Transanal endoscopic microsurgery (TEM) is a minimally invasive technique for the excision of rectal tumors located between 8 and 18 cm above the anal verge. Lesions located below 8 cm can usually be approached with traditional instruments, whereas exposure using a traditional transanal approach becomes difficult above this point. TEM provides excellent exposure of tumors up to 18 cm from the anal verge. When compared to the techniques of low anterior resection, parasacral approach, or sphincter-splitting approaches, TEM offers greater precision, lower morbidity, and a shorter hospital stay. The technique is not widely established because of the specific instrumentation required, the unusual technical demands of the approach, and the specific patient selection criteria.
The main indication for the TEM procedure is any benign lesion that cannot be removed by the traditional endoscopic snare technique. These consist mainly of lipomas, leiomyomas, tubular or villous adenomas, or carcinoids. Depending on the skill of the surgeon, any size benign lesion can be considered for TEM; however, “optimal” tumor size ranges from 2 cm in diameter to three fourths of the lumen circumference. TEM is also used for the removal of malignant neoplasms in patients with low-risk cancers (T1). In a high-risk operative candidate or as a palliative modality, these criteria can be extended. Careful selection and meticulous operative technique is critical for the outcome when operating with curative intent. A full-thickness resection is recommended to ensure a negative surgical margin. This resection technique also decreases the risk of missing small rectal cancers that may be located deep inside a villous adenoma that might occur if only a mucosectomy is performed.
Lymph-node invasion is the primary factor limiting the effectiveness of local treatment in early rectal cancer. The lymph-node metastasis rate of T1 rectal tumors lies between 0% and 15.4% depending on tumor grade. Cancer recurrence rates after TEM are generally lower than those reported with conventional transanal surgery. The indications for neo-adjuvant or adjuvant radiation chemotherapy following local resection of rectal cancer by TEM remain controversial.
TEM may be also be an alternative for the resection of rectal stenoses within 5 to 15 cm of the dentate line (i.e., inflammatory stenoses after high fistulae or colorectal anastomotic stenoses).
Preoperative assessment of type, stage, and grade of the tumor is crucial to the success of TEM. This generally consists of a clinical examination with rectoscopy and biopsy, endorectal ultrasound, and a sphincter function test. Patients with known benign lesions do not need evaluation by computed tomography (CT). Lesions should be assessed for mobility. Patients with malignant lesions greater than 3 cm or with ulcerated appearance on sigmoidoscopy are at greater risk of penetration through the rectal wall and must be carefully evaluated before TEM is attempted. Patients should also be asked about fecal or urinary incontinence, which may be made worse by a TEM procedure.