Dr. Jared Gardner and resident Dr. Iman Sarami review ten unknown soft tissue pathology cases, focusing on challenging diagnoses that require integrating histology, immunohistochemistry (IHC), and molecular findings. The session covers common sarcomas (Synovial Sarcoma, Myxoid Liposarcoma) defined by specific translocations, internal organ-based tumors (Gastric Glomus Tumor, Paraganglioma), and several reactive or pseudo-malignant entities (Nodular Fasciitis, Proliferative Myositis, Ischemic Fasciitis) which mimic malignancy but are benign. The discussion emphasizes microscopic pitfalls, such as the bland appearance of translocation sarcomas and the alarming cellular atypia seen in reactive processes, as well as the importance of recognizing specific growth patterns (Zellballen, tissue culture, honeycomb fat entrapment, delicate branching vasculature).
Soft Tissue Unknowns Set #1: Key Diagnoses and Features
Details: 25 y/o male with a lower thigh mass. Features include monotonous, plump spindle cells, high cellularity, cystic formation, and dilated, staghorn-like vessels (mimicking Solitary Fibrous Tumor, SFT).
Details: 30 y/o female with a stomach nodule. Characterized by round/cuboidal cells surrounding dilated vessels, often showing a nuclear halo. These cells are pericytes, not true endothelial cells.
Key Takeaways: Glomus tumors are often found in the nail bed, but internal sites (like the stomach) are known. They stain positive for smooth muscle actin (SMA) and are negative for endothelial markers (ERG, CD31). Unlike peripheral glomus tumors, size and depth in the stomach do not strongly predict poor behavior.
Details: 25 y/o female with a 10 cm deep soft tissue mass in the thigh. Histology shows hypocellular, bland spindle cells in a myxoid matrix with a crucial feature: delicate, branching "chicken wire" vessels (where RBCs pass single-file).
Key Takeaways: This translocation sarcoma (FUS-DDIT3) often lacks features of malignancy (pleomorphism, necrosis), making it easy to miss and mistake for a benign myxoma. Location (deep thigh muscle) and the specific vascular pattern are key diagnostic clues.
19:24 Case 4: Deep Angiomyxoma (Aggressive Angiomyxoma)
Details: 35 y/o female with a large (15β20 cm) vulvar/pelvic mass. The lesion shows an edematous background, vessels of varied wall thickness, and subtle, thin smooth muscle cells branching away from the vessels.
Key Takeaways: This lesion is locally aggressive due to its infiltrative nature but is not classically malignant. It is typically large and deep, which distinguishes it from small superficial stromal polyps of the genital tract.
Details: 55 y/o female with a carotid body lesion. Microscopic pattern is characterized by Zellballen (tight nests of round neuroendocrine cells) separated by rich vascular channels. Nuclei show characteristic "salt and pepper" chromatin and often non-malignant random pleomorphism.
Key Takeaways: Diagnosis is confirmed by neuroendocrine markers (Synaptophysin, Chromogranin). S100 highlights the supportive sustentacular cells surrounding the nests. Malignancy is defined only by the presence of metastasis.
Details: 25 y/o female with a rapidly growing, small (3 cm) forearm lesion. Shows a feathery, "tissue culture" appearance, composed of plump, stellate myofibroblasts in a loose mixoid background, often with microcysts and abundant mitoses (which are not atypical).
Key Takeaways: Despite its aggressive clinical and cellular appearance, it is benign and often self-regressing. It is linked to the MYH9-USP6 translocation. Do not overcall as sarcoma; size is an important differentiator (usually <3 cm).
Details: 60 y/o male with a rapidly growing shoulder lesion. A form of fasciitis that infiltrates and splays apart individual skeletal muscle fibers ("checkerboard pattern"). Contains large, alarming "ganglion-like" cells (rhabdoid appearance) within a fasciitis-like background.
Key Takeaways: This is a key diagnostic pitfall; the splaying of muscle (rather than pushing it aside, as sarcomas usually do) is a crucial low-power clue indicating a benign, reactive process.
43:53 Case 8: Ischemic Fasciitis (Atypical Decubital Fibroplasia)
Details: 80 y/o man with a painful hip mass (over a bony prominence). Shows a distinct zonal pattern: a central area of ischemic fibrinoid necrosis, surrounded by a reactive zone of granulation tissue and fasciitis-like proliferation (including ganglion-like cells), transitioning to scar tissue.
Key Takeaways: Associated with chronic pressure/ischemia (like a decubitus ulcer). The combination of necrosis, fat infiltration, and highly reactive cells can easily lead to a sarcoma misdiagnosis (e.g., Mixofibrosarcoma or Liposarcoma).
Details: 10 y/o female with multiple soft tissue nodules, showing multiple elongated nodules connecting together (grossly a "bag of worms"). Microscopic features include thin, wispy collagen ("shredded carrot") and S-shaped/buckled schwann cell nuclei.
Key Takeaways: Pathognomonic for Neurofibromatosis Type 1 (NF1). Pathologists must be certain of the diagnosis due to the clinical implications.
Details: 15 y/o male with a skin nodule, diffusely growing into the dermis/subcutis, encasing adnexal coils and entrapping fat in a "honeycomb" pattern. Bland cells.
Key Takeaways: The honeycomb fat entrapment mimics DFSP. IHC is essential: Diffuse Neurofibroma is S100 positive (Schwann cells), while DFSP is S100 negative. May also contain Wagner-Meissner bodies (tactoid bodies).