This comprehensive pathology video reviews 13 distinct soft tissue and bone tumor entities, emphasizing pattern recognition, key morphological features, and critical differential diagnoses. The discussion spans benign lesions (like hibernoma, myofibroma, and chondroma of soft parts) and aggressive sarcomas (including alveolar soft part sarcoma, synovial sarcoma, chordoma, and high-grade liposarcomas). The presentation utilizes classic diagnostic triads and memorable visual analogies ("mulberry cells," "ramen noodle sign," "apple bite cells," "splattered cells," "bicycle chain") shared by mentors in the field. Key takeaways highlight the importance of recognizing translocation sarcomas (which often exhibit uniform cellularity) and using ancillary techniques (IHC for Brachyury, MDM2, MUC4, TFE3, and molecular studies like FISH) to resolve complex differentials, particularly those mimicking metastatic carcinoma or other benign/malignant entities.
Review of Soft Tissue and Bone Tumors: Cases and Diagnostic Pearls
0:35 Case 1: Hibernoma: A benign tumor of brown fat, characterized by a lobulated pattern and three distinct cell types: mature white fat, multi-vacuolated cells (mulberry cells, non-pleomorphic), and granular pink cells packed with heat-producing mitochondria. Clinically, it is often large, deep-seated, and hot on PET scans, sometimes mimicking liposarcoma.
6:07 Case 2: Alveolar Soft Part Sarcoma (ASPS): An exceptionally rare sarcoma showing pseudo-alveolar spaces (cells falling apart centrally) with large, plump, epithelioid cells. The differential includes metastatic renal cell carcinoma (RCC) and alveolar rhabdomyosarcoma (ARMS). Key features include nuclear notches ("apple bite cells," 14:39) and positivity for TFE3 (molecularly defined by the X-17 translocation). ASPS is notorious for vascular invasion (18:17) and metastasis to unusual sites (e.g., brain).
21:11 Case 3: Spindle Cell Hemangioma (Benign): Characterized by a biphasic pattern combining large cavernous vascular spaces and cellular areas of spindle/epithelioid endothelial cells. A diagnostic feature is the presence of small vacuoles that resemble "mini fat cells" (24:45). It is benign, often multifocal, and defined by IDH1 mutations.
29:37 Cases 4 & 5: Myofibroma: A benign tumor characterized by a biphasic pattern of hypocellular "myoid nodules" (often blue/pseudochondroid/mixoid) and highly cellular spindle areas containing staghorn-like hemangiopericytic vessels (31:33). It is related to myopericytoma and typically benign in children and adults.
37:16 Case 6: Fibrous Hamartoma of Infancy (FHI): A benign lesion of infants/young children defined by a triad of (1) mature fat, (2) organized fascicles of spindle fibroblasts with wavy collagen ("ramen noodle sign," 39:15), and (3) round/blue nodules of immature mesenchyme (the key diagnostic feature, 39:53). This pattern helps distinguish it from DFSP and desmoid fibromatosis.
44:57 Case 7: Desmoplastic Fibroblastoma (Collagenous Fibroma): A benign, hypocellular, circumscribed tumor often found in distal extremities. It features highly collagenous/mixoid stroma containing sparse, stellate fibroblasts described as "splattered cells" (47:25). It requires differentiation from low-grade fibromyxoid sarcoma (MUC4 stain useful) and must not be confused with sclerotic fibroma (plywood fibroma).
58:45 Case 8: Cutaneous Myxoma (Superficial Angiomyxoma): A multi-lobulated, hypocellular mixoid tumor in the skin/subcutis featuring prominent branching vessels, bland "potbelly bean-shaped" cells (59:56), and often scattered neutrophils (1:00:10). It may entrap adnexal structures (1:00:30) and is associated with Carney complex. Local recurrence is common.
1:05:00 Case 9: Synovial Sarcoma: A malignant translocation sarcoma (uniform, monotonous spindle cells). The biphasic form is classic, exhibiting glandular/epithelioid structures within the cellular spindle stroma (1:06:21).
1:09:43 Cases 10 & 11: Chordoma: A malignant tumor of the axial skeleton (clivus or sacrococcygeal, 1:09:51) showing a multi-nodular, mixoid pattern. Cells are epithelioid/clear, arranged in chains or cords ("bicycle chain" analogy, 1:16:14), including pathognomonic physiliferous cells (bubbly, vacuolated cells, 1:12:38). Diagnosis is confirmed by co-expression of Keratin and S100, and nuclear positivity for Brachyury (1:14:17).
1:18:33 Case 12: Extraskeletal Myxoid Chondrosarcoma (EMC): A rare soft tissue-only sarcoma (not related to bone chondrosarcoma). Morphology includes round blue cells in cords and chains within a mixoid matrix. It is a translocation sarcoma (EWS-R1/NR4A3 fusion).
1:23:11 Case 13: Chondroma of Soft Parts: A true benign cartilage tumor arising entirely in soft tissue (often hands/feet). The histology can be cellular and bizarre, but should not be mistaken for chondrosarcoma unless there is aggressive radiological evidence of bone destruction (1:24:16).
1:28:40 Case 14: Myxoid Liposarcoma (MLS): A high-risk translocation sarcoma (FUS-DDIT3) typically found deep in the thigh. Characterized by bland, spaced-out cells in a mixoid matrix (respect social distancing, 1:29:10) and delicate, thin-walled branching vessels (1:33:01). High Grade/Round Cell component is defined by >5% hypercellularity (1:36:17) and worsens the prognosis. MLS metastasizes to unusual sites, including bone (spine) and other soft tissues (1:42:13).
2:01:06 Case 15: Differentiated Liposarcoma (DDLPS): Defined by the transition from well-differentiated liposarcoma (WDLPS—mature fat, fibrous bands, atypical cells, 2:02:03) to a high-grade sarcoma (like UPS, 2:02:37). DDLPS frequently expresses heterologous differentiation (e.g., osteosarcoma, 2:04:07). Molecularly defined by MDM2/CDK4 amplification (2:08:08).
2:10:00 Case 16: Chondroid Lipoma: A very rare benign entity featuring lipoblast-like vacuolated cells, often in cords, set against a vaguely chondromixoid background. This case highlights that lipoblasts alone do not equal liposarcoma. This lesion is often associated with hemorrhage, fibrin, and prominent hyalinization (2:14:24).