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Table 1 Histological and immunohistochemical differential diagnosis of rectal GIST involving prostate

From: Collision tumour involving a rectal gastrointestinal stromal tumour with invasion of the prostate and a prostatic adenocarcinoma

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