Alveolar rhabdomyosarcoma of the head and neck region in older adults: genetic characterization and a review of the literature

The Small Round Blue Cell Tumors of Sinonasal Tract: Pathologists Grey Zone

Background

One of the most challenging diagnostic categories in the sinonasal tract includes small-blue-round-cell tumors. These are malignant tumors which show many overlapping histomorphology and immunohistochemistry (IHC) findings. Limited, small biopsy of these not completely excisable tumors adds to the diagnostic confusion.

Materials and Methods

A cross-sectional study was done for 2 years (January 2018-December 2020) in a tertiary care institute, which included 70 cases of tumors of which 49 cases were malignant. All paraffin-embedded blocks were subjected to hematoxylin and eosin stain and IHC followed by molecular study wherever needed.

Results

Of the total cases, small-blue-round-cell tumor constituted the major category comprising 20 rare and interesting cases which included sinonasal undifferentiated carcinoma (4 cases), malignant lymphoma (2 cases of diffuse large B-cell lymphoma and 2 cases of extranodal natural killer/T-cell lymphoma), rhabdomyosarcoma (2 cases), olfactory neuroblastoma (2 cases), malignant melanoma (2 cases), plasmacytoma (2 cases), atypical Ewing's sarcoma (EWS) (1 case), EWS (1 case), nuclear protein in testis (NUT) carcinoma (1 case), and small-cell neuroendocrine carcinoma (1 case).

Conclusion

Tumors of the sinonasal tract are very diverse, more so in small-round-cell tumor which present with a undifferentiated morphology. Thus, accurate diagnosis needs clinicoradiological parameters and special ancillary techniques such as IHC and molecular study in addition to histopathology for early diagnosis and therapy to prevent significant morbidity and mortality caused in these tumors.

Small biopsies in the head and neck: Bone and soft tissue

Bone and soft tissue lesions in the head and neck encompass not only a broad morphologic spectrum but also significant inherent clinicopathologic overlap. Epidemiology, radiology, and location - similar to the diagnostic assessment in other sites - are especially important considerations in the context of an established mesenchymal proliferation. Herein, the approach towards diagnosis is stratified by morphology (spindle, sarcomatoid, epithelioid, round cell), cellular lineage (fibroblastic, nerve sheath, rhabdomyogenic), and tumor grade (benign, low- to high-grade malignant) as the basis of further immunohistochemical or molecular investigation.

Top 10 Histological Mimics of Neuroendocrine Carcinoma You Should Not Miss in the Head and Neck

BACKGROUND

The spectrum of neuroendocrine neoplasia (NEN) of the head and neck region is wide-ranging and diverse, including a variety of diagnoses stretching from benign and low-malignant tumor forms to highly proliferative, poor prognosis neuroendocrine carcinoma (NEC). Moreover, there are several non-neuroendocrine differential diagnoses to keep in mind as well, displaying various degree of morphological and/or immunohistochemical overlap with bona fide neuroendocrine lesions.

METHODS

Review.

RESULTS

While the growth patterns may vary, well-differentiated NEN usually display a stippled "salt and pepper" chromatin, a granular cytoplasm, and unequivocal expression of neuroendocrine markers such as chromogranin A and synaptophysin. However, these features are often less pronounced in NEC, which may cause diagnostic confusion-not the least since several non-NEC head and neck tumors may exhibit morphological similarities and focal neuroendocrine differentiation.

CONCLUSION

As patients with NEC may require specific adjuvant treatment and follow-up, knowledge regarding differential diagnoses and potential pitfalls is therefore clinically relevant. In this review, the top ten morphological and/or immunohistochemical mimics of NEC are detailed in terms of histology, immunohistochemistry, and molecular genetics.

Molecular profile of head and neck rhabdomyosarcomas: A systematic review and meta-analysis

OBJECTIVE

This systematic review aimed to identify the molecular alterations of head and neck rhabdomyosarcomas (HNRMS) and their prognostic values.

STUDY DESIGN

An electronic search was performed using PubMed, Embase, Scopus, and Web of Science with a designed search strategy. Inclusion criteria comprised cases of primary HNRMS with an established histopathological diagnosis and molecular analysis. Forty-nine studies were included and were appraised for methodological quality using the Joanna Briggs Institute Critical Appraisal tools. Five studies were selected for meta-analysis.

RESULTS

HNRMS predominantly affects pediatric patients (44.4%), and the parameningeal region (57.7%) is the most common location. The alveolar variant (43.2%) predominates over the embryonal and spindle cell/sclerosing types, followed by the epithelioid and pleomorphic variants. PAX-FOXO1 fusion was observed in 103 cases of alveolar RMS (79.8%). MYOD1 mutation was found in 39 cases of sclerosing/spindle cell RMS (53.4%). FUS/EWSR1-TFCP2 gene fusions were identified in 21 cases of RMS with epithelioid and spindle cell morphologies (95.5%). The 5-year overall survival rate of patients was 61.3%, and MYOD1 mutation correlated with significantly higher mortality.

