— SPECIALTY CONFERENCE —

Clinical History

A 21 year old college student presented with jaw pain, fullness under the left angle of the jaw, diminished hearing in the left ear, trismus and 15 pound weight loss. This initially was felt to represent unilateral otitis media, but when myringotomy was unsuccessful an MRI scan was obtained, which showed a large parapharyngeal mass displacing the left tonsil medially and extending to the skull base. Transoral fine needle aspiration was performed. The images are from Diff-Quik and Pap stained direct smears and H&E stained cell block from the aspiration.

Pathological/Microscopic Findings and any Immunohistochemical or Other Studies:

The Diff-Quik smears are cellular and are composed of epitheliod to short spindled cells with inconspicuous cytoplasm and large, open nuclei. Loose clustering of the cells is seen. Focally, cytoplasmic vacuolization is identified. On the Papanicolaou stained slides the cells are more overtly spindled shaped. The nuclear detail is more apparent, and the chromatin is largely smooth with several small nucleoli or chromocenters seen in most of the cells. Karyorrhexsis and apoptosis are abundant. The H&E stained cell block showed both epitheloid and spindled cell groups with numerous single cells in the background. Pale, eosinophilic cytoplasm and acellular matrix material is seen in the background. A few of the nuclei have pale intranuclear inclusions.

Immunostains performed:
Desmin: positive
Smooth Muscle actin: positive
Actin: positive
Myogenin: negative
CD99: weakly positive
Pankeratin: negative
Mucin: negative
P63: negative
CD45: negative
S100: negative
HMB45: negative
NSE: negative
Chromogranin: negative
Synaptophysin: negative
CD34: negative

Differential Diagnoses:

Clinical differential: 1. Lymphoma 2. Sarcoma (especially rhabdomyosarcoma) 3. Malignant tumor of the deep lobe of the parotid (especially mucoepidermoid carcinoma)

Pathologic differential: 1. Rhabdomyosarcoma 2. Paraganglioma 3. Malignant peripheral nerve sheath tumor 4. High grade mucoepidermoid carcinoma

Final Diagnosis:

Final cytologic diagnosis: Rhabdomyosarcoma

Final histologic diagnosis: Sclerosing rhabdomyosarcoma

Case Discussion:

The #1 lesson I learned from my mentor is to always be mindful of the clinical findings when evaluating the pathologic material. Nowhere is this more important than in cytology, since the material available for morphology is usually fragmented and limited, and the amount of cells available for special studies may be small. Thus prioritizing immunostains (and molecular/cytogenetic studies) on the basis of pathologic and clinical probabilities of the various diagnoses in the differential is essential.

This case was clinically almost certainly malignant, based on the rapid onset of symptoms, size of the mass and the radiologic findings. The two leading clinical diagnoses were lymphoma and sarcoma (especially rhabdomyosarcoma), given the rate of growth of this tumor, the young age of the patient and the location in the head and neck. The cytologic features suggested either an epitheloid or spindled cell neoplasm, most likely sarcoma, but carcinoma was not outside the realm of possibility. The aspirate did not have the features of lymphoma, as the cells were frequently spindled and cohesive and not lymphoid appearing, and lymphoglandular bodies were present in the background. Among sarcomas, rhabdomyosarcoma and malignant peripheral nerve sheath tumor were the best contenders. Paraganglioma was also a consideration on cytologic grounds, due to the nested character of the cells and the mixed appearance of epitheloid and spindled cells. Among carcinomas arising from the deep lobe of the parotid, mucoepidermoid carcinoma was considered, as these can be poorly differentiated squamoid carcinomas without overt glandular differentiation. Some of the groups on the cell block had the appearance of a poorly differentiated squamous cell carcinoma.

The immunostains enumerated above were performed on the cell block (luckily the cell block was of generous size permitting this number of immunostains), and results clearly pointed to the diagnosis of rhabdomyosarcoma. Desmin, smooth muscle actin and actin are positive supporting this impression, with negative epithelial markers, mucin, S100 and neuroendocrine markers, helping rule out the other entities in the differential diagnosis. For completeness sake, CD45 and melanoma markers were also performed and were negative.

One curiosity of the case was the eosinophilic material present on the cell block, not apparent on the direct smears. Possibilities included necrotic debris, mucus, serum, collagen or matrix material produced by the tumor. The material appeared without cellular debris, so did not seem to be necrosis. It was negative with mucicarmine stain, ruling out mucus; and more intensely stained than usual for serum. That left the possibility that this might by collagen or some type of matrix material intrinsic to the tumor.

Material from the FNA was submitted to cytogenetics at the time of the procedure, but took two weeks for final diagnosis. The cytogenetics culture grew cells with a complex abnormal karyotype. There were three related hypertriploid clones with relative gains of chromosomes 7,15,18,19,21,22 and relative loses of chromosomes 3 and 9. The FOXO1 (13q14) rearrangement was not seen, but there are extra copies of this gene, consistent with the triploid karyotype. The cytogenetics results favored the interpretation of embryonal rhabdomyosarcoma. The myogenin immunohistochemical results also favored embryonal over alveolar rhabdomyosarcoma. Myogenin is usually much more strongly expressed in alveolar rhabdomyosarcoma than embryonal rhabdomyosarcoma.

This patient’s oncologist wished to enroll him on a COG clinical trial a few days after the FNA, but needed to know the subtype of rhabdomyosarcoma to select the appropriate trial, and pressed for a core biopsy to get this information. The rapid growth rate of the mass was so worrisome that the oncologist was unwilling to wait for final cytogenetics and wanted to commence chemotherapy. Core biopsy showed sclerosing rhabdomyosarcoma, which is an unusual subtype of rhabdomyosarcoma dominated by eosinophilic heavily collagenized matrix. Without immunohistochemistry, it may be easily mistaken for hyalinizing chondrosarcoma or osteosarcoma. Cases are usually in the extremities or in the head and neck. Originally this entity was described in adults, but has now been described in all ages, including children and adolescents. Sclerosing rhabdomyosarcoma usually has cytogenetic abnormalities most associated with embryonal rhabdomyosarcoma, as this case was eventually shown to have. By the time the clinical trial reviewed the pathology the cytogenetics material was available, and the patient was managed on the ARST 0531 COG trial for intermediate risk rhabdomyosarcoma, as there was no evidence of metastatic disease. The patient has done well and is about two thirds completed with the protocol.

Review of the Literature/Treatment Options:

Rhabdomyosarcoma is the most common soft tissue sarcoma of children and adolescents, affecting males slightly more than females (3:2 ratio). There are three main subtypes, embryonal, alveolar and pleomorphic, though other rare variants occur, include the sclerosing rhabdomyosarcoma represented by this case. Pleomorphic rhabdomyosarcoma is largely a disease of adults, with embryonal and alveolar subtypes involving children and teens. Embryonal subtype is more common in children, and alveolar more in adolescents. Embryonal rhabdomyosarcoma is usually detected in the head and neck or urogenital tract. When embryonal carcinoma involves a hollow viscus such as the nose, bladder or urethra, the tumor has room to expand and makes an exuberant, “bunch of grapes”-like mass which is the botryoid pattern of embryonal rhabdomyosarcoma. Alveolar rhabdomyosarcoma is most common in the trunk and extremities, but can occur in the head and neck. Pleomorphic rhabdomyosarcoma is usually found in the extremities in adults.

The diagnosis of rhabdomyosarcoma can be made by fine needle aspiration cytology in conjunction with other studies; it is not purely a morphologic diagnosis. Embryonal and alveolar rhabdomyosarcoma sampling both yield a “small-, round-, blue-cell tumor”, and other tumors with this morphology are often in the differential (often neuroblastoma, nephroblastoma and lymphoma are considerations). Subtle differences have been reported in the cytologic features of the embryonal and alveolar subtypes, but most pathologists would not find these differences dependable enough to distinguish between these types. Strap cells are more frequently found, chromatin is usually finer and cytoplasm is less abundant in the embryonal subtype than in the alveolar subtype. But given that the distinction between the subtypes is a critical therapeutic and prognostic decision, additional studies are needed. Immunostains to establish the skeletal muscle differentiation of the cells (desmin, actins, myo D1) are essential. As discussed above, myogenin is variably expressed between the subtypes, and may be a helpful clue. Following the trend of many other tumors, the distinction is now principally a molecular one, and identification of the characteristic cytogenetic PAX3/FOXO1 gene fusion is a hallmark of the diagnosis of alveolar rhabdomyosarcoma and the driver of the poorer prognosis of this subtype. Tumors with alveolar architecture but lacking the gene fusion behave more similarly to embryonal rhabdomyosarcoma.

Conclusion(s):

1. Reviewing cytology slides in the context of the clinical impression can direct the workup of a difficult case, suggest entities to be considered in the differential diagnosis and prompt the pathologist to collect material for adjuvant studies.

2. The principle subtypes of rhabdomyosarcoma are embryonal, alveolar and pleomorphic, with distinct epidemiologic, histologic, molecular and therapeutic differences between them.

3. Sclerosing rhabdomyosarcoma is an unusual variant of rhabdomyosarcoma in which the neoplastic tumor cells are set in a densely hyalinized matrix. While not one of the common subtypes, it is most closely related to embryonal rhabdomyosarcoma.

References

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