Rare indications for transanal endoscopic microsurgery

INTRODUCTION

Transanal endoscopic microsurgery (TEM), introduced by Buess in the 1980s, and its successor, the transanal endoscopic operation (TEO), have profoundly expanded the armamentarium of minimally invasive rectal surgery^[1]. These rigid platforms provide a stable pneumorectum, magnified stereoscopic vision, and a dedicated set of instruments that enable precise excision of rectal lesions. Although initially conceived for benign adenomas and early-stage rectal carcinoma, the platforms have progressively been applied to a much broader spectrum of rectal conditions. Their versatility lies not in a single procedure but in their role as enabling platforms upon which various dissection strategies can be executed, including full-thickness resections and submucosal dissections. This adaptability allows TEM and TEO to address a range of rare rectal pathologies that were once the domain of radical resection or managed suboptimally by conventional transanal excision^[2].

The core advantages of TEM and TEO derive from their capacity to overcome the inherent limitations of traditional transanal surgery. Conventional transanal excision is limited by poor exposure, a restricted working distance, and inadequate instrumentation, factors that collectively compromise the oncological adequacy of resections, particularly for more proximal or larger lesions^[3]. By contrast, transanal endoscopic platforms provide consistent exposure throughout the rectum, enabling surgeons to extend organ-preserving surgery far beyond what was previously achievable. These characteristics are particularly valuable when facing rare rectal tumours, where histological diagnosis is often uncertain preoperatively and an oncologically sound local excision may be both diagnostic and therapeutic^[4].

The spectrum of rare lesions amenable to TEM and TEO is broad and heterogeneous, addressing conditions that develop both within and beyond the rectum^[5]. Gastrointestinal stromal tumours (GISTs), neuroendocrine neoplasms (NENs), rectal melanoma, squamous cell carcinoma, and even metastatic lesions to the rectum represent oncological entities that require en bloc excision with negative margins for diagnosis and local control. In parallel, developmental and benign conditions such as duplication cysts, hamartomas, neuronal dysplasia, and heterotopias can also be excised with precision, thereby avoiding the morbidity associated with abdominal approaches. The ability of TEM and TEO to facilitate both full-thickness resections, ensuring accurate staging and margin clearance, as well as submucosal dissections, which preserve deeper structures when appropriate, exemplifies their adaptability to such diverse pathologies^[6].

Equally important is the safety profile demonstrated by these platforms. Across indications, perioperative morbidity remains low, with peritoneal entry and postoperative bleeding representing the most frequent but manageable complications. Mortality directly attributable to TEM or TEO is exceedingly rare^[7]. Hospitalisation is usually brief, often limited to only a few days, underscoring the minimal physiological burden of the approach. For patients with rare rectal lesions, many of whom are elderly or carry comorbidities, this reduced invasiveness is especially advantageous.

Despite their advantages, the application of TEM and TEO to rare indications has historically been under-reported. Most literature focuses on adenomas and early adenocarcinomas, which represent the bulk of clinical practice and have been extensively studied in terms of long-term oncological outcomes^[8]. Reports of rarer entities are typically limited to case reports and small series, scattered across different disciplines. As a result, evidence regarding outcomes in these unusual settings remains fragmented, and clinicians often rely on extrapolation from more common indications. A systematic description of experience across multiple rare histologies, using a consistent platform and operative technique, is therefore of particular value to both surgeons and multidisciplinary teams.

Another crucial point is the role of TEM and TEO in organ preservation. Radical rectal resection carries substantial morbidity, including risks of anastomotic leakage, urinary and sexual dysfunction, and the potential need for permanent stoma formation^[9]. In contrast, transanal endoscopic local excision avoids these burdens while still ensuring pathological diagnosis and local control. In an era where quality of life and functional preservation are increasingly recognised as key endpoints alongside oncological safety, the expansion of TEM and TEO into rare indications further consolidates their importance within rectal surgery.

