ArticlesAccuracy of magnetic resonance imaging in prediction of tumour-free resection margin in rectal cancer surgery
Introduction
After “curative” resection of the rectum for rectal cancer, local recurrence rates vary from 3% to 32%.1 Incomplete removal of the lateral spread of the tumour is now generally accepted as the reason for most of these recurrences.2, 3, 4 Perioperative radiotherapy reduces local recurrence rates.5 In Europe there is a preference for preoperative radiotherapy, based on the results of several trials; the Swedish Rectal Cancer Trial showed the most convincing results, with a local recurrence rate of 11% after radiotherapy compared with a rate of 27% in the controls, and improved survival.6 In the USA, on the other hand, the standard practice since 1990 has been postoperative chemotherapy for T3 and/or N1 disease.7, 8
Attention has also been directed at the surgical technique itself as a determinant of local recurrence rates.9 Histology of resection specimens has shown that the frequency of local recurrence greatly decreases when a tumour-free circumferential resection margin of more than 1 mm can be obtained.2, 3, 4 Surgery alone should, therefore, be able to achieve local cure in virtually all T1 and T2 tumours and in many T3 tumours. When the rectum and the surrounding mesorectal fat are removed by sharp dissection along the mesorectal fascia, a procedure called total mesorectal excision (TME), overall local recurrence rates are below 10% without radiotherapy.9, 10, 11 Preoperative radiotherapy could therefore be limited to patients in whom an involved or close resection margin is expected,12 if these patients could be identified by a reliable preoperative imaging method that predicted the distance of the tumour from the circumferential resection plane of a TME specimen.
Many studies have reported the accuracy of imaging techniques in staging the local extension of rectal cancer. Endorectal ultrasonography is currently regarded as the most accurate imaging modality, with accuracy for tumour staging ranging from 64% to 94%.13, 14, 15 The mesorectal fascia, however, is difficult to identify on ultrasonography because of the limited soft-tissue contrast resolution and limited field of view; prediction of the circumferential resection margin with this imaging method is therefore difficult.
The accuracy of conventional magnetic resonance imaging (MRI) techniques for tumour staging has been disappointing owing to insufficient spatial resolution.16, 17 Modern MRI techniques, with endorectal and phased-array coils, have led to better spatial resolution and accuracy for predicting the tumour stage.18, 19, 20, 21 Although the tumour stage is an important prognostic factor, preoperative assessment is of little benefit when it does not affect preoperative or operative management. Prediction of the circumferential resection margin, by contrast, could be clinically useful to select patients for preoperative radiotherapy. In one MRI study, the mesorectal fascia was visualised but the researchers did not attempt to predict the circumferential resection margin.20 With postoperative MRI of the resected specimen, Blomqvist and colleagues were able to predict a tumour-free lateral resection margin.22 These two studies suggest that preoperative MRI should be able to predict the circumferential resection margin.
The aim of our study was to assess the accuracy of phased-array MRI for preoperative staging of rectal carcinoma and the accuracy for predicting the distance of the tumour to the circumferential resection margin in a TME.
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Participants
The study was approved by the local institutional review board. Between January, 1998, and May, 2000, 85 patients with primary rectal cancer underwent palliative or curative resection in our department of surgery. No preoperative MRI could be done in nine patients, mainly for logistic reasons. After giving informed consent, the other 76 patients took part in the study. There were 58 men and 18 women, with a mean age of 65 years (range 30–85). In 60 patients, the tumour was clinically mobile in
Results
All patients tolerated the MRI examination. Three scans showed motion artefacts, but the images were of sufficient quality to allow a good assessment. The final histological staging showed T1 tumours in seven patients, T2 in 13, T3 in 40, and T4 in 16.
For observer 1, the MRI tumour stage agreed with the histological stage in 63 (83%) of 76 patients (weighted κ=0·77 [95% CI 0·66–0·89]). The sensitivity and specificity for the prediction of T2 were 38% (five of 13) and 94% (59 of 63), for T3 95%
Discussion
In this study, high-spatial-resolution MRI had accuracy for predicting the tumour stage of 67% and 83%, for the two observers. Although the phased-array MRI technique is more accurate than body-coil MRI16, 17 or computed tomography,24 it is far from perfect, and there is substantial variability within and between observers. Most staging failures occurred in differentiation of T2 and borderline T3 lesions. On MRI, distinction of spiculation in the perirectal fat caused by fibrosis only (stage
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