This presentation by Dr. Jared Gardner provides an in-depth review of eleven challenging and educational cases in bone and soft tissue pathology, focusing on critical differential diagnoses and diagnostic pitfalls. The lecture covers a broad spectrum of entities, ranging from reactive processes like Myositis Ossificans (demonstrating its maturation path over nine months) and benign lesions like Non-Ossifying Fibroma and Plantar Fibromatosis, to complex malignant tumors. Key malignant cases discussed include Monophasic Synovial Sarcoma and Malignant Peripheral Nerve Sheath Tumor (MPNST), emphasizing features like the "herringbone" pattern and the use of modern ancillary markers (e.g., H3K27me3 loss in MPNST). Dr. Gardner also highlights the high-risk diagnosis of Epithelioid Sarcoma (characterized by INI1 loss) and a rare variant of Rhabdomyosarcoma. Emphasis is placed on integrating morphology, specific immunohistochemical profiles, molecular data, and crucial clinical/radiographic correlation for accurate diagnosis, particularly in rare or high-grade sarcomas.
A Structured Approach to Challenging Bone and Soft Tissue Pathology Cases
0:49 Myositis Ossificans (MO): A rapidly growing, painful, deep soft tissue mass (Case 1). Characterized by edema in surrounding muscle and a zonal arrangement: loose fasciitis-like center transitioning to woven bone laid down at the periphery.
Takeaway: The zonal growth pattern (benign bone formation on the outside, cellular activity in the center) and rapid growth followed by stabilization are key. Follow-up showed maturation into lamellar bone and resolution of the central cellular area (12:56).
17:37 Enchondroma vs. Chondrosarcoma in Small Bones (Cases 2 & 3): Two cartilage tumors are compared. A cellular, but radiographically contained lesion in a finger (Case 2) is a benign Enchondroma (Chondrosarcoma is rare in small bones).
22:52 Malignancy Criterion: A lesion in the foot destroying the bone, invading soft tissue, and displaying permeative growth (Case 3) is diagnostic of Chondrosarcoma (Grade 2). Malignant growth pattern is paramount over cellularity in small bones.
29:19 Plantar Fibromatosis (Lederhose’s Disease) (Case 4): A slowly growing plantar foot mass. Shows broad, sweeping fascicles of bland spindle cells separated by fine collagen. Collagen shows an extreme wavy pattern ("ramen noodle sign") (30:00).
Takeaway: Cellularity and mitotic activity are acceptable in Plantar Fibromatosis. Cross-sections of fascicles appear as sheets of round cells (34:39), a known pitfall. Can be confirmed by nuclear Beta-Catenin (often difficult to interpret) (37:59).
40:23 Monophasic Synovial Sarcoma (Case 5): A deep thigh mass in an elderly adult. Characterized by high cellularity, nuclei packed closely together, and the classic "herringbone" or "fishbone" fascicular growth pattern (41:47).
Takeaway: Monophasic is more common than biphasic; diagnosis relies on morphology (cellular fascicles, uniform nuclei) and ancillary studies (Keratin, EMA, TLE1, or SS18 fusion/IHC for fusion product) (44:00).
48:43 Malignant Peripheral Nerve Sheath Tumor (MPNST) (Case 6): An arm mass arising from a nerve/plexiform neurofibroma (NF1 patient). Shares the herringbone pattern but exhibits more pleomorphism, mixoid marbling of cellular areas, and specific epithelioid change clustering around vessels (51:09).
Takeaway: MPNST lacks specific unique features; diagnosis requires a constellation of findings plus one of three criteria: arising from a nerve/neurofibroma, or occurring in an NF1 patient. Loss of H3K27me3 is a key modern ancillary tool (53:11).
58:43 Epithelioid Sarcoma (Case 7): A hand mass with sheets of large epithelioid cells, pale cytoplasm, and pleomorphic nuclei. IHC shows strong Pan-keratin/EMA positivity.
1:00:26 Diagnostic Marker: Diagnosis is confirmed by the loss of nuclear expression of INI1 (SMARCB1) in the tumor cells, contrasted with positive background control cells.
1:04:56 Pigmented Villonodular Synovitis (PVNS) (Case 8): Involves the knee joint, presenting with thickened, shaggy, dark brown/black synovium. Microscopically, displays villous architecture, abundant hemosiderin, foamy cells, and multinucleated giant cells.
Takeaway: Mitotic activity is common and does not indicate malignancy. Key features include the "ladybug cells" (histiocytes with hemosiderin halos) (1:09:47) and the tumor’s ability to be PET-scan hot, which can lead to misdiagnosis.
1:12:59 Non-Ossifying Fibroma (NOF) (Case 9): A lytic bone lesion (cortical defect) in a young patient. Histology shows bland spindle cells in a storiform pattern, abundant hemosiderin, and foamy histiocytes (1:15:09). Radiographically characterized by eccentric location and sclerotic, scalloped borders.
1:16:57 Gaucher's Disease (Case 10): A large bone lesion replaced by sheets of histiocytes. The cytoplasm has a unique fibrillary/striated appearance ("wrinkled tissue paper") due to lysosomal storage of glucocerebroside.
1:20:33 Sclerosing Rhabdomyosarcoma (Case 11): Thigh mass with abundant dense, sclerotic collagen, hyperchromatic spindle cells arranged in cords, and microalveolar/pseudovascular structures. IHC markers include Desmin and MyoD1 (1:23:08).
Takeaway: This rare variant can mimic leiomyosarcoma or vascular tumors. MyoD1 is the preferred myogenic marker, as Myogenin is often only focally positive (1:26:39). Prognosis varies by age and molecular drivers (fusion in infants, MyoD1 mutation in adults).
1:29:12 Q&A on Fibrosarcoma: Dr. Gardner stresses that adult-type Fibrosarcoma is vanishingly rare; most such diagnoses are actually other entities (Synovial Sarcoma, MPNST, DFSP). The herringbone pattern is the classic sign of fibrosarcoma morphology, but it is not specific (1:30:01).
1:31:39 Q&A on MPNST: MPNST diagnosis is most comfortable when a precursor (Neurofibroma) or NF1 history is known, but modern markers like H3K27me3 loss can aid diagnosis in isolated cases (1:32:01).