Endometriosis is a gynecologic disorder that is defined histologically by the presence of normal endometrial glands and stroma outside the endometrial cavity and uterine musculature [1].   The normal endometrium, it consists of the functionalis, the superficial endometrial layer, lost during menstruation and the basalis, the deep layer of more monomorphic glands that regenerates the functionalis after shedding. The stromal cells range from small and spindled, during the proliferative phase, to large, eosinophilic and polygonal during the secretory phase secondary to hormonal effect [1].

Endometriosis is primarily located in the pelvic cavity, often presenting clinically as a pelvic mass or pain.  However, it can also involve extra-pelvic sites, including the abdominal wall, umbilicus, urinary tract, inguinal region, and gastrointestinal tract. In the gastrointestinal tract, it can occur anywhere from mucosa to serosa, and it can be subject to biopsy during colonoscopy. [1].  The mean patient age at the time of presentation is  36 years (range 27–49 years), and the main  complaints are pelvic pain, dysmenorrhea and irregular bleeding [1]. Histologically, endometriosis consists of well-circumscribed foci of endometrial glands embedded in variable amounts of endometrial stroma [2]. Glands are lined by pseudostratified, columnar, proliferative-type endometrium. Nucleoli can rarely be prominent [2]. The stroma closely surrounds the glandular epithelium and is composed of densely packed small fusiform cells. Pax-8 and estrogen receptor (ER) immunohistochemical stains are positive in endometrial glandular epithelium, while CD10 and ER immunostains are positive in endometrial stromal cells [2].  Cytologically, the smears are variably cellular, ranging from low to abundant with prominent epithelial and stromal populations. The cytological diagnosis of endometriosis requires the identification of at least two of the following 3 three features: endometrial type glands, endometrial type stroma and evidence of chronic bleeding as hemosiderin-laden macrophages [2]. In our case, the glandular component consisted  of epithelial cells arranged in an orderly distribution as syncytial tissue fragments and large cohesive sheets, occasionally  associated with  round to oval and evenly spaced endometrial stroma nuclei with occasional nuclear crowding and a fine, evenly distributed chromatin pattern.

In the appropriate clinical setting, the cytological diagnosis, is often straightforward, especially when the three expected features are present [2,3]. Findings of glandular atypia in cytology may falsely raise the concern for malignancy and represents a serious potential diagnostic pitfall [2]. On the other hand, the presence of isolated spindle to oval stromal cells on cytologic smears may also be a source of diagnostic error and lead to a diagnosis of a Gastrointestinal stromal tumor (GIST) [2,3]. Our case is especially worrisome for GIST because the endometrial implant was confined to the muscularis propria, without any involvement of any of the other layers of the colon wall [2,3]. However GISTs lacks glandular cells as well as significant hemorrhage and hemosiderin-laden macrophages typical of endometriosis [1-3]. Additionally, GISTs cells contain eosinophilic cytoplasm which often contains vacuoles, a feature not found in the stromal cells of endometriosis. Although rare, malignant transformation of endometriosis has been documented and associations between endometriosis and ovarian carcinomas have been established, especially for clear cell and endometriod carcinomas [2].  Worth mentioning it that clear cell and endometrioid carcinomas share overlapping molecular changes with endometriosis, including PI3KCA, ARID1A, and p53 mutations [1]. Despite this, endometriosis does not appear to be associated with tumor stage or predict prognosis in these ovarian cancers [1]. Finally, regarding risk factors, early age at menarche, shorter menstrual lengths, and taller height, are associated with a higher risk of endometriosis, while parity, higher body mass index (BMI) and smoking are associated with decreased risk [4].