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Mucinous adenocarcinoma represents a potentially poor prognostic subgroup of rectal cancer. A consensus on the effect of mucinous cancer on outcomes following neoadjuvant chemoradiotherapy and curative resection for rectal cancer has not been reached.The aim of the current study is to use meta-analytical techniques to assess the association between mucinous histology and response to neoadjuvant chemoradiotherapy in rectal cancer.A comprehensive literature search of PubMed, Embase, and The Cochrane Library was performed.All studies examining the effect of mucinous histology on chemotherapeutic response in rectal cancer were included.No direct interventions were performed.Outcomes of mucinous rectal adenocarcinoma were compared with nonmucinous tumors by using random-effects methods to analyze data. Data are presented as ORs with 95% CIs. The main outcomes measured were the rates of pathological complete response, tumor and nodal downstaging, positive resection margin rate, local recurrence, and overall mortality.Eight comparative series describing outcomes in 1724 patients were identified, 241 had mucinous tumors (14%). Mucinous tumors had a reduced rate of pathological complete response (OR, 0.078; 95% CI, 0.015–0.397; p = 0.002) and tumor downstaging (OR, 0.318; 95% CI, 0.185–0.547; p < 0.001) following neoadjuvant chemoradiotherapy with an increased rate of positive resection margin (OR, 5.018; 95% CI, 3.224–7.810; p < 0.001) and poorer overall survival (OR, 1.526; 95% CI, 1.060–2.198; p = 0.023) following resection. Mucin expression did not significantly affect nodal downstaging (OR, 0.706; 95% CI, 0.295–1.693; p = 0.435) or local recurrence (OR, 1.856; 95% CI, 0.933–3.693; p = 0.078). There was no across-study heterogeneity for any end point.Most studies were retrospectively designed, and there were variations in patient populations and duration of follow-up.Mucinous rectal adenocarcinoma represents a biomarker for poor response to preoperative chemoradiotherapy and is an adverse prognostic indicator.