, we believe that the value of the 1-mm cutoff is in providing an objective measurement predicting the likelihood of successful R0 resection with EID, as demonstrated in our study.

Second, we agree that the specificity of 69% reflects a risk of understaging. In both our original analysis and further review of cancers with invasion into the longitudinal layer (pT2_long) or deeper, no significant tumor- or patient-level factors explained misclassification. However, MRI quality did influence diagnostic accuracy. We hypothesize that standardization of MRI protocols to achieve consistently high-resolution imaging may enhance performance. While endorectal ultrasound may offer complementary staging details about the depth of invasion, its limited integration into routine practice currently restricts its broader utility.

Publication History

Article published online:
24 September 2025

• References

• 1 van der Schee L, Carten R, Albers SC. et al. Assessing the accuracy of magnetic resonance imaging in identifying early rectal cancers suitable for endoscopic intermuscular dissection. Endoscopy 2025; 57
• 2 Violi V, Cobianchi F, Adami M. et al. Human defunctionalized colon: a histopathological and pharmacological study of muscularis propria in resection specimens. Dig Dis Sci 1998; 43: 616-623
• 3 MERCURY Study Group. Extramural depth of tumor invasion at thin-section MR in patients with rectal cancer: results of the MERCURY study. Radiology 2007; 243: 132-139
• 4 Brown G. Preoperative staging of rectal cancer using magnetic resonance imaging [MD Thesis]. Cardiff: University of Wales College of Medicine; 2002