MRI Staging of Anorectal Malignancy—A Reporting Dilemma: Is It Adenocarcinoma or Squamous Cell Carcinoma?

Abstract Aim  Magnetic resonance imaging (MRI) of anorectal malignancy is often reported assuming low rectal adenocarcinoma (LRC). The biopsy may, however, reveal squamous cell carcinoma (SCC). Thus, the aim was to compare the imaging findings of SCC and LRC. Methods  This was a retrospective study of patients who underwent staging MRI for anorectal malignancy (<5 cm from the anal verge) for adenocarcinoma or squamous cell carcinoma between 2016 and 2021. Two radiologists blinded to biopsy reviewed MRI. Imaging findings and apparent diffusion coefficient (ADC) values were compared between SCC and LRC. Results  We studied 137 patients ( n  = 60 SCC, n  = 77 LRC) with a mean age of 50.4 (standard deviation: 12.4) years and tumor length of 5.6 ± 1.9 cm. SCC patients were older, and their distal tumor margin was closer to the anal verge (5.3 vs. 22 mm for LRC; p <0.001). T2 intermediate signal and diffusion restriction was seen in 97 and 98.2% of SCC and 75.3 and 77% of LRC, respectively. SCC had lower ADC values (0.910 × 10 −3 mm 2 /s) than LRC (1.126 × 10 −3 mm 2 /s; p  < 0.001). But there was no difference in the ADC values when T2 hyperintense tumors were excluded ( p  = 0.132). Extramural vascular invasion (EMVI) was more frequent in LRC (35.1 vs. 16.7%; p  = 0.013). A combination of distance from the anal verge of less than 11 mm, absent EMVI, and the presence of internal iliac and inguinal nodes had an area under the curve (95% confidence interval) of 0.810 (0.737–0.884). Conclusion  ADC values are unhelpful in differentiating SCC and LRC. Tumors closer to anal verge, absence of EMVI, and the presence of inguinal and internal-iliac nodes may point towards SCC.