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Core tip: The experience and evidences regarding the use of transanal total mesorectal excision is still scarce but promising. Preliminary data showed excellent results, without sacrificing the pathological and oncological outcomes. Whilst still in its infancy, further investigations should be encouraged. Data from large registries and randomized trials are awaited before to draw definitive conclusions. INTRODUCTION There is no doubt that low anterior resection (LAR) and total mesorectal excision (TME) have revolutionized the management of rectal cancer and improved its oncological outcomes[1]. On the other hand, the introduction of minimally invasive surgery for oncological rectal resection has not yet completely convinced the most skeptical open surgeons. Whilst potentially better short-term outcomes have been published favoring laparoscopic approach[2,3], the recent ALaCaRT and ACOSOG Z6051 trials failed to show the non-inferiority of laparoscopic LAR in comparison to open surgery[4,5]. Indeed, there is still a degree of uncertainty, notably regarding the risk of incomplete TME specimen, positive margins, and worse long-term oncological outcomes. To fuel the debate further, other large randomized series did not show inferior pathological or oncological outcomes following laparoscopic LAR[6]. Meanwhile, even the amazing introduction of robotics has not significantly improved the outcomes[7].

E specimen, positive margins, and worse long-term oncological outcomes. To fuel the debate further, other large randomized series did not show inferior pathological or oncological outcomes following laparoscopic LAR[6]. Meanwhile, even the amazing introduction of robotics has not significantly improved the outcomes[7]. To overcome the challenges posed by abdominal TME surgery, a transanal approach has been developed over the last decade, with promising early outcomes. There is growing evidence available including our recent review of transanal TME (TaTME) showing excellent results[8]. However, TaTME is still in its infancy and definitively requires more robust data and longer follow-up. Since the first description of TaTME, a number of relatively large series have been published, showing not only the feasibility of the approach, but also its safety[9,10] even in challenging patients. In our own experience, we have recently shown a low conversion rate, low R1 rate, and an excellent completeness of TME[11-13]. Several parameters and factors pose technical challenges and need special consideration when considering planning TME surgery: (1) dealing with “difficult anatomy” (male, obese, narrow pelvis, post radiation); (2) increasing the sphincter-preserving rate; (3) performing a safe distal rectal stapling; (4) avoiding positive margins; (5) reducing the risk of incomplete TME; (6) improving the oncological outcomes; and (7) offering adequate functional outcomes.

ith “difficult anatomy” (male, obese, narrow pelvis, post radiation); (2) increasing the sphincter-preserving rate; (3) performing a safe distal rectal stapling; (4) avoiding positive margins; (5) reducing the risk of incomplete TME; (6) improving the oncological outcomes; and (7) offering adequate functional outcomes. TaTME seems to offer a solution for most of these parameters/factors. The narrow pelvis with a bulky irradiated specimen in an obese male patient is no longer a relative contra-indication to laparoscopic surgery. Starting the most difficult part of the dissection (the lowest part of the pelvis) from the distal end offers obvious advantages. First of all, the distal margin can be assessed precisely and secured with a purse-string before performing the rectotomy. This in turns avoids the need for distal cross-stapling, which can be laparoscopically challenging due to the limited angle of the endoscopic stapler and the pelvic morphology. This often results in multiple firing to complete the transection with the associated risk of anastomotic leak after more than 2 reloads[14]. With TaTME, this is no longer a challenge. Different anastomotic techniques have been proposed, guaranteeing a safe and efficient way to rejoin the bowel[15]. Although, this may increase the rate of sphincter-preserving surgery, it is at the cost of a higher rate of coloanal anastomosis. Beyond these technical considerations, the interest to proceed with a complete TME is important.

