Musculoskeletal System (MSK) Module — 3rd Year MBBS

📌 AIM Study Tip

This chapter follows the KMU learning outcomes and explains soft-tissue and muscle tumours in a logical sequence. First understand how each tumour behaves and appears morphologically, then revise the high-yield distinguishing features.

3rd Year MBBS KMU Curriculum AIM Learning Cycle

📖 AIM Learning Material

Topic 11 — Soft-Tissue and Muscle Tumours

MSK Module — Classification, clinical behaviour and morphology of adipocytic, fibrous, fibroblastic and muscle tumours

Topic Introduction

Soft-tissue tumours arise from the tissues that support, connect and move the body, including adipose tissue, fibrous tissue and skeletal or smooth muscle. They range from common, harmless lesions such as lipoma to aggressive malignant tumours such as liposarcoma and rhabdomyosarcoma. Their names usually reflect the type of tissue or cell that the tumour resembles. Understanding these lesions requires three linked ideas: the tissue of origin, whether the tumour is benign or malignant, and its characteristic gross and microscopic appearance. This chapter explains the classification and salient clinical features of soft-tissue tumours, followed by the important morphology and behaviour of lipoma, liposarcoma, nodular fasciitis, fibromatoses, rhabdomyosarcoma, leiomyoma, leiomyosarcoma and fibrosarcoma.

A. Classification and General Behaviour of Soft-Tissue Tumours

Soft-tissue tumours are classified mainly according to the normal adult tissue or cell type that they resemble. This classification does not always prove the exact cell from which a tumour originated. Instead, it describes the direction of differentiation shown by the tumour cells. Each major group may contain benign, locally aggressive or malignant lesions. Benign tumours usually remain localized and resemble normal mature tissue. Malignant soft-tissue tumours are called sarcomas. They may infiltrate surrounding structures, recur after removal and spread through the bloodstream, particularly to the lungs.

Basic classification

Adipocytic tumours: lipoma and liposarcoma.
Fibrous and fibroblastic tumours: nodular fasciitis, fibromatoses and fibrosarcoma.
Skeletal muscle tumours: rhabdomyoma and rhabdomyosarcoma.
Smooth muscle tumours: leiomyoma and leiomyosarcoma.

Salient clinical features

Most soft-tissue tumours present as a mass or swelling. A benign lesion is often superficial, mobile, slowly growing and painless. A malignant lesion may be deep, progressively enlarging, firm or fixed. Pain may occur when the tumour compresses nerves, invades surrounding structures or undergoes necrosis.

Superficial, small and mobile masses are more often benign.
Deep-seated or rapidly enlarging masses raise concern for malignancy.
Local recurrence is especially important in infiltrative lesions such as fibromatosis.
Blood-borne metastasis is a major feature of many sarcomas.
The final diagnosis depends on morphology and, where necessary, immunohistochemical or molecular confirmation.

Diagnostic clue: The depth, rate of growth and mobility of a soft-tissue mass help estimate its behaviour, but histopathology is required for definite classification.

B. Tumours of Adipose Tissue: Lipoma and Liposarcoma

Adipocytic tumours show differentiation towards fat cells. A lipoma is a benign tumour composed mainly of mature adipocytes, whereas a liposarcoma is a malignant tumour showing adipocytic differentiation. Despite their similar names, liposarcoma usually develops independently and is not the usual result of malignant transformation within a lipoma.

Lipoma

Lipoma is one of the most common benign soft-tissue tumours. It usually arises in the subcutaneous tissue of adults, particularly over the trunk, neck and proximal limbs. Because it grows slowly and remains well circumscribed, it commonly presents as a soft, freely mobile and painless swelling.

Gross morphology

Usually a well-circumscribed or thinly encapsulated mass.
Soft, yellow and lobulated.
The cut surface resembles normal adipose tissue.
Most lesions are located in superficial subcutaneous tissue.

Microscopic morphology

Composed of mature adipocytes arranged in lobules.
The cells closely resemble normal fat cells.
Nuclei are small and peripheral without significant atypia.
Fibrous septa may separate the lobules.
Mitotic activity and tumour necrosis are absent.

The close resemblance to normal fat explains the soft consistency and yellow colour of the lesion. Complete removal is usually curative, although recurrence can occur if the lesion is incompletely removed.