CONCLUSION

The genotypic profile of histologic variants of HNRMS is widely variable, and MYOD1 mutation could be a potential prognostic factor, but more studies are required to establish this.

Primary Cutaneous Alveolar Rhabdomyosarcoma in an Elderly Adult: A Rare Potential Mimic of Merkel Cell Carcinoma

ABSTRACT

Rhabdomyosarcoma (RMS) rarely arises as a primary skin tumor. It is also very rare in older adults, especially the alveolar type. We report an 80-year-old White woman who presented with a painful, erythematous, raised lesion (2 × 3.5 cm) above the left knee that was fixed within the skin, yet mobile about underlying soft tissue. A punch biopsy showed monotonous malignant round blue cells involving the dermis. Immunostains showed diffuse expression of CD56, focal chromogranin, focal dot-like pancytokeratin, CK7, and neurofilament, but negative for synaptophysin, CK20, SOX-10, MUM-1, CD43, TTF-1, and CD99. A CK20-negative variant of Merkel cell carcinoma was initially favored, but given the unusual immunophenotype and the presence of cellular dyscohesion, desmin and myogenin stains were performed, both of which were diffusely positive. Molecular testing revealed rearrangement of PAX3 and FOXO1 loci, confirming the diagnosis of alveolar RMS. PET/CT showed a probable 1.9-cm left inguinal lymph node metastasis; no internal or deep soft tissue primary tumor mass was identified, supporting a true primary cutaneous origin. Alveolar RMS may express keratins and neuroendocrine markers, making it easy to confuse with Merkel cell carcinoma on those exceptionally rare instances, when it arises in the skin of older adults.

Clinicopathological study of hepatic mesenchymal hamartoma and undifferentiated embryonal sarcoma of the liver: a single center study from Iran

Background

Undifferentiated embryonal sarcoma of liver (UESL) and hepatic mesenchymal hamartoma (HMH) are two rare entities which mainly affect the pediatric population. The aim of this investigation was to provide a comprehensive overview of the clinicopathologic characteristics of the patients diagnosed with these two conditions in a tertiary referral center in Iran.

Methods

In this retrospective study patients diagnosed with UESL or HMH between 2012 and 2020 were studied. A comprehensive histopathologic evaluation of the cases along with immunohistochemistry evaluation using a panel of antibodies was conducted. Furthermore, clinical, paraclinical, and treatment data and follow up information was collected.

Results

A total of 16 patients (8 UESL, 8 HMH) were studied in this investigation. Patients with UESL had a significantly (p = 0.002) higher age at diagnosis compared with those with HMH. Histologically, UESL cases were characterized by anaplastic cells with eosinophilic cytoplasm and bizarre nuclei and frequent atypical mitosis and spindling in a myxoid stroma while disordered arrangement of hepatic parenchyma, bile ducts, and primitive mesenchyme was seen in HMH. Furthermore, small round cells and extramedullary hematopoiesis were seen in 2 UESL and 3 HMH cases, respectively. Concurrent HMH was also seen in two UESL cases. Immunohistochemistry panel showed positive staining for Vimentin, Glypican-3, Desmin, CD56, CD10, and BCL2 in UESL cases and immunoreactivity for Vimentin, HepPar 1, Glypican-3, SMA, CD56, BCL2, and CD34 in various components of HMH.

Conclusions

In this study, the clinicopathologic features of UESL and HMH cases are presented. We also evaluated the utility of an immunohistochemistry panel in the diagnosis of these two rare entities and suggested novel markers. Our study corroborated the findings of previous investigations and expanded the clinicopathologic features of these two rare entities with diagnostic and potential therapeutic implications.

Alveolar rhabdomyosarcoma of the mandible in an adult invading the parotid gland: A rare case report

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Rhabomyosarcoma (RMS) can be localized in the cervico-facial region and in an adult.

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Its diagnosis is histological with an immunohistochemical study.

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RMS in adults require neoadjuvant chemotherapy combined with locoregional treatment by tumor excision surgery followed by radiotherapy.

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RMS of the head and neck of adults have a poor prognosis.

Introduction

Rhabdomyosarcoma (RMS) is a malignant mesenchymal neoplasm uncommon in adults.

Presentation of case

We report the case of an alveolar rhabdomyosarcoma in a rare location and in an adult. It concerns a 46-year-old woman with a mandibular location invading the parotid gland. The Immunohistochemical analysis revealed intense positivity for myogenin and desmin, favoring the diagnosis of alveolar rhabdomyosarcoma. The patient had a right hemi-mandibulectomy with parotidectomy and received chemotherapy with radiotherapy. The evolution was good up to 2 years postoperatively then the patient relapsed with a recurrence of tumour rapidly progressing and metastases in the cervical spine.

