Residency Program - Case of the Month

Answer:

Alveolar rhabdomyosarcoma (Stage IV)

Discussion:

Rhabdomyosarcomas (RMS) are the most common soft tissue sarcoma occurring in children younger than 15 years and are divided into subtypes embryonal, alveolar and pleomorphic. The embryonal rhabdomyosarcomas (E-RMS) occur most frequently in the head and neck region followed by the genitourinary system and often affect younger children from bi¬rth and rarely occurring after age 10¹. Alveolar rhabdomyosarcoma (ARMS) more often present in the extremities of older children and young adults between the ages 10-25 years. Breast involvement of either subtype is a rare clinical finding. The alveolar type has a significantly worse progrnosis than the embryonal subtype; therefore identification is critical for therapy.

There are three histomorphologic patterns of ARMS: typical pattern, solid pattern and mixed embryonal-alveolar pattern. All patterns show the same round cell cytomorphology reminiscent of lymphoma, but with primitive myoblastic differentiation.

The diagnosis of RMS is made by a combination of histomorphology, immunohistochemistry and FISH studies. Tumor cells in ARMS classically are positive for both mygenin and myo-D1, which are both highly sensitive and specific for rhabdomyosarcoma¹. Other muscle markers such as desmin and muscle specific actin (HHF-35) may be used, they are less specific and are expressed in other cells of myogenic origin, including myoepithelial cells, myofibroblasts, cardiac and smooth muscle.

Our case presented a particular challenge due to the expression of CD56 and CD57. CD56, also called neural cell adhesion molecule (N-CAM) is a membrane glycoprotein expressed on neural and muscle tissues that is involved in homotypic adhesive interactions. This marker is classically used for detection of NK-cell tumors, neuroendocrine tumors and plasma cell dyscrasias. CD57 is a glycoprotein that also functions in cell adhesion. The utility of CD57 is similar to CD56, however it has a less specific staining pattern, being expressed in other small round blue cell tumors and carcinomas. CD56 and CD57 have been studied using flow cytometry to evaluate for rhabdomyosarcoma and neuroblastoma involvement of bone marrow and the cases of RMS show CD45-/CD56+/CD57+/-². Although RMS often expresses CD56, it may be a potential diagnostic pitfall and does not show consistent reactivity. 

WT-1 tumor supprossor located at 11p13 and is a nuclear marker for Wilm’s tumor.  Recent evidence has supported the diagnostic utility of WT-1 in differentiating peripheral neuroectodermal tumor (PNET), RMS and Wilm’s tumor based on the immunohistochemical staining pattern³. The research by Carpentieri et al showed that in ARMS, there is specific cytoplasmic WT-1 staining pattern. This is in contrast to the required nuclear stain for Wilm’s tumor and the absence of staining in PNET. Our case nicely illustrates the classic cytoplasmic immunohistochemical staining pattern in ARMS. 

Alveolar rhabdomyosarcomas demonstrate a specific translocation at chromosome 13 (13q14) partnered with two exclusive PAX/FKHR conditions: PAX3/FKHR fusion (present in 60% of cases) and PAX7/FKHR fusion (present in 20%). The fusion protein is not present in 20% of cases⁴. The two genes are members of the paired box transcription factor family (PAX) and become paired with FKHR, a member of the fork head transcription factor family. This generates PAX3-FKHR and PAX7-FKHR chimeric genes that are expressed as chimeric transcripts that encode chimeric proteins, which function to activate transcription from PAX-binding⁵.

This cases demonstrates multiple challenging areas. The differential diagnosis must include metastatic lymphoma based on the initial CBC findings and the cytomorphology of the tumor cells. This in combination with the challenging immunohistochemical pattern (CD45-/CD56+) and cytoplasmic WT-1 staining pattern were used to come to properly identify the tumor as alveolar rhabdomyosaracoma. FISH studies are pertinent in this case and confirm the diagnosis. 

References:

1. Fletcher C.D.M. UKK, Mertens F. World Health Organization Classification of Tumours. Pathology and Genetics of Tumours of Soft Tissue and Bone. Lyon: IARC Press; 2002.
2. Bozzi F, Collini P, Aiello A, et al. Flow cytometric phenotype of rhabdomyosarcoma bone marrow metastatic cells and its implication in differential diagnosis with neuroblastoma. Anticancer research. May-Jun 2008;28(3A):1565-1569.
3. Carpentieri DF, Nichols K, Chou PM, Matthews M, Pawel B, Huff D. The expression of WT1 in the differentiation of rhabdomyosarcoma from other pediatric small round blue cell tumors. Modern pathology : an official journal of the United States and Canadian Academy of Pathology, Inc. Oct 2002;15(10):1080-1086.
4. Parham DM, Qualman SJ, Teot L, et al. Correlation between histology and PAX/FKHR fusion status in alveolar rhabdomyosarcoma: a report from the Children's Oncology Group. The American journal of surgical pathology. Jun 2007;31(6):895-901.
5. Barr FG. Gene fusions involving PAX and FOX family members in alveolar rhabdomyosarcoma. Oncogene. Sep 10 2001;20(40):5736-5746.