Liposarcoma

Liposarcoma is a malignant adipocytic tumour that usually occurs in adults. Unlike lipoma, it commonly develops in deep soft tissues, especially the thigh or retroperitoneum. Retroperitoneal tumours may become very large before detection because there is considerable space for expansion. Liposarcoma includes several morphological patterns. Their clinical behaviour varies from relatively indolent, locally recurrent tumours to highly aggressive sarcomas with metastatic potential.

Important morphological forms

Type	Important morphology	General behaviour
Well-differentiated liposarcoma	Mature adipocytes with variation in cell size, atypical stromal cells and occasional lipoblasts	Mainly locally aggressive; recurrence is important
Myxoid liposarcoma	Abundant myxoid matrix, delicate branching capillaries and primitive cells with lipoblasts	Intermediate malignant potential
Pleomorphic liposarcoma	Marked cellular pleomorphism, bizarre tumour cells and pleomorphic lipoblasts	High-grade aggressive sarcoma

Lipoblast

A lipoblast is a tumour cell showing adipocytic differentiation. It contains one or more cytoplasmic lipid vacuoles that indent or scallop the nucleus. Lipoblasts support the diagnosis of liposarcoma, but the diagnosis depends on the overall architecture and cytological features rather than on a single cell alone.

Clinical features and complications

Presents as a deep, enlarging and often painless mass.
Retroperitoneal lesions may produce abdominal fullness or pressure symptoms.
Local recurrence may occur because deep tumours can be difficult to remove completely.
High-grade forms can metastasize through the bloodstream.
Large tumours may compress adjacent organs, vessels or nerves.

Exam distinction: Lipoma contains uniform mature fat cells, whereas liposarcoma shows atypia, variable cellularity or lipoblasts according to its subtype.

C. Fibroblastic Proliferations: Nodular Fasciitis and Fibromatoses

Nodular fasciitis and fibromatoses are fibroblastic or myofibroblastic proliferations. They are important because both can grow rapidly or infiltrate surrounding tissue and may therefore be mistaken for sarcoma. However, nodular fasciitis is a self-limited benign proliferation, while fibromatoses are locally aggressive lesions that do not metastasize.

Nodular fasciitis

Nodular fasciitis is a benign, rapidly growing proliferation of fibroblasts and myofibroblasts. It often affects young adults and commonly occurs in the superficial fascia of the upper limbs, trunk, head or neck. Its rapid appearance can create concern for malignancy, but the lesion is usually small and well circumscribed.

Clinicopathological features

Rapidly growing, solitary mass developing over a short period.
Usually superficial and relatively small.
May be tender or painful.
A history of preceding trauma may occasionally be present, but it is not required.
Does not metastasize and may regress or be cured by local excision.

Microscopic morphology

Plump spindle-shaped fibroblasts and myofibroblasts.
Cells arranged in short, irregular bundles producing a tissue-culture-like pattern.
Myxoid background with extravasated red blood cells.
Numerous mitotic figures may be present.
Mitotic figures are usually typical rather than atypical.
Marked pleomorphism and destructive necrosis are absent.

The high mitotic activity reflects rapid growth, not necessarily malignancy. The combination of rapid growth and numerous mitoses is the main reason nodular fasciitis can be confused with a sarcoma.

Fibromatoses

Fibromatoses are clonal fibroblastic proliferations that lie between benign fibrous lesions and malignant fibrosarcoma. They do not metastasize, but they can infiltrate surrounding skeletal muscle, fascia and other structures. This infiltrative growth makes complete surgical removal difficult and explains their tendency to recur.

Classification

Superficial fibromatoses: arise in superficial fascia and are usually smaller and less aggressive.
Deep fibromatosis: also called a desmoid tumour; arises in deep musculoaponeurotic tissues and is more infiltrative.

Superficial fibromatoses

These lesions form firm nodules or bands in superficial fascia. Examples include palmar and plantar fibromatosis. Their contraction may produce deformity or limitation of movement because the abnormal fibrous tissue shortens over time.

Deep fibromatosis

Deep fibromatosis commonly presents as a firm, poorly defined mass in the abdominal wall, shoulder girdle, trunk or limbs. It consists of bland fibroblasts embedded in abundant collagen. Although the cells appear cytologically benign, the tumour sends long extensions into adjacent muscle and soft tissue.

Morphology

Firm, grey-white and poorly circumscribed mass.
Long sweeping fascicles of uniform spindle cells.
Abundant collagen between tumour cells.
Minimal cytological atypia.
Low or moderate mitotic activity without atypical mitoses.
Infiltration of surrounding skeletal muscle and soft tissue.