METHODS

We conducted a retrospective observational cohort study at a single tertiary referral centre, examining consecutive patients who underwent local excision of rectal lesions with TEM or TEO between 1993 and 2024. Cases were identified from a prospectively maintained database. Inclusion criteria were: (i) local excision performed using TEM or TEO; (ii) definitive pathology showing a rare rectal lesion; and (iii) availability of core perioperative and follow-up data. Exclusion criteria were: (i) standard indications such as rectal adenomas or adenocarcinomas; (ii) non-rectal lesions; and (iii) incomplete records preventing outcome assessment. For clarity of reporting, pathologic reports were grouped a priori into the following categories: GIST, neuroendocrine tumours (NETs), neuroendocrine carcinomas (NECs), ulcerative solitary rectal syndrome (USR), hamartoma, duplication cyst, neuronal dysplasia combined with schwannoma, leiomyoma combined with leiomyosarcoma, squamous cell carcinoma combined with nodal metastasis from squamous carcinoma, gastric heterotopia, and melanoma. For each patient, we collected demographics, lesion size and site, operative details (including platform, resection plane, patient positioning, peritoneal entry, conversion, and resection margins), and postoperative outcomes (including complications, length of stay, further intervention, and follow-up with recurrence). Data were drawn directly from the electronic registry, with verification against operative and pathology reports when required. We summarised lesion size as mean with range, whereas time-based outcomes are reported as median with interquartile range. No formal hypothesis testing was planned, as the study’s intent was descriptive and exploratory, focusing on rare entities.

RESULTS

DISCUSSION

This study demonstrates that transanal endoscopic platforms provide a practical and versatile approach for the excision of rare rectal lesions, with favourable safety and oncological profiles. It should be stressed that TEM and TEO represent surgical platforms rather than procedures in themselves. The distinction is essential, as the platforms are designed to offer a stable pneumorectum, precise instrumentation, and magnified vision, which can then be employed to perform different types of dissection according to the pathology and the surgeon’s intent. The procedure is therefore defined not by the platform, but by the dissection plane and the surgical strategy undertaken.

In this cohort, both full-thickness and submucosal dissections were undertaken, each chosen according to the biological behaviour and expected depth of the lesion. Full-thickness resection was performed in the overwhelming majority of patients, particularly where diagnostic certainty was required or where oncological adequacy was paramount. Submucosal dissection was reserved for lesions in which the pathology or imaging suggested confinement to the mucosa or superficial submucosa. The availability of both strategies within the same platform underscores the adaptability of TEM and TEO. Compared with conventional transanal excision, TEM has consistently been associated with better specimen quality, higher R0 rates, and lower local recurrence in rectal lesions, and these same technical advantages likely explain its performance in uncommon histologies^[2,3,11].

Oncological adequacy is a central consideration in the treatment of rare rectal tumours^[12], many of which pose unique biological challenges. In this series, clear resection margins were achieved in nearly all patients, with only a handful of R1 resections. Recurrence was exceptional, and when it did occur, it was documented several years later, underscoring that local control is reliably achievable when these platforms are employed. These findings align with larger series reporting high R0 rates for more common indications, such as adenomas and early adenocarcinoma, where TEM and TEO consistently outperform conventional transanal surgery in terms of margin status and recurrence rates^[6]. The current data extend this principle into much rarer histologies, including GIST, NEN, melanoma, squamous carcinoma, and hamartomas, confirming the broad utility of the approach^[13-15].

Complications were infrequent and generally minor. The most common intraoperative issue, peritoneal entry, occurred in only three patients and was always manageable within the platform without conversion to open or laparoscopic surgery. Overall postoperative morbidity comprised four events: one bleeding episode requiring transfusion and reoperation and three additional non-fatal complications recorded in the database. Mortality was nil. This confirms the widely accepted safety profile of transanal endoscopic surgery, which is characterised by extremely low rates of significant complications and minimal physiological stress on the patient^[16]. In addition, the length of stay was very short, with a median of two to five days across all subgroups, reflecting both the minimally invasive nature of the intervention and the rapid recovery it affords. This stands in stark contrast to abdominal resections, which, even in modern laparoscopic series, still require longer hospitalisation, higher morbidity, and a greater risk of stoma formation^[17].

