High-power light microscopy of a benign gastrointestinal stromal tumor (GIST) reveals spindle-shaped cells arranged in interlacing fascicles. The neoplastic cells are slender with occasional tapered ends and minimal cytoplasm, producing a homogeneous, eosinophilic background. Nuclei are elongated, with small, inconspicuous nucleoli and lack of pronounced nuclear waviness, pleomorphism, or hyperchromasia. The stroma is dominated by bland, eosinophilic collagen, and mitotic figures are not evident in the field, supporting a low proliferative activity. This spindle cell GIST subtype typically shows uniform cytology and cohesive fascicles, distinguishing it from schwannian or myogenic tumors, which may exhibit nuclear palisading or wavy nuclei. The absence of jagged nuclear contours or notable cytoplasmic vacuolization further supports a benign course in small lesions. Clinically, small GISTs are frequently asymptomatic and discovered incidentally during imaging or surgery. Immunohistochemical confirmation (CD117/c-KIT and DOG1 positivity) and mutational analysis (KIT or PDGFRA) are routinely pursued to establish diagnosis and guide risk stratification, though histology alone can strongly suggest GIST when classic spindle morphology is present. The image highlights diagnostic features relevant to surgical pathology, including tumor size, cellular blandness, and microenvironment, which influence management decisions and prognosis in gastrointestinal stromal tumors. This context supports accurate diagnosis and appropriate surgical planning.

Histopathology: A formalin-fixed, paraffin-embedded tissue section stained with Hematoxylin and Eosin (H&E) shows interlacing fascicles of spindle-shaped smooth muscle cells with elongated, blunt-ended or cigar-shaped nuclei and abundant eosinophilic cytoplasm. The cells are uniform in size, with minimal nuclear atypia and rare mitotic figures; there is no evident necrosis. The architectural pattern is consistent with a benign leiomyoma arising from the smooth muscle layer, such as the muscularis propria of the gastrointestinal tract or uterine myometrium, depending on anatomic origin. Immunohistochemistry demonstrates tumor cells that are strongly positive for desmin and smooth muscle actin (SMA), confirming smooth muscle differentiation. The neoplastic cells are negative for gastrointestinal stromal tumor markers CD34, CD117 (c-KIT), and DOG1, effectively excluding GIST. Diagnostic significance: the combination of characteristic spindle cell morphology and desmin/SMA positivity with CD34/CD117/DOG1 negativity supports a diagnosis of leiomyoma over GIST or leiomyosarcoma. Clinical correlation: these lesions are typically benign with favorable prognosis following complete excision; differential diagnoses include leiomyosarcoma (increased mitotic activity, atypia, necrosis) and GIST (c-KIT or DOG1 positivity). This image exemplifies classical smooth muscle tumor histology and IHC profile, and is useful for educational purposes in surgical pathology, GI/hepatic pathology, and mesenchymal tumor differential diagnosis. Educational diagnostic utility.

Immunohistochemical slide of a gastrointestinal stromal tumor (GIST) showing robust cytoplasmic/membranous brown staining on a hematoxylin counterstain background. The image likely represents a tissue section stained for CD117 (c-KIT) and/or DOG1, markers that confer high sensitivity and specificity for GIST. In this field, CD34 may be variably positive and SMA positivity can be focal, complicating differential diagnosis with leiomyosarcoma. The tissue is FFPE, sectioned at 4 μm, and stained with DAB chromogen to yield brown immunoreactivity. The tumor cells display spindle-cell morphology in fascicles embedded in a collagenous stroma; nuclei are ovoid to elongated with variable mitotic activity. The immunophenotype—predominant CD117 (c-KIT) and DOG1 positivity—supports the diagnosis of GIST, with DOG1 often providing superior specificity. The combined histology and IHC pattern differentiates GIST from smooth muscle and neural neoplasms. Clinically, this immunoprofile directs targeted therapy options (tyrosine kinase inhibitors such as imatinib or sunitinib) and informs mutational testing for KIT or PDGFRA to predict treatment response. This image is valuable for education in oncologic pathology, GI pathology, and diagnostic immunohistochemistry, illustrating how a mesenchymal GI tumor is identified by characteristic immunostains. It is suitable as a reference for immunophenotype, differential diagnosis, and molecular correlation in GIST research and teaching.

Imaging modality: Histopathology with bright-field light microscopy of a hematoxylin and eosin (H&E) stained section from a gastrointestinal tract neoplasm. Anatomic context: gastrointestinal tract wall (GI tract), smooth muscle tissue, muscularis propria region. Visual features: densely cellular fascicles of spindle-shaped cells with elongated, blunt-ended cigar-shaped nuclei and eosinophilic cytoplasm; minimal pleomorphism; absence of conspicuous mitotic figures within this field; variable stromal collagen and small, serpiginous blood vessels. The cells exhibit classic smooth muscle morphology consistent with leiomyosarcoma; adjacent areas elsewhere in the tumor may reveal higher mitotic activity and pleomorphism. Pathology: malignant neoplasm with smooth muscle differentiation; immunohistochemistry typically shows desmin and smooth muscle actin (SMA) positivity with h-caldesmon expression; in this field, morphological features strongly support leiomyosarcoma over benign leiomyoma or other spindle cell tumors such as GIST. Diagnostic significance: histology indicates a primary GI smooth muscle sarcoma; prognosis is influenced by mitotic rate, tumor size, necrosis, and location; clinical correlation includes surgical resection planning and potential adjuvant therapy; differential diagnoses include GIST, leiomyoma, fibrosarcoma, and sarcoma NOS. Potential clinical use: reference for tumor morphology in GI spindle cell neoplasms, educational cases in surgical oncology, pathology sign-out, and radiopathologic correlation in GI oncology.