Histopathological reporting of soft tissue tumour

AN APPROACH TO HISTOPATHOLOGICAL REPORTING
OF SOFT TISSUE TUMOUR

Classification of Soft Tissue Tumour: click

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The pathologist should answer the following questions in a step-wise manner before making the final diagnosis:

Is it a primary tumour?

Could it be an inflammatory process (eg. pseudotumour or abscess) ?

Is it a metastatic tumour or involvement by lymphoma or leukaemia?

Is it a pure soft tissue tumour or a lesion like spindle cell carcinoma?

Does the tumour have any recognizable differentiation?

Is it benign, locally aggressive but non-metastasizing or malignant?

Is the excision complete with adequate margins?

Are there any other histological prognostic data present?

The following clinical details should be provided by the clinician:

I ABOUT THE PATIENT
Age and sex
Preoperative symptoms
Previous medical or surgical history

II ABOUT THE TUMOUR
Location including anatomical planes-
(Cutanous,subcutaneous or deep)
Superficial soft tissue- Benign tumours and rarely sarcomas (myxoid MFH and leiomyosarcoma)
Deep fascia - Sarcoma
Size and duration - Eg. Sarcoma is larger with longer history. Fasciitis is smaller and of recent onset
Any history of pain or tenderness.
Whether single or multiple,
Whether underlying structures are involved
Whether rapid or slow growing tumour.

HISTOCHEMISTRY

PAS is positive-
-Granular cell tumour
-Epithelial mucin in biphasic synovial sarcoma
-Glycogen in smooth and skeletal muscle tumour, chondroid tumours & liposarcoma
-Diastase resistant inclusions in alveolar soft part sarcoma
Alcian blue for sulphated mucin in Myxoid chondrosarcoma.
Masson trichrome- Collagen and muscle
PTAH- Cross striations in rhabdomyosarcoma.
Reticulin- Demonstrate architecture and vascular pattern.

Following tumours may be under- diagnosed as a benign lesions :
Malignant tumours like epithelioid sarcoma, low grade fibromyxoid sarcoma and myxofibrosarcoma, atypical lipomatous tumour and well differentiated leiomyosarcoma .

This may be avoided by-
1. Careful assessement of architecture, cytological character, tumour stromal interphase and associated reaction ;
2. Immunohistochemical findings ;
3. Awareness of other diagnostic possibilities.

Following reactive soft tissue lesion may be overdiagnosed as sarcomas:
Nodular fasciitis and its variants
Proliferative fasciitis ; Proliferative myositis;
Myositis ossificans, Fracture callus

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IMMUNOHISTOCHEMISTRY:

(Visit: The role of immunohistochemistry in the differential diagnosis of Soft tissue tumors Carlos A Muro-Cacho MD PhD )

Vimentin- Positive for most soft tissue tumours. Rarely some carcinomas and lymphomas are present.
Cytokeratin- (EMA and pancytokeratin) -Epithelial marker.
Desmin, smooth muscle actin display muscle differentiation.
CD31 and von Willebrand factor-vascular marker
LCA, B and T cells markers- lymphoid tumour marker
S100- Demonstrate neural, adipocytic, and cartilaginous differentiation.
HMB45 -Demonstrate clear cell sarcoma and angiomyolipoma.
CD99, neuron specific enolase, (NSE) and PGP9.5- primitive neuroectodermal tumours/Ewing's sarcoma.
CD34- dermatofibrosarcoma protuberans and solitary fibrous tumour
Myoglobin-Display skeletal muscle differentiation.

The histopathology report should include the following features:

    HISTOLOGICAL PATTERNS-
   Spindle cell
   Epithelioid
   Fibrohistiocytic
   Fascicular
   Adipocytic
   Vascular
   Myxoid
   Mixed patterns

STROMA-
1. Hyaline
2. Fibrous
3. Myxoid
4. Osteocartilaginous

            CYTOPLASMIC FEATURES-
I. CYTOPLASMIC STAIN:
1.Eosinophilic -Indicate smooth muscle , myofibroblastic or fibrohistiocytic forms
2.Basophilic - Suggest neural , fibroblastic or synovial forms.
II   CYTOPLASMIC VACUOLES:
1.Intracytoplasmic vacuoles with RBCs- Epithelioid vascular tumour
2.Well defined vacuoles indenting the nucleus- Lipoblasts
III   CYTOPLASMIC INCLUSION:
1. Eosinophilic round inclusions (actin)- Infantile digital fibroma
2. PAS positive crystalline structures-Alveolar soft part sarcoma.

NUCLEAR FEATURE
I SHAPE:
1. Cigar shaped- Smooth muscle tumour
2. Wavy with pointed end- Neural tumour
3.Tapered,pointed ends- Fibroblasts
4. Ovoid, crenated ;small distinct nucleolus - Myofibroblast

II. CHROMATIN:
Hyperchromatic dense chromatin-
eg- degenerative changes in neural and adipocytic tumours.

GIANT CELLS
Giant cells are noted in some soft tissue tumours:
TYPES:
Mutinucleated lipoblasts - Liposarcoma
Ganglion like giant cells-Proliferative myositis /fasciitis
Floret giant cells- Pleomorphic lipoma
Wreath like giant cells- Alveolar rhabdomyosarcoma
Touton giant cell- Juvenile xanthogranuloma

VASCULAR PATTERN
1. Chicken wire or crow's feet branching- Myxoid liposarcoma
2. Staghorn branching- Haemangiopericytoma
3. Hyaline vessels - Schwannoma, aggressive angiomyxoma
4. Sieve- like space- Kaposi's sarcoma, Spindle cell haemangioendothelioma

MITOTIC COUNT
Numerous bizarre mitotic figures may be present in a benign tumour like Atypical fibroxanthoma.
Mitotic count may be low in a high grade malignant tumour like round cell liposarcoma. Any mitosis in neurofibromatous tumour implies malignancy, in contrast fairly numerous mitoses is noted in cellular schwannoma.
Mitoses is the main criteria for the diagnosis of malignancy in smooth muscle tumour.

EDGE OF TUMOUR
1. True or pseudocapsule
2. Well circumscribed or infiltrative margin.

ADEQUACY OF EXCISION
In all locally aggressive and malignant tumours it is essential to comment on adequacy of excision.
Local recurrence may occur in case of sarcomas where the excision margins are less than 1-2 cm.

OTHER FEATURES:
1. Multicentricity
2. Necrosis
3. Vascular invasion
4. Cystic change

NOTE:

Spindle cell carcinoma can be ruled out by appropriate immunostains.
Spindle cell melanoma can closely resemble many sarcomas specially leiomyosarcoma , MPNST and MFH.
Soft tissue lymphoma must be considered in any large round cell tumour in soft tissue.

Criteria of malignancy depends on the cell type:
Smooth muscle tumour- Presence of mitoses or necrosis in deep lesions are consistent with malignancy.
Nerve sheath tumour- Encapsulation and widespread S100 positivity are indicative of Cellular schwannoma.
Myofibroblastic tumours- pleomorphism, cytologic and mitotic atypia with large nucleoli and necrosis indicate malignancy.
Light microscopy is usually the only useful technique for distinguishing reactive lesions or benign tumours from malignant tumours.
Examples :
Well differentiated liposarcoma may have areas resembling its benign counterpart.
Fibromatosis can be difficult to distinguish from low grade sarcoma.