This video, presented by Jared Gardner and Edie Fulton, provides a detailed microscopic and clinical overview of the Pleomorphic Hyalinizing Angiectatic Tumor (PHAT), a rare soft tissue tumor categorized as intermediate malignancy. PHAT is characterized by three main features: striking angiectatic (dilated) vessels with hyalinized/fibrinoid walls, pleomorphic spindle cells, and very low mitotic activity. The video heavily focuses on the critical differential diagnoses: distinguishing PHAT from the benign schwannoma (PHAT is S100 negative) and from high-grade pleomorphic sarcoma (PHAT lacks significant mitotic activity). PHAT, which typically occurs in the lower leg, has a high local recurrence rate but an excellent prognosis compared to sarcoma. The video also discusses the relationship between PHAT and Hemosiderotic Fibrolipomatous Tumor (HFLLT), suggesting a precursor relationship supported by shared molecular rearrangements (TGFBR3 and MGEA5).
Pleomorphic Hyalinizing Angiectatic Tumor (PHAT): Pathology and Differential Diagnosis
0:00:13 Definition and Behavior: PHAT is an unusual and rare tumor of intermediate malignancy. Most cases are indolent, but local recurrence is common (30-50%). Metastasis is rare, and the prognosis is good.
0:01:22 Characteristic Vessels (Angiectatic and Hyalinizing): The most important feature is the presence of big, dilated, angiectatic vessels. Their walls are expanded, homogeneous, and pink (hyalinized) due to fibrin or collagen deposition.
0:02:15 Pleomorphism: The tumor exhibits pleomorphic, hyperchromatic spindle cells. Pleomorphism can be judged by comparing nuclear size; if the largest tumor nucleus is at least three times bigger than a neighboring smaller nucleus, it is considered pleomorphic.
0:03:08 Distinction from Schwannoma: PHAT is easily confused with schwannoma due to similar vessel appearance. However, PHAT is S100 negative, whereas schwannoma is positive, and PHAT generally lacks nuclear palisading or Verocay bodies.
0:03:17 Immunohistochemistry (IHC): PHAT typically stains CD34 positive but is S100 negative.
0:04:03 Distinction from High-Grade Sarcoma: Due to pleomorphism, PHAT can be confused with high-grade sarcomas like Undifferentiated Pleomorphic Sarcoma (UPS). The key differentiator is the mitotic rate; PHAT usually has less than one mitosis per 50 high power fields, despite the pleomorphism. This distinction is crucial for prognosis.
0:06:54 Location and Growth: PHAT most commonly occurs in the lower leg (foot or ankle), usually in the subcutis. Its infiltrative growth pattern contributes to the high local recurrence rate.
0:08:18 Relationship to HFLLT: The tumor's infiltrative periphery ("Early PHAT") is microscopically identical to Hemosiderotic Fibrolipomatous Tumor (HFLLT). This pattern is defined by bland spindle cells, hemosiderin deposition, and infiltration of adjacent adipose tissue, suggesting HFLLT may be a precursor.
0:09:50 Evolving Molecular Data: Recent studies suggest that PHAT and HFLLT share gene rearrangements in TGFBR3 and MGEA5, indicating a potential molecular relationship between these entities.
0:14:09 Cytological Detail: Tumor cells in PHAT frequently exhibit nuclear pseudo-inclusions (vacuoles of cytoplasm inside the nucleus).
0:15:01 Key Takeaway: The main take-home point is to utilize S100 negativity and very low mitotic rate to differentiate PHAT from the benign schwannoma and the highly aggressive undifferentiated pleomorphic sarcoma.