General Characteristics and Macroscopic Appearance
Rhabdomyosarcoma can occur at any age and affect a wide variety of anatomical regions, presenting as an organ-nonspecific neoplasm. Macroscopically, this tumor has the appearance typical of most sarcomas: on cross-section, its tissue looks like "fish flesh".
A critically important feature determining surgical management is that the neoplasm lacks a capsule and is characterized by pronounced infiltrative growth. Due to deep invasion into surrounding normal structures, complete radical excision of the tumor within healthy tissue margins is often unfeasible.
Microscopic Structure and Rhabdomyoblasts
The key diagnostic criterion for the disease is the detection of rhabdomyoblasts in histological sections. These tumor elements undergo the same stages of morphogenesis as normal fetal muscle cells:
- Pre-myoblast stage: cells mimic primitive mesenchyme. Nuclei are round and centrally located. There is scant cytoplasm, and due to the formation of short processes, cells can appear stellate.
- Onset of myogenesis: cytoplasm volume increases markedly. Due to the accumulation of muscle filaments, it stains intensely pink with eosin. During myoblastic differentiation, cells often elongate, resembling "tadpoles" or "spider cells", although some retain a round shape with a central nucleus.
- Fiber formation: at subsequent stages of development, tumor elements fuse. Multinucleated muscle fibers form, containing thin and thick filaments in their cytoplasm (these changes are particularly well visualized in cell culture).
Morphological Classification
Identifying a specific tumor variant is critically important because morphology directly correlates with favored location, typical patient age, and profoundly influences biological behavior and prognosis.
| Variant | Age | Typical Location | Biological Behavior |
|---|---|---|---|
| Embryonal | < 5 years | Head and neck region, abdominal cavity, urogenital system | Intermediate |
| Botryoid | < 5 years | Urinary system, vagina, bile ducts | Relatively favorable |
| Spindle cell | 5–20 years | Orbit, paratesticular region | Relatively favorable |
| Alveolar | Any (often > 5 years) | Extremities, meninges, nasal sinuses | Aggressive |
Features of Individual Morphological Variants
Each subtype has its unique structural features:
- Embryonal rhabdomyosarcoma is the most common. Histologically, it is formed by a myxoid, edematous stroma strongly resembling primitive mesenchyme. Scattered within this stroma are foci of forming embryonic muscle with polymorphic cells and abundant eosinophilic cytoplasm.
- Botryoid rhabdomyosarcoma (from Greek botryoeides — resembling a bunch of grapes) macroscopically forms characteristic polypoid masses. Externally, they closely resemble bunches of grapes.
- Spindle cell rhabdomyosarcoma consists of fascicles of elongated cells with spindle-shaped nuclei. Under microscopy, it can mimic fibrosarcoma or leiomyosarcoma. The main differential diagnostic feature is the presence of areas with clear myoblastic differentiation.