The rarity of the lesions presented here deserves particular attention. By their very nature, tumours such as rectal GIST, NEN, duplication cysts, or rectal melanoma are seldom encountered in clinical practice, which makes drawing generalisable conclusions difficult. However, the present series confirms that the platform-based strategy is robust across a heterogeneous range of histologies. The ability to maintain margin negativity, avoid conversion, and minimise complications across this spectrum supports the idea that TEM and TEO can be considered the standard rigid transanal endoscopic platforms for rare rectal lesions when local excision is deemed appropriate. This has implications for centres that may otherwise consider such lesions unsuitable for transanal management, potentially expanding organ-preserving options for patients. Beyond rare histotypes, the versatility of these platforms is also illustrated by atypical indications reported in the literature, such as treatment of complete rectal prolapse using the TEO platform^[18].

Another important point concerns the learning curve and dissemination of the technique. TEM has historically been associated with a steep learning curve, partly due to the rigid rectoscope and dedicated instruments. TEO was developed to provide a shorter, more accessible system with a high-definition camera and more ergonomic handling^[19]. Both platforms, however, offer the same essential benefits of stability and magnification. The rarity of the cases in this series suggests that centres must maintain broad expertise and a culture of multidisciplinary discussion to integrate these rare pathologies into practice. Training curricula are increasingly incorporating simulation and structured assessment, and the application of these tools to TEM and TEO will likely further shorten the learning curve and ensure the reproducibility of outcomes^[20].

From a comparative perspective, alternative methods exist for the local excision of rectal lesions, most notably transanal minimally invasive surgery (TAMIS) and endoscopic submucosal dissection (ESD). TAMIS utilises a disposable transanal port and laparoscopic instruments, offering flexibility and ease of setup. However, its stability and visualisation are often inferior to TEM and TEO, particularly for more proximal lesions^[21]. ESD, on the other hand, is a purely endoscopic approach that excels in mucosal and superficial submucosal lesions but is less reliable for full-thickness resections and cannot provide the same degree of control once the muscularis propria is entered. Nevertheless, the superiority of one device over the other for mucosal lesion is still under discussion^[22]. For rare tumours with uncertain behaviour or need for definitive histological staging, TEM and TEO therefore remain the gold standard, bridging the gap between minimal invasiveness and oncological adequacy.

Limitations of the present study must be acknowledged. The data derive from a retrospective series spanning over 20 years of surgical activity, and which is susceptible to information bias and missing data. Follow-up was heterogeneous, with some patients followed for many years and others censored shortly after surgery. The rarity of the lesions means that numbers are small in each subgroup, limiting the ability to detect infrequent outcomes. Moreover, detection of recurrence was based on clinical records rather than a protocolised surveillance programme, which may underestimate true incidence. Nonetheless, the strengths include the consecutive nature of the series, the prospective maintenance of the database, and the detailed operative documentation, which provide a reliable description of practice and outcomes in this unusual setting.

Looking to the future, new technologies may further extend the scope of transanal platforms. Robotic assistance has already been applied in the form of robotic TAMIS and robotic-assisted TEO, offering improved dexterity, three-dimensional vision, and potentially more precise suturing^[23]. Whether these advantages translate into measurable clinical benefits for rare lesions remains to be established, but early reports suggest feasibility and safety. Similarly, flexible transanal platforms are under development, which may combine the advantages of endoscopic manoeuvrability with the stability of rigid systems. Artificial intelligence and computer vision may also play a role in enhancing safety and training by providing real-time feedback on dissection planes, instrument handling, and error recognition^[24]. These innovations may ensure that even the rarest rectal tumours can be managed with increasing consistency and safety.

In conclusion, the present findings reinforce the broader principle of organ preservation in rectal surgery. Although the numbers are small, the capacity to manage unusual tumours with low morbidity, nil mortality, and rapid recovery supports the philosophy of avoiding unnecessary radical resections when a safe and oncologically adequate local excision can be achieved. For patients, this means not only reduced operative risk but also preservation of bowel function and quality of life, outcomes that are particularly valuable in a population often facing complex oncological decisions. The challenge for the surgical community is to ensure that such opportunities are not missed due to a lack of familiarity or access to the appropriate platforms.