Case 489 -- A 20-month-old girl with a left calf mass

Contributed by Julia Kofler, MD and Sarangarajan Ranganathan, MD
Published on line in December 2006


The patient is a 20 month old girl with an unremarkable past medical history. She presented with a 1 month history of a non-tender mass in her left calf.

An MRI demonstrated a soft tissue mass within the muscles of the left leg located posterior to tibia and fibula (Figure 1; T1 weighted image with contrast).


A needle biopsy was performed and showed a hypercellular mass that was infiltrating into skeletal muscle (Figure 2). The tumor cells had small round to irregularly shaped nuclei with a minimal amount of pale eosinophilic to amphophilic cytoplasm and were arranged in patternless sheets. Scattered mitotic figures were present (Figure 3). Desmin (Figure 4) and HHF-35 (Figure 5) immunohistochemical stains highlighted surrounding and entrapped muscle fibers. Scattered tumor cells were also positive, but the majority was negative for desmin and HHF-35. Most tumor cells showed strong nuclear staining for myogenin (Figure 6), while the skeletal muscle was negative. Molecular studies revealed a PAX7/FKHR fusion transcript by RT-PCR.


Following the biopsy, the patient underwent several cycles of chemotherapy. Restaging by MRI (Figure 7) and PET scan revealed a decrease in size of the tumor mass, but there was still viable residual tumor present. Approximately 3 months after the initial diagnosis, the patient underwent resection of her mass.


The resection specimen consisted of a 5.1 x 3.3 x 2.5 cm well circumscribed mass that was almost completely surrounded by skeletal muscle. On the deep margin, the tumor was only covered by a thin fibrous layer representing a fascial plane. On cut surface, the tumor was pink-tan with rare cystic spaces (Figure 8).


Microscopic examination revealed a rather well circumscribed tumor that was in some areas surrounded by a fibrous pseudocapsule, but in other areas infiltrated into the adjacent skeletal muscle. The mass was transected by rare irregular fibrovascular septae. The tumor cells were predominantly arranged in patternless sheets and showed a highly variable histologic appearance as seen on low power (Figure 9). Some areas were hypercellular and exhibited similar histologic features as the biopsy specimen with small, crowded cells with round to irregular nuclei and minimal pale eosinophilic to amphophilic cytoplasm (Figure 10). In other areas, variable numbers of tumor cells contained abundant brightly eosinophilic cytoplasm (Figure 11). These cells had a round or elongated shape, and bi-and multinucleated forms were present (Figures 12 and 13). Rare cells exhibited a more fibrillary cytoplasm (Figure 14). Other areas of the mass were more hypocellular with loosely arranged stellate tumor cells (Figure 15). Rare cells contained larger hyperchromatic nuclei, but anaplasia was not a prominent feature. Focally, the tumor exhibited an alveolar growth pattern (Figure 16). Myogenin immunohistochemical stain showed variable nuclear staining of tumor cells (Figures 17, 18 and 19).


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