# Measurement of the distance between tumor micro-foci and gross tumor in rectal cancer pathological specimens: implication on margin distance of clinical target volume treated with high-dose radiotherapy for rectal cancer

## TL;DR

This study measures the distance between tumor micro-foci and the main tumor in rectal cancer to help determine safe radiation therapy margins.

## Contribution

The study introduces corrected micro-foci distances using retraction factors for more accurate clinical target volume margin planning in rectal cancer radiotherapy.

## Key findings

- The 95th percentile micro-foci distances were 6.44 mm peripherally, 5.54 mm inferiorly, and 5.42 mm superiorly after retraction correction.
- Half of the patients had lateral micro-foci, while 28.6% and 25% had inferior and superior micro-foci, respectively.
- Retraction factors during specimen processing were consistently lower than in vivo–in vitro retraction factors.

## Abstract

To measure the micro-foci distance away from gross tumor and to provide reference to create the clinical target volume (CTV) margin for boost radiotherapy in rectal adenocarcinoma.

Twenty-eight rectal cancer surgical specimens of only total mesorectal excision were collected. The pathological specimens were retrospectively measured, and the nearest distance between the tumor micro-foci and gross tumor was microscopically measured. The “in vivo–in vitro” retraction factor was calculated as the ratio of the deepest thickness laterally and the vertical height superior/inferiorly of the rectal tumor measured in MRI and those measured in immediate pathological specimens. The retraction factor during pathological specimen processing was calculated as the distance ratio before and after dehydration in the lateral, superior, and inferior sides by the “knot marking method.” The distances of tumor micro-foci were individually corrected with these two retraction factors.

The mean “in vivo–in vitro” tumor retraction factors were 0.913 peripherally and 0.920 superior/inferiorly. The mean tumor specimen processing retraction factors were 0.804 peripherally, 0.815 inferiorly, and 0.789 superiorly. Of 28 patients, 14 cases (50.0%) had 24 lateral micro-foci, 8 cases (28.6%) had 13 inferior micro-foci, and 7 cases (25.0%) had 19 superior micro-foci. The 95th percentiles of the micro-foci distance for 28 patients were 6.44 mm (peripheral), 5.54 mm (inferior), and 5.42 mm (superior) after retraction correction.

The micro-foci distances of 95% of rectal adenocarcinoma patients examined were within 6.44 mm peripherally, 5.54 mm inferiorly, and 5.42 mm superiorly. These findings provide reference to set the boost radiotherapy CTV margin for rectal cancer.