Study design and patients
This was a respective study that investigated the clinical value of intra-tumor heterogeneity evaluated by DWI in LRC. The present study included patients with rectal adenocarcinomas that were mainly located under the peritoneal reflection who underwent curative surgery with regional lymph node dissection between April 2009 and March 2017 at the Division of Digestive Surgery of Kyoto Prefectural University of Medicine (KPUM). Exclusion criteria were as follows: (1) patients diagnosed with T1 LRC before treatment initiation; (2) patients diagnosed with distant metastases before treatment initiation; (3) patients who did not undergo MRI with DWI before treatment and surgery. One hundred and forty-six cases fulfilled the inclusion criteria and 114 were subsequently excluded; therefore, data collected from 58 LRC cases were analyzed in the present study (Figure 1).
All surgeries were performed or supervised by surgeons with sufficient experience and certificated by The Japan Society of Endoscopic Surgery and The Japanese Society of Gastrointestinal Surgery. The surgical procedure was selected based on the Japanese colorectal cancer guidelines . Lateral lymph node dissection (LLND) was performed for LRC with 5-mm or larger lateral lymph nodes in the short axis diameter in pre-treatment CT or MRI. Pre-operative CRT was performed for LRC clinically diagnosed as T3 or N(+) in order to secure CRM.
Clinical and pathological diagnoses
Clinical and pathological TNM staging was performed according to the Japanese Classification System . The clinical diagnosis of LRC was based on the findings of the conventional modalities, endoscopy, contrast-enhanced CT, and MRI. The pathological diagnosis was decided by at least two experienced pathologists and evaluated according to the Japanese Classification System. Briefly, lymph nodes in the mesorectum, around the inferior mesenteric artery, and in both lateral areas were defined as the regional lymph nodes of LRC. The tumor differentiation grade was classified according to the differentiated type, which was the main cellular component of tumors.
Pre-operative and pre-treatment MRI
The MRI protocol employed was similar to that in our previous study . Imaging was performed with a 1.5 or 3.0 T pelvic MRI with pelvic phased-array coils at KPUM or related medical centers. T2-weighted axial images with a section thickness of 5–7 mm and sagittal images with fast spin-echo sequences were acquired. An axial diffusion-weighted sequence with background body signal suppression (DWIBS, b-values 800-1000 s/mm2) was also obtained. Cases without pre-operative CRT underwent MRI for staging within 2 months before surgery, and those with pre-operative CRT underwent MRI for pre-treatment staging and re-staging in order to diagnose therapeutic responses almost 4 to 7 weeks after the completion of CRT. Positive CRM evaluated by MRI (mrCRM) was defined as a tumor, lymph node, extramural vascular invasion (EMVI), or tumor deposit located <1 mm from the mesorectal fascia [15, 16]. EMVI evaluated by MRI (mrEMVI) was diagnosed according to the 5-scale EMVI scoring system suggested by Smith et al. , and scores of 3 and 4 were defined as positive mrEMVI. MRI findings were interpreted by a digestive surgeon based on the radiological reports of experienced radiologists.
Quantification of intratumor heterogeneity on DWI images
The intra-tumor heterogeneity of DWI images was quantified as described in our previous study . The heterogeneity was evaluated in only MRI before treatment initiation. Briefly, DWI were evaluated for the maximum cut surface of an axial image of the rectal tumor, which was identified by a T2-weighted axial image (Figure 2A). The distribution of signal intensities in this high-intensity area was evaluated, and the maximum (MAX) and minimum (MIN) values of signal intensities in the tumor were measured (Figure 2B, C). The intra-tumor heterogeneity of the signal intensity on DWI was quantified using the following formula: MRI heterogeneous score of DWI (mrHSD) = [(MAX value of signal intensity) - (MIN value of signal intensity)] / [(MAX value of signal intensity) + (MIN value of signal intensity)].
Comparisons were performed between both groups using the Mann-Whitney U test or Fisher’s exact test. P values <0.05 were considered to be significant. Survival curves were constructed using the Kaplan–Meier method, and differences in survival were examined using the Log-rank test. Uni- and multivariate analyses of the factors influencing survival were performed using Cox’s proportional hazard model. Differences were considered to be significant when the relevant p value was <0.1 in Cox’s proportional hazard model. These analyses were performed using JMP statistical software (JMP version 10).