Diffusion weighted MRI in the evaluation of tumour response after preoperative chemoradiotherapy in patients with locally advanced rectal cancer

Introduction: To determine diagnostic performance of diffusion weighted magnetic resonance imaging for assessment of tumor response after radiation therapy combined with chemotherapy in patients with local advanced rectal cancer by means of apparent diffusion coefficient (ADC) measurements and to compare diffusion weighted imaging and T2 weighted MR volumetry.Materials and Methods: Forty-three patients (mean age: 62.5 years; 19 women, 24 men) who had biopsy proven rectal carcinoma and who underwent preoperative CRT were prospectively examined for the study. DW and T2 weighted imaging was performed with a 1.5-T MR imager in all patients before and after the therapy. Three different diffusion gradients (b values were b=0, b=600 and b=1000 s/mm²) were applied in three different directions (x, y, z) using single shot echo-planar SE with 1.5 T MR imager. Diffusion weighted images and ADC maps were obtained. The mean tumor volume (cm3) and mean ADC values were measured for b 600 and b1000 before and after the therapy and compared statistically by Paired sample t test. ADC increase rate and volume reduction ratio (%) were compared between the responders and the non-responders using two reference standards: T downstaging and tumor regression grade (TRG) by using Mann- Whitney- U test.Results: Patients were assigned to the tumor downstaged group (n=18) and the tumor non-downstaged group (n=25) on the basis of histopathologic examination results following the surgery. Before the CRT, the mean tumor ADC in the downstaged group was significantly lower than that in the non-downstaged group. (b 600 : 0.63 (±0,10) x 10-3 mm²/sn; p< 0.001, b 1000: 0.87 (± 0.15) x10-3 mm²/sn). At the end of the CRT; the mean tumor ADC increased significantly from b 600 0.63 (±0.11) x 10-3 mm²/sn to 1.26 (±0.23) x 10-3 mm²/sn; b 1000: 0.87(±0.15) x10-3 mm²/sn to 1.53 (±0,34) x10-3 mm²/sn in the downstaged group. The mean percentage of tumor ADC change in the downstaged group was significantly higher than that in the non-downstaged group. The mean tumor volume reduction rate of responders was significantly higher than that of non-responders (76.02% and 41.72%, p= 0.001).