MRI assessment of rectal cancer response to neoadjuvant therapy: a multireader study

Abstract

Objectives: A watch and wait strategy with the goal of organ preservation is an emerging treatment paradigm for rectal cancer following neoadjuvant treatment. However, the selection of appropriate patients remains a challenge. Most previous efforts to measure the accuracy of MRI in assessing rectal cancer response used a small number of radiologists and did not report variability among them.

Methods: Twelve radiologists from 8 institutions assessed baseline and restaging MRI scans of 39 patients. The participating radiologists were asked to assess MRI features and to categorize the overall response as complete or incomplete. The reference standard was pathological complete response or a sustained clinical response for > 2 years.

Results: We measured the accuracy and described the interobserver variability of interpretation of rectal cancer response between radiologists at different medical centers. Overall accuracy was 64%, with a sensitivity of 65% for detecting complete response and specificity of 63% for detecting residual tumor. Interpretation of the overall response was more accurate than the interpretation of any individual feature. Variability of interpretation was dependent on the patient and imaging feature investigated. In general, variability and accuracy were inversely correlated.

Conclusions: MRI-based evaluation of response at restaging is insufficiently accurate and has substantial variability of interpretation. Although some patients' response to neoadjuvant treatment on MRI may be easily recognizable, as seen by high accuracy and low variability, that is not the case for most patients.

Key points: • The overall accuracy of MRI-based response assessment is low and radiologists differed in their interpretation of key imaging features. • Some patients' scans were interpreted with high accuracy and low variability, suggesting that these patients' pattern of response is easier to interpret. • The most accurate assessments were those of the overall response, which took into consideration both T2W and DWI sequences and the assessment of both the primary tumor and the lymph nodes.

Keywords: MRI; Neoadjuvant treatment; Organ preservation; Rectal cancer; Watch and wait.

Fig. 1

Accuracy, sensitivity, and specificity of radiologists’ categorization of the overall response to NAT as either complete or incomplete. The dark blue diamonds indicate the accuracy parameter for the group of radiologists as a whole. Light blue boxes indicate the interquartile range. Circles and triangles indicate the accuracy parameters of individual radiologists. Circles indicate more than 10 years attending experience, and triangles indicate 10 or less years of attending experience.

Fig. 2

Accuracy, sensitivity, and specificity of radiologists’ interpretations of the primary tumor response according to T2W images, DW images, and the combination of features of T2W images and features of DW images. The dark blue diamonds indicate the accuracy parameter for the group of radiologists as a whole. Colored boxes indicate the interquartile range. Circles and triangles indicate the accuracy parameters of individual radiologists. Circles indicate more than 10 years attending experience, and triangles indicate 10 or less years of attending experience.

Fig. 3

Variability among the 12 radiologists in interpreting three key features and in categorizing the overall response to NAT. Patients are listed from left to right in decreasing order of selection of complete response in the overall assessment. The top row includes the reference standard (complete response in green; incomplete response in yellow). The color intensity corresponds to the proportion of observers (dark red 100% and dark blue 0%) who selected complete response for the overall assessment and for each one of the features (tumor on T2W sequences alone, tumor on both T2W and DWI sequences, and tumor on DWI sequences alone).