Histological processing variability in the determination of lateral resection margins in rectal cancer

Involvement of the lateral resection margin (LRM) has been shown to be a reliable predictor of local recurrence of rectal cancer. Accurate determination of the LRM status is crucial in selecting patients for postoperative radiotherapy. However, variability in processing factors may affect the measurement of the LRM.

To investigate how formalin fixation and laboratory processing affects the measurement of the LRM.

For this study, rectal cancer specimens (n = 9) were fixed in formalin for 4 days and then sectioned transversally, and one half of the specimen was sent for processing. The effacing tumours were placed back in formalin for another 3 days. At day 7, the effacing tumour block (mirror image) was sent for processing. The longest and the shortest perpendicular resection margins for each of the day 4 and day 7 specimens were measured. In a second experiment, control tissue (colon; n = 40), length 10 (0.05) mm, was also processed from a normal sigmoid colon. Specimens were retained in formalin for 24 h (n = 12), 48 h (n = 12), 72 h (n = 9) and 96 h (n = 7). The degree of tissue shrinkage was then recorded. Variations in the recorded LRM and length of colonic tissue are presented as a median (interquartile range) and data were compared using analysis of variance.

In the cases of rectal cancer, the variation in measured LRM between day 4 and day 7 specimens was 3.2 (1.5-5) mm. In 30 of the 37 comparisons, the day 7 LRM increased in length, whereas in the remaining 7 it decreased. In the second experiment, control tissue of the original length 10 (0.05) mm increased in length to 10.9 (8.9-13.0) mm, p<0.01.

These results suggest that the fixation period/laboratory processes result in measurable differences in the reported LRM. This degree of variation has implications for the reliable reporting of the LRM, predicting local recurrence rates and planning subsequent adjuvant radiotherapy.