Background: Pelvic anatomy and tumour features play a role in the difficulty of the laparoscopic approach to total mesorectal excision in rectal cancer. The aim of the study was to analyze whether these characteristics also influence the quality of the surgical specimen. Material and Methods: We performed a prospective study in consecutive patients with rectal cancer located less than 12 cm from the anal verge who underwent laparoscopic surgery between January 2010 and July 2013. Exclusion criteria were T1 and T4 tumours, abdominoperineal resections, obstructive and perforated tumours, or any major contraindication for laparoscopic surgery. Dependent variables were the circumferential resection margin (CMR) and the quality of the mesorectum. Results: Sixty-four patients underwent laparoscopic sphincter-preserving total mesorectal excision. Resection was complete in 79. 1% of specimens and CMR was positive in 9. 7%. Univariate analysis showed tumor depth (T status) (P=0. 04) and promontorium-subsacrum angle (P=0. 02) independently predicted CRM positivity. Tumor depth (P 0. 05) and promontorium-subsacrum axis (P 0. 05) independently predicted mesorectum quality. Multivariate analysis identified the promontorium-subsacrum angle (P= 0. 012) as the only independent predictor of CRM. Conclusion: Bony pelvis dimensions influenced the quality of the specimen obtained by laparoscopy. These measurements may be useful to predict which patients will benefit most from laparoscopic surgery and also to select patients in accordance with the learning curve of trainee surgeons. Keywords: rectal cancer, total mesorectal excision, laparoscopy, pelvimetry, quality of mesorectum, circumferential resection margin Introduction Total mesorectal excision (TME) has been the gold standard for rectal cancer surgery since 1982 when Heald et al 1,2 published their seminal paper showing that this approach led to a sharp decrease in the local recurrence rate and an increase in survival. TME preserves the integrity of the mesorectal fascia as dissection is based on the anatomical embryological planes, and it provides an optimal mesorectum, reducing the recurrence rate by 5-10% 3. Many studies have tried to show the advantages of the laparoscopic approach in TME. Regarding short-term outcome, several series and prospective randomized trials, performed by specifically trained surgeons, have demonstrated the immediate postoperative clinical advantages of the minimally invasive approach. They have also shown that features of pathological specimens obtained by the laparoscopic approach are similar to those obtained in open surgery 4,5. These positive results, however, are difficult to achieve in general surgical practice6. As long-term results of large prospective randomized trials are lacking, it is not yet clear whether the incidence of local recurrence (LR) differs in open and laparoscopic approaches. Despite the advantages of the laparoscopic dissection technique, this approach may be specially challenging in certain situations, and a conversion rate of up to 25% has been reported 3. In a previous study we investigated the factors that had an impact on immediate clinical outcome, showing that BMI, tumor size and gender had a significant influence on operative time, conversion and morbidity7. On the basis of these previous findings, we hypothesized that intraoperative technical difficulties may have an impact on the quality of the pathological specimen. The aim of the present study was to analyze preoperatively whether pelvic dimensions and tumour characteristics also influence the quality of the specimen. Material and Methods Patient selection We performed a prospective study in consecutive patients with rectal cancer who underwent TME by laparoscopy with curative intent from January 2010 to July 2013. The inclusion criteria were rectal cancer located at or below 12 cm from the anal verge. Exclusion criteria were patients with a T1 tumor treated by local excision, preoperative clinical evidence of locally advanced disease (T4), abdominoperineal resections, obstructive and perforated tumours, and any major contraindication for laparoscopic surgery. All the patients included in the study were evaluated by a multidisciplinary team of surgeons, medical oncologists, radiotherapists, radiologists and pathologists. Table 1 shows the demographic parameters, pelvimetry measurements, and surgical and pathological features. All the patients were analyzed on an intentionto- treat basis, including those converted to open surgery. The protocol was approved by the ethics committee at our institution and the study was performed in accordance with the Declaration of Helsinki. Preoperative staging All patients underwent a CT scan, an MRI, and a total colonoscopy that included tumour biopsy. According to the hospital protocol, patients with Stage III rectal cancer received a long course of preoperative radiochemotherapy (45 Gy for 5 weeks, plus capecitabine (Xeloda®) at a dose of 825 mgr/m2/12 hours/day or continuous infusion of 5-fluorouracil 225-250 mgr/m2). Patients treated with neoadjuvant therapy underwent a repeat MRI immediately before surgery to determine downsizing or downstaging. In patients treated with preoperative radiochemotherapy, pelvimetrics and tumour measurements were performed in the MRI post-neoadjuvant treatment. Radiologic study Preoperative pelvic MRI images were acquired on a Philips Intera 1. 5 Tesla and Achieva 3 Tesla (Philips Medical System). Pelvis was studied in the three spatial planes from iliac crests to ischiatic tuberosities using a 3 mm slice thickness. We measured the promontorium-retropubic, subsacrum-retropubic, promontorium-subsacrum, and intertuberous axes, the circumferential resection margin (CRM), and maximum and minimum pelvic diameters, at the tumour level. We also measured tumour and prostate volume in men. MRI pelvimetry angles measured were the promontorium-subsacrum, and the sagittal and coronal depth. The sagittal depth angle was calculated from two lines drawn tangentially from the sacral promontory and the anterosuperior tip of the pubis to the superior and inferior edges of the tumour8. Coronal depth angle was calculated by two lines drawn from the inferior edge of the two sacroiliac joints to the lateral edges of the tumour. Volumetric analysis of minor pelvis, rectal ampulla, rectal tumour and prostate was analyzed using specific software (Osirix, Apple, Cupertino, CA, USA). a DICOM file was used to make a 3-dimensional reconstruction from planar slices obtained during the MRI reconstruction . Pelvimetry measurements were performed by two consultant radiologists (J. C. P and D. H) Surgical treatment Surgery was performed 8 weeks after completion of the neoadjuvant treatment. All patients in the series underwent laparoscopic sphincter-preserving low anterior resection (LAR) with TME. The characteristics of the technique are described in our previous study7. A protective ileostomy was made in all cases. Pathological analyses Specimens were examined following the scheme proposed by Quircke9, including fresh examination of the integrity of the mesorectum and its corresponding classification (complete: intact mesorectum with only minor irregularities of a smooth mesorectal surface; nearly complete: rude mesorectum defects, not disrupting the muscle layer; incomplete: little bulk to mesorectum with defects down to muscularis propia). To simplify the data, we grouped mesorectal quality into two subgroups: optimal (complete) and suboptimal (nearly complete and incomplete groups). The mesorectum surface was stained and fixed in formalin block. The resection specimens were sectioned in parallel cuts of 0. 5 cm perpendicular to the length of the bowel, allowing measurement of the deepest point of tumour invasion. Microscopic study was conducted and pCRM and ypCRM were measured. CRM were considered positive if the tumour was located less than 1 mm from the rectal fascia. Statistical analysis Results are given as number of cases and percentages for categorical data, as mean and standard deviation for quantitative variables. Data were analyzed by use of bivariate analysis, contingency tables, and the chi squared or Fisher's exact test for categorical variables and ANOVA or t test for quantitative variables. The statistical significance level was set at 5% (alpha= 0. 05). All variables that were significant in univariate analyses and those considered clinically relevant were entered in a multivariate stepwise (forward selection/backward elimination) to determine which variables were significant independent risk factors. All the analyses were performed using SPSS software (version 21. 0; SPSS. Inc. Chicago, IL).