Apparent diffusion coefficient measurement covering complete tumor area better predicts rectal cancer response to neoadjuvant chemoradiotherapy.

Neoadjuvant long course combined chemoradiotherapy (CRT), as a first step of a standard treatment for locally advanced rectal cancer, aims to downsize and downstage tumor before surgery in order to increase the number of complete resections with no microscopic residual tumors (R0 resections) and thereby reduce local recurrence rate. In locally advanced tumors, this treatment often leads to excellent response. In up to 24% of patients, no residual tumor tissue is found in the resection specimen, corresponding to complete tumor response (1). These patients have a better prognostic outcome than patients with residual tumor. Also, in patients with complete or significant response to CRT, organ saving treatment may be applied (1-6). To safely perform less invasive strategies, it is crucial to accurately select patients and reliably assess tumor response to treatment (7,8). Tumor response is estimated by different imaging techniques. Since conventional magnetic resonance imaging (MRI) is not sufficient for this purpose, functional MRI techniques, such as diffusion-weighted imaging (DWI) and dynamic-contrast enhanced MRI, have been used (9-12). DWI analyzes extracellular movement of water protons, which depends on tissue microarchitecture and can be quantitatively measured as apparent diffusion coefficient (ADC). It allows assessment of tumor response to treatment due to its superb tumor tissue definition as high signal intensity areas and quantitative information reflecting tissue cellularity. ADC has become an established oncologic biomarker, as lower ADC values indicate malignant tissues (13-16). ADC measurements are performed by positioning a region of interest (ROI) in the tumor area and measuring ADC values within the ROI. However, it is still debatable whether the measurements should be performed within the most cellular tumor parts represented as markedly restricted diffusion zones or in the larger tumor areas with inclusion of necrotic and fibrotic tumor parts. Therefore, the aim of our study is to determine the impact of two different ADC measurement techniques on DW MR images on the assessment of rectal cancer response to neoadjuvant CRT.