Distal resection margins before and after fixation: a comparative analysis in rectal cancer surgery

Formalin fixation may cause measurable changes in the dimensions of surgical specimens, which may potentially affect the pathological assessment of distal margins in rectal cancer surgery. This study aimed to evaluate the differences between distal margin measurements obtained from fresh specimens and those recorded after fixation.

Rectal cancer specimens from patients who underwent anterior or low anterior resection were retrospectively analyzed. Distal margins were measured macroscopically on fresh specimens during intraoperative pathological assessment and then remeasured after formalin fixation. Differences between the prefixation and postfixation measurements were calculated, and the shrinkage rates were determined.

Sixty patients were included in this analysis. The mean distal margin decreased from 4.39 ± 1.95 cm in fresh specimens to 3.90 ± 2.04 cm after fixation, corresponding to a mean reduction of 0.49 cm (95% CI: 0.26-0.71 cm) and an average shrinkage of 13.2%. No specimen had a distal margin shorter than 1 cm on fresh measurement; however, after fixation, two specimens were reported to have margins below this threshold. During a median follow-up of 46.5 months, exploratory analyses did not identify a clear association between the postfixation distal margin category (<2 cm vs. ≥ 2 cm) and oncological outcomes; however, these analyses were based on small subgroups and should be interpreted with caution. Among the two patients with a distal margin <1 cm after fixation, neither developed a local recurrence, although one developed a distant recurrence during follow-up.

Formalin fixation results in a measurable reduction in the distal margin length in rectal cancer specimens. Although this reduction is generally modest, it may occasionally influence the pathological interpretation of distal clearance, particularly in borderline cases. These findings should be interpreted in the context of specimen processing and do not necessarily indicate a true oncological inadequacy. Larger studies evaluating long-term oncological outcomes are required to further clarify the clinical implications of fixation-related margin shortening.