Introduction
Since total mesorectal excision (TME) was first described in the early 1930s and later
popularised by Heald [1], efforts have been made to standardise the technique, following
the correct embryological planes and using appropriate landmarks. Laparoscopic and
robotically assisted approaches to the rectum have gained popularity during recent
years, compelling colorectal surgeons to develop their skills and knowledge. Transanal
TME (TaTME) is a new addition to the approaches in rectal surgery. Despite being associated
with several benefits in selected patients, TaTME requires advanced technical skills
and, more importantly, knowledge of the pelvic structures, planes and spaces as they
are encountered moving cephalad from the perineum. Magnetic resonance imaging (MRI)
is the gold standard for imaging of the pelvis and pelvic floor, but understanding
of relevant anatomy when performing a new technique may be hampered by difficulty
in interpretation of two-dimensional (2D) images when considering three-dimensional
(3D) structures. We describe a new tool that could help understanding of TaTME planes
and preoperative planning.
Materials and methods
Two cases were used to demonstrate our technique. Both patients were scheduled for
TaTME and had undergone a preoperative MRI.
Standard axial T2-weighted spectral attenuated inversion recovery (SPAIR) and sagittal
T2-weighted MRI sequences were obtained, and digital imaging and communications in
medicine (DICOM) images were imported into a validated open-source segmentation software
[2]. A specialist consultant gastrointestinal radiologist manually segmented the following
structures: sphincter complex, rectosigmoid colon, levator plate, pelvis, mesorectal
fascia, bladder, ureters, urethra, seminal vesicles and prostate. Each mesh was imported
into another open-source system, MeshLab V1.3.3.1 as stereolithography (STL) files
for mesh smoothing to be applied. Individual labels were applied to each anatomical
structure.
Results
Segmentation of patient images took approximately 15 min per case. A further 10 min
was required for smoothing and applying colour and transparency of the anatomical
structures to emphasise surgically relevant anatomy.
Patient 1 was a male with low rectal cancer who had TaTME. Relevant anatomy shown
in Fig. 1a provides an overall overview of the pelvis and mesorectal fascia; Fig. 1b
highlights the location at which the tumour penetrates the rectal wall; Fig. 1c demonstrates
the proximity of the tumour to the prostate and adjacent urinary system, but also
the clearance between them; and Fig. 1d is an angled view showing the relationship
between the tumour and the urethra.
Fig. 1
3D reconstructions for a patient with a low rectal cancer. a Overview of the pelvis
and mesorectal fascia; b tumour penetrating the rectal wall; c tumour proximity to
the prostate and adjacent urinary system; d angled view of tumour and the urethra
Patient 2 was a male who had a combined single incision laparoscopy (SILS) and TaTME
completion proctectomy and ileoanal pouch formation for ulcerative colitis. Figure 2a
provides an overview of the anatomy showing a relatively straight and posterior direction
of the rectum as it descends into the pelvis. Figure 2b provides insight into the
relation between internal sphincter/rectum and the prostate/urethra. Distance between
structures and relative proximity can be easily understood. Figure 2c shows the clearance
between the low rectum and both ureters, whilst Fig. 2d shows an anterior oblique
view of the sphincter complex and the urethra.
Fig. 2
3D reconstructions for a patient with ulcerative colitis undergoing a completion proctectomy
and ileoanal pouch formation. a Overview of the pelvis; b relation between internal
sphincter/rectum and the prostate/urethra; c low rectum and the both ureters; d angled
view of the sphincter complex and the urethra
The 3D images can be rotated and the various structures inserted and removed so that
the radiologist or surgeon can examine any structure form any angle, examine their
relationships and determine distances and angles to facilitate safe dissection.
Discussion
We provide two examples that demonstrate the utility of 3D modelling in surgical planning
for TaTME, demonstrating how this technique is feasible and can be derived from manipulation
of standard DICOM images from routine 2D MRI.
Transanal minimally invasive removal of the rectum, with or without TME, has gained
popularity over the last decade. Specifically, TaTME offers better access to the distal,
horizontal rectum in low-lying rectal cancers in patients with a narrow pelvis, bulky
tumours or a large prostate, thereby allowing high-quality resection even under these
circumstances [3]. Nevertheless, mastering the anatomy of the pelvis is demanding,
and even surgeons familiar with TaTME may benefit from improved knowledge and understanding
of important anatomical landmarks. Moreover, a TME might not always be necessary,
for example in benign conditions such proctectomy for inflammatory bowel disease,
further highlighting the importance of visualising the desired extent of resection
ahead of surgery. TaTME involves a different approach to the routine for rectal surgery,
necessitating a thorough appreciation of the pelvic anatomy to facilitate proficiency
gains with the technique and minimise morbidity.
