The use of FDG-PET/CT and diffusion-weighted magnetic resonance imaging for response prediction before, during and after preoperative chemoradiotherapy for rectal cancer

Abstract

Purpose. To investigate the use of FDG-PET/CT before, during and after chemoradiotherapy (CRT) and diffusion-weighted magnetic resonance imaging (DW-MRI) before CRT for the prediction of pathological response (pCR) in rectal cancer patients. Material and methods. Twenty-two rectal cancer patients treated with long course CRT were included. An FDG-PET/CT was performed prior to the start of CRT, after 10 to 12 fractions of CRT and five weeks after the end of CRT. The tumor was delineated using a gradient based delineation method and the maximal standardized uptake values (SUV_max) were calculated. A DW-MRI was performed before start of CRT. Mean apparent diffusion coefficients (ADC) were determined. The ΔSUV_max during and after CRT and the initial ADC values were correlated to the histopathological findings after total mesorectal excision (TME). Results. ΔSUV_max during and after CRT significantly correlated with the pathological response to treatment (during CRT: ΔSUV_max = 59% ± 12% for pCR vs. 25% ± 27% if no pCR, p=0.0036; post-CRT: 90% ± 11 for pCR vs. 63% ± 22 if no pCR p=0.013). ROC curve analysis revealed an optimal threshold for ΔSUV_max of 40% during CRT and 76% after CRT. The initial ADC value was also significantly correlated with pCR (0.94 ± 0.12 × 10⁻³ mm²/s for pCR vs. 1.2 ± 0.24 × 10⁻³ mm²/s, p=0.002) and ROC curve analysis revealed an optimal threshold of 1.06 × 10⁻³ mm²/s. Combining the provided ΔSUV_max thresholds during and after CRT increased specificity of the prediction (sensitivity 100% and specificity 94%). The combination of the thresholds for the initial ADC value and the ΔSUV_max during CRT increased specificity of the prediction to a similar level (sensitivity of 100% and specificity of 94%). Conclusions. The combination of the different time points and the different imaging modalities increased the specificity of the response assessment both during and after CRT.