Complications

Repeated local recurrence after incomplete removal.
Compression or entrapment of nerves and vessels.
Functional impairment due to invasion of muscle or fascia.
Serious local effects when the lesion involves vital anatomical structures.
No distant metastasis.

Diagnostic clue: Bland spindle cells do not always indicate harmless behaviour. Fibromatosis appears bland microscopically but grows infiltratively and frequently recurs.

D. Skeletal Muscle Tumours and Rhabdomyosarcoma

Skeletal muscle tumours show differentiation towards striated muscle. They are broadly classified into benign rhabdomyoma and malignant rhabdomyosarcoma. Rhabdomyoma is uncommon, while rhabdomyosarcoma is an important malignant tumour of childhood. Rhabdomyosarcoma may arise in locations where mature skeletal muscle is sparse because the tumour develops from primitive mesenchymal cells capable of skeletal muscle differentiation.

Etiology and pathogenesis

Rhabdomyosarcoma develops when primitive mesenchymal cells acquire genetic alterations that disturb normal growth and muscle differentiation. The abnormal cells continue to proliferate while showing partial development towards skeletal muscle. The exact molecular abnormality varies among its morphological types.

Pathogenesis chain Genetic alteration in primitive mesenchymal cell → uncontrolled proliferation → incomplete skeletal-muscle differentiation → infiltrative soft-tissue mass → local destruction and possible metastasis

Clinical features

Rhabdomyosarcoma occurs mainly in children and adolescents. The presenting feature depends strongly on the anatomical site. Common locations include the head and neck region, genitourinary tract and extremities.

Head and neck: may produce a rapidly enlarging facial, orbital or nasopharyngeal mass.
Genitourinary tract: may present with urinary obstruction, bleeding or a polypoid mass.
Extremities: usually presents as a deep enlarging soft-tissue mass.
Local invasion may cause pain, obstruction or loss of function.
Metastasis may occur through blood or lymphatic channels.

Major morphological types

Embryonal rhabdomyosarcoma

Embryonal rhabdomyosarcoma is the most common pattern in younger children. Its cells resemble developing embryonic skeletal muscle. The tumour may contain small primitive cells together with larger cells showing eosinophilic cytoplasm and greater skeletal-muscle differentiation.

Commonly affects the head and neck or genitourinary region.
Shows variable cellularity in a myxoid stroma.
Contains primitive cells and rhabdomyoblasts.
Rhabdomyoblasts may have deeply eosinophilic cytoplasm.
Cross-striations may occasionally be visible but are not always present.

Botryoid variant: When embryonal rhabdomyosarcoma arises beneath a mucosal surface, such as in the urinary bladder or vagina, it may form a grape-like polypoid mass. This is called sarcoma botryoides.

Alveolar rhabdomyosarcoma

Alveolar rhabdomyosarcoma occurs more often in older children and adolescents, particularly in the deep soft tissues of the extremities. Tumour cells form nests separated by fibrous septa. Cells in the centre of the nests may lose attachment, producing spaces that resemble pulmonary alveoli.

Nests of relatively uniform tumour cells.
Fibrous septa divide the tumour into compartments.
Central loss of cohesion creates an alveolar pattern.
Usually behaves more aggressively than typical embryonal tumours.

Pleomorphic rhabdomyosarcoma

Pleomorphic rhabdomyosarcoma mainly occurs in adults. It contains highly atypical and bizarre tumour cells with evidence of skeletal-muscle differentiation. It is a high-grade malignant tumour.

Complications

Destruction of adjacent muscles and soft tissues.
Obstruction of hollow organs when arising in the genitourinary tract.
Neurological or visual problems in head and neck lesions.
Local recurrence.
Lymphatic and blood-borne metastasis.

Exam distinction: Embryonal tumours resemble developing muscle, whereas alveolar tumours show nests separated by fibrous septa with central loss of cellular cohesion.

E. Smooth Muscle Tumours: Leiomyoma and Leiomyosarcoma

Smooth muscle tumours are classified as benign leiomyoma and malignant leiomyosarcoma. They may arise wherever smooth muscle is present, including the uterus, gastrointestinal tract, walls of blood vessels and skin. Their distinction depends mainly on cellular atypia, mitotic activity, tumour-cell necrosis and infiltrative behaviour.

Leiomyoma

Leiomyoma is a benign tumour composed of well-differentiated smooth muscle cells. The uterus is the most common site. Uterine leiomyomas may be single or multiple and often form sharply circumscribed masses within or around the uterine wall.