Improving the surgeon’s understanding of the relation of the pelvic organs to each
other and of the pathology may protect patients from injury, especially at the start
of the learning curve. An important risk is that of urethral injury, often in the
pre-prostatic region, which may occur during the anterior dissection. The rate of
injury was 1% in the international TaTME registry [4], but voluntary enrolment and
selection bias may mean this is an underestimate of its true incidence. In particular,
there are certain situations where the anatomy can be further distorted, such as post-chemoradiation
[4], and in these instances adjunctive imaging through 3D reconstructions could be
beneficial. Another potential cause for morbidity with TaTME is vaginal wall injury,
which may also occur during anterior rectal dissection. Other smaller structures,
such as the neurovascular bundle of Walsh, with capsular arterial branches, or the
autonomic nerves, pose similar challenges in TaTME surgery. Both structures are difficult
to appreciate using routine rectal cancer sequences, but where deemed necessary, dedicated
sequences may be obtained to delineate this anatomy, allowing for 3D reconstruction.
Dissection in a plane deep to the endopelvic fascia can result in injury to the inferior
hypogastric plexus and bleeding from presacral veins. Surgeons must be familiar with
the concept of the false “pneumodissection” plane and avoid following a plane deep
to the nerve plexus [5].
Another matter to be considered carefully is the ideal route of specimen extraction.
It is pivotal to select patients that may benefit from transabdominal extraction of
the specimen, in order to avoid shearing of the mesentery with tumour cell exfoliation
and shear stress to the marginal artery, with subsequent risk of ischaemia if an anastomosis
is performed [4]. 3D allows one to follow each structure of the pelvis, including
the urethra and surface of the prostate, detailing this delicate anatomy. The ability
to rotate the 3D reconstruction into the same position as that of the patients on
the table allows surgeons to assess the angles of dissection both anteriorly and posteriorly.
This, combined with the facility to remove overlying structures, allows further appreciation
of threatened margins, assessment of the optimum route of dissection and an awareness
of abnormal anatomy.
Preoperative 3D modelling is a useful adjunct to routine preoperative planning. Notwithstanding
the importance of adequate training and teaching in TaTME, it can also be a useful
tool for the mentoring/proctoring surgeon to assess the knowledge of the mentee, and
to discuss with them the detailed surgical strategy before the actual operation in
each specific case. This is even more relevant when considering the possibility of
accessing the reconstruction remotely. The measurement of anatomical factors such
as the anorectal angle, anal canal length, buttock depth and interspinous distances
would not only add further insight but allow the surgeon to assess the appropriate
platform to be used. The 3D imaging can also be used to print patient-specific models,
which could also be used during consultation with patients themselves, in order better
to explain management strategies and obtain informed consent. Future models will aim
to provide interactive elements so that the user can take full advantage of this platform,
such as augmented or virtual reality, importantly, with haptic feedback. These innovations
will revolutionise surgical rehearsal and also provide benefits during surgery itself,
to improve training and patient outcomes.
3D modelling aids individualisation of treatment and surgical approaches. Identification
of ideal surgical planes of excision, particularly in patients who do not need TME,
in order to reduce the risk of collateral injury. It can be useful to address the
extent of multivisceral resections in locally advanced cancers, and to assess patient
suitability for the procedure. There are certain instances where conventional 2D MRI
is favourable, such as in determining beyond TME approaches in cases where the circumferential
resection margin is threatened. For example, one may identify particular parts of
the mesorectal fascia or Denonvilliers’ fascia, which require en bloc excision, depending
on tumour position. Future work will aim to improve segmentation techniques and add
enhanced sequences to better understand this. In addition, comparison of anatomical
factors such as tumour bulk, prostate volume, mesorectal volume and their influence
on clinical outcomes would be interesting. Nevertheless, 3D rendering and the possibility
of assessing each organ/structure separately represent an invaluable tool and adjunct.
Conclusions
Surgeons currently use a combination of MRI scans, reports and discussion with radiologists
to better understand anatomy and plan surgery. 3D reconstructions present an opportunity
to improve a surgeon’s understanding of the information from 2D MRI, allowing for
preoperative rehearsal of complex cases and to improve skill acquisition in innovative
and existing surgical techniques. More experience using this technique is required
before conclusions can be drawn on the impact of 3D imaging and its suggested benefits
on technical error and complication.