Table 1.

Histological and immunohistochemical differentialdiagnosis of rectal GISTinvolving prostate

Diagnosis	GIST (rectum/ prostate)	Leiomyoma (rectum/prostate)	Leiomyosarcoma (rectum/prostate)	Schwannoma (rectum/prostate)	Fibromatosis (mesenteric)	Solitary fibrous tumour (prostate)	Malignant melanoma (rectum/ prostate)
Growth Pattern	Fascicles, storiform or palisading	Fascicles	Fascicles	Antoni A/ Antoni B	Parallel fascicles, collagenous background	“Patternless pattern”, collagenous background, staghorn vessels	Diffuse / nested
Cell Type	Spindle cell, high cellularity, no perinuclear vacuolization	Spindle cell, “cigar-shaped” nuclei, perinuclear vacuolization, low cellularity	Spindle cells, “cigar-shaped nuclei, atypia and intense nuclear pleomorphism, vacuolization, high cellularity	Spindle cells, wavy nuclei, nuclear atypia may be present	Bland spindle or strellate-cells, low to moderate cellularity	Spindle cells, low or high cellularity, pleomorphism (variable)	Epithelioid (rarely spindle cell)
Mitosis	Few to many	Few	Frequent	Rare	Rare	0-15 / 10 HPF	Frequent
Necrosis/ Hemorrhage	May be present	Absent	Present	Sometimes present	Absent	Variable	Usually present
CD 117	Consistenly positive (cytoplasmic and membranous)	Negative	Negative	Negative	Positive sometimes (cytoplasmic)	Negative	Rarely positive
CD34	Consistenly positive	Rarely positive	Negative	Sometimes focal	Very rare	Positive	Negative
Actina (SMA)	Sometimes positive	Positive	Positive	Negative	Positive	Rarely	Negative
Desmina	Rarely	Positive	Positive	Negative	Very rare	Negative	Negative
S-100	Rarely	Very rare	Very rare	Strongly positive	Negative	Negative	Positive
HMB-45	Negative	Negative	Negative	Negative	Negative	Negative	Positive
Beta-Catenin	Negative	Negative	Negative	Negative	Positive	Negative	Negative