TaTME seems to offer a solution for most of these parameters/factors. The narrow pelvis with a bulky irradiated specimen in an obese male patient is no longer a relative contra-indication to laparoscopic surgery. Starting the most difficult part of the dissection (the lowest part of the pelvis) from the distal end offers obvious advantages. First of all, the distal margin can be assessed precisely and secured with a purse-string before performing the rectotomy. This in turns avoids the need for distal cross-stapling, which can be laparoscopically challenging due to the limited angle of the endoscopic stapler and the pelvic morphology. This often results in multiple firing to complete the transection with the associated risk of anastomotic leak after more than 2 reloads[14]. With TaTME, this is no longer a challenge. Different anastomotic techniques have been proposed, guaranteeing a safe and efficient way to rejoin the bowel[15]. Although, this may increase the rate of sphincter-preserving surgery, it is at the cost of a higher rate of coloanal anastomosis. Beyond these technical considerations, the interest to proceed with a complete TME is important. The threat of incompleteness of mesorectal excision was recently shown to be significant after LAR and APE (36% and 13% respectively)[16]. The lowest part of the mesorectum is at risk of being left behind, which is unacceptable from an oncological point of view. Again, starting the dissection from below might help to obtain a more complete TME specimen. Moreover, comparative studies have shown better pathological outcomes after TaTME in comparison to laparoscopic TME[17,18]. The awaited results from the large multicenter registry study (LOREC) should hopefully help to draw more definitive conclusions.

ction from below might help to obtain a more complete TME specimen. Moreover, comparative studies have shown better pathological outcomes after TaTME in comparison to laparoscopic TME[17,18]. The awaited results from the large multicenter registry study (LOREC) should hopefully help to draw more definitive conclusions. The main challenges for the future of TaTME can be summarized in three different categories: (1) the long-term oncological outcomes; (2) the functional outcomes; and (3) the safe introduction of this approach. Obviously, the technique is still in its infancy and long-term outcomes are not yet available. Early oncological data seem promising[13], but it is too early to draw definitive conclusions. The COLOR III study[19], evaluating TaTME vs laparoscopic TME, should provide a more comprehensive overview of the added value of the transanal approach. In addition, quality of life and functional outcomes will be assessed. Based on previous reports[20-22], adequate function has been reported. However, still a high rate of coloanal anastomosis is performed and the risk of worse functional outcomes is possible.

sive overview of the added value of the transanal approach. In addition, quality of life and functional outcomes will be assessed. Based on previous reports[20-22], adequate function has been reported. However, still a high rate of coloanal anastomosis is performed and the risk of worse functional outcomes is possible. As for any new surgical technique, the danger of widespread rapid and unmonitored adoption without proper training exists. The development of a dedicated curriculum should be established in order to avoid unnecessary preventable complications during the early phase of a surgeon-s learning curve. As already mentioned for robotic surgery and other surgical innovations, training is probably the biggest challenge[23]. Dedicated theoretical and practical courses including cadaver workshops as well as live cases proctoring are key to ensuring the safe introduction of a new surgical technique[24]. In conclusion, TaTME is a promising approach, aiming to overcome the limitations of laparoscopic TME. So far, the published data support its use. Excellent pathological and acceptable short-term clinical outcomes have been reported, however long-term oncological and functional data are still awaited. There is no doubt that TaTME will play a significant role in the evolution of rectal surgery as the drive to perfecting TME and improving outcomes continues. Conflict-of-interest statement: Roel Hompes is a regular faculty member for TAMIS courses organized by Applied Medical. The other authors have no financial disclosure or conflict of interest. Manuscript source: Invited manuscript

In conclusion, TaTME is a promising approach, aiming to overcome the limitations of laparoscopic TME. So far, the published data support its use. Excellent pathological and acceptable short-term clinical outcomes have been reported, however long-term oncological and functional data are still awaited. There is no doubt that TaTME will play a significant role in the evolution of rectal surgery as the drive to perfecting TME and improving outcomes continues. Conflict-of-interest statement: Roel Hompes is a regular faculty member for TAMIS courses organized by Applied Medical. The other authors have no financial disclosure or conflict of interest. Manuscript source: Invited manuscript Specialty Type: Oncology Country of Origin: United Kingdom Peer-Review Report Classification Grade A (Excellent): A Grade B (Very good): 0 Grade C (Good): C Grade D (Fair): 0 Grade E (Poor): 0 Peer-review started: June 28, 2016 First decision: August 5, 2016 Article in press: September 23, 2016 P- Reviewer: Agresta F, Campos FG S- Editor: Ji FF L- Editor: A E- Editor: Lu YJ