Background: Mucinous rectal carcinoma has been reported to have a lower survival rate and a poorer histologic response to chemoradiotherapy(CRT). Magnetic resonance imaging (MRI) can accurately evaluate the amount of mucin pools (MP) in primary cancer tissue. We compared the degree of MP on MRI before and after CRT with the histologic findings of resected specimens to investigate the predictors of response to CRT.
Methods: The study group comprised 205 patients with rectal adenocarcinoma who received preoperative CRT. MPs were measured on MRI before and after CRT and in resected specimens. The degree of MP was classified into five classes according to the MP area ratio: 0%, class I; 1 to 19%, class II; 20 to 49%, class III; and 50% or higher, class IV.
Results: The degree of MP on MRI was largely unchanged after CRT; however, the MP on MRI after CRT was underestimated in 26.3% of patients as compared with that in resected specimens. A pathological complete response was obtained in patients who initially had no MP or had an MP ratio of less than 20%. The tumor volume was significantly greater, and the rates of tumor shrinkage and T downstaging were significantly lower in patients who had an MP area ratio of 20% or higher before CRT than in those who had an MP area ratio of less than 20%.
Conclusions: The MP area ratio measured on MRI before treatment was closely associated with the response to CRT and is a potentially useful predictor of treatment response.
Keywords: Chemoradiotherapy; Mucin pool; Mucinous carcinoma; Pathological response; Rectal cancer; Tumor shrinkage.