Ultrasound elastography in patients with rectal cancer treated with chemoradiation

Abstract

Objective

The current literature has described several predictive markers in rectal cancer patients treated with chemoradiation, but so far none of them have been validated for clinical use. The purpose of the present study was to compare quantitative elastography based on ultrasound measurements in the course of chemoradiation with tumor response based on T stage classification and the Mandard tumor regression grading (TRG).

Materials and methods

We prospectively examined 31 patients with rectal cancer planned for high dose radiochemotherapy. The tumor and the mesorectal fat elasticity were measured using the Acoustic Radiation Force Impulse to generate information on the mechanical properties of the tissue. The objective quantitative elastography shear wave velocity was compared to the T stage classification and TRG.

Results

The baseline mean tumor elasticity was 3.13 m/s. Two and six weeks after the start of chemoradiation the velocities were 2.17 m/s and 2.11 m/s, respectively. The difference between baseline velocity and velocities during the treatment course was statistically significant, (p < 0.0001). Patients with tumor confined to the rectal wall at histopathology (ypT1-2) had a mean elasticity measurement after two weeks of treatment of 1.95 m/s, whereas tumors invading the mesorectal fat (ypT3-4) had a velocity of 2.47 m/s, (p < 0.05). The mean elasticity tended to be lower (1.99 m/s) after two weeks in patients with TRG 1–2 responses in contrast to 2.24 m/s in those with TRG 3–4.

Conclusion

Ultrasound elastography after two weeks of chemoradiation seems to hold early predictive information to the pathological T stage.

Introduction

The aims of preoperative chemoradiation treatment in locally advanced tumors are to reduce the risk of local relapse, improve resectability to enable complete resection, and preserve sphincter function in low located tumors [1]. A high radiation dose increases the rate of major response in T3 tumors without increasing toxicity or surgical complications [2]. Transrectal ultrasound has shown to be an accurate method in preoperative staging of rectal cancer [3], [4], [5]. New methods using semi-quantitative elastography [6] have recently been used successfully to differentiate between rectal adenomas and carcinomas by measuring strain ratios [7]. The current literature has described several predictive bio-markers in rectal cancer patients treated with neoadjuvant chemoradiation [8], but so far ultrasound elastography has not been used in response evaluation in rectal cancer.

Objective measurements of tumor elasticity were enabled with the advent of acoustic radiation force impulse (ARFI) imaging. ARFI uses a short pulse less than 1 millisecond, producing shear waves in the target tissue. These shear waves are detected by the system, and the velocity is measured in m/s. Slow shear waves correspond to soft tissue [9]. A recent study on liver tumors has shown promising results in distinguishing haemangiomas from metastases using the shear wave technique [10]. At present it is not possible to use the shear wave technique on endoluminal transducers, but tumors in patients with low lying locally advanced rectal cancers are accessible via transgluteal ultrasonography approach [11]. The purpose of the present study was to prospectively compare quantitative elastography based on ultrasound measurements in the course of chemoradiation with tumor response based on ypTN-classification and the Mandard tumor regression grading (TRG) [12].