Limitation of radiological T3 subclassification of rectal cancer due to paucity of mesorectal fat in Chinese patients.

Objectives: To describe the thickness of mesorectal fat in local Chinese population and its impact on rectal cancer staging.

Design: Case series.

Setting: Two local regional hospitals in Hong Kong.

Patients: Consecutive patients referred for multidisciplinary board meetings from January to October 2012 were selected.

Main Outcome Measures: Reports of cases that had undergone staging magnetic resonance imaging for histologically proven rectal cancer were retrospectively retrieved and reviewed by two radiologists. All magnetic resonance imaging examinations were acquired with 1.5T magnetic resonance imaging. Measurements were made by agreement between the two radiologists. The distance in mm was obtained in the axial plane at levels of 5 cm, 7.5 cm, and 10 cm from the anal verge. Four readings were obtained at each level, namely, anterior, left lateral, posterior, and right lateral positions.

Results: A total of 25 patients (16 males, 9 females) with a median age of 69 (range, 38-84) years were included in the study. Mean thickness of the mesorectal fat at 5 cm, 7.5 cm, and 10 cm from the anal verge was 3.1 mm (standard deviation, 3.0 mm), 9.8 mm (5.3 mm), and 11.8 mm (4.2 mm), respectively. The proportions of patients with mean mesorectal fat thickness of <15 mm were 100%, 84%, and 75% at 5 cm, 7.5 cm, and 10 cm from the anal verge, respectively. The thickness of mesorectal fat was the least anteriorly, and <15 mm at all three arbitrary levels (P<0.001).

Conclusion: The thickness of mesorectal fat was <15 mm in the majority of patients and in most positions. Tumours invading 10 mm beyond the serosa on magnetic resonance imaging may paradoxically threaten the circumferential resection margin in Chinese patients. Use of T3 subclassification of rectal cancer in Chinese patients may be limited.