Discussion

RMS is an aggressive but rare disease that is one of the most common malignant head and neck tumors in children. The predilection sites of adult rhabdomyosarcoma are the extremities. Current treatment includes a combination of ablative surgery, chemotherapy, and radiation therapy.

Conclusion

The prognosis of head and neck rhabdomyosarcomas in adults remains very poor given the often initially advanced stage of the disease and the high metastatic potential.

Mandibular embryonal rhabdomyosarcoma with cartilaginous metaplasia: report of a case and review of literature

Rhabdomyosarcoma (RMS) is a malignant tumor of skeletal muscle origin and frequently involves the head and neck region. It represents the second most common pediatric soft tissue sarcoma and accounts for 3% of all childhood cancers. Here, we report a case of embryonal RMS presenting as a right-sided facial swelling in a 7-year-old boy. Histologically, the tumor consisted of classic embryonal rhabdosarcomatous areas with metaplastic cartilage, in both initial biopsy and final resection specimens. Cartilaginous metaplasia arising in the background of RMS is a rare finding, thus raising a diagnostic challenge. To the best of our knowledge, this represents the first case of RMS with cartilaginous metaplasia in the oral cavity.

Stage 1 embryonal rhabdomyosarcoma of the female genital tract: a retrospective clinical study of nine cases

Background

The aim of the study is to investigate the clinical features, treatments, and prognosis of stage 1 embryonal rhabdomyosarcoma of the female genital tract.

Methods

A retrospective analysis was performed on nine cases of stage 1 embryonal rhabdomyosarcoma of the female genital tract. Clinical characteristics, treatments, recurrence, and prognosis were analyzed.

Results

Of the nine patients with embryonal rhabdomyosarcoma, three originated from the vagina and six from the cervix. For the eight patients who initially received surgery, the median survival time was 88 months. As for the six patients that received adjuvant chemotherapy, five of them who received six or more cycles of treatment achieved tumor-free survival and the survival time ranged from 9 to 228 months. The remaining patient, who declined further treatment after two cycles of chemotherapy, relapsed 11 months following the surgery and died 3 months later. Out of the nine patients, only one was initially treated with chemotherapy, and achieved complete remission, but relapsed 21 months later. After a combination of surgery and chemotherapy, this patient remained tumor-free for total of 117 months.

Conclusions

Patients with early stage embryonal rhabdomyosarcoma of the female genital tract have good prognosis, and the combination of surgery and chemotherapy can lead to better outcomes.

Small round blue cell tumors of the sinonasal tract: a differential diagnosis approach

One of the most challenging diagnostic categories within tumors of the sinonasal tract is the small round blue cell tumors. Biopsies are usually small and limited, resulting in considerable diagnostic difficulty for practicing surgical pathologists. These tumors share several overlapping histologic and immunophenotypic findings while also showing considerable variation within and between cases. Specific tumor site of origin, imaging findings, and clinical findings must be combined with the histology and pertinent ancillary studies if the correct diagnosis is to be reached. Discrimination between neoplasms is critical as there are significant differences in therapy and overall outcome. It is important to have a well developed differential diagnosis for this category of tumors, where each of the diagnoses is considered, evaluated, and either confirmed or excluded from further consideration. In an undifferentiated tumor, showing a small round blue cell morphology, using the mnemonic 'MR SLEEP' helps to highlight tumors to consider: melanoma, mesenchymal chondrosarcoma, rhabdomyosarcoma, sinonasal undifferentiated carcinoma, squamous cell carcinoma (including NUT carcinoma), small cell osteosarcoma, lymphoma, esthesioneuroblastoma (olfactory neuroblastoma), Ewing sarcoma/primitive neuroectodermal tumor, pituitary adenoma, and plasmacytoma. A panel of pertinent immunohistochemistry studies, histochemistries and/or molecular tests should aid in reaching a diagnosis, especially when taking the pattern and intensity of reactions into consideration.

Sinonasal Small Round Blue Cell Tumors: An Immunohistochemical Approach

Although clinical history and morphologic appearance should be the initial considerations when evaluating small round blue cell tumors of the sinonasal tract, the final diagnosis often hinges on immunohistochemical findings. Unfortunately, interpretation of stains in these tumors is fraught with numerous pitfalls and limitations. This article presents an approach to sinonasal small round blue cell tumors based on four common immunohistochemical patterns: cytokeratin positivity, squamous marker positivity, neuroendocrine marker positivity, and cytokeratin negativity.

Orbito-ethmoidal rhabdomyosarcoma in an adult patient: a case report and review of the literature

We report a patient who presented to the ENT service complaining of nasal obstruction, exophthalmos, edema and ipsilateral facial congestion. Imaging studies revealed an aggressive noncalcified solid mass centered in the left nasoethmoidal region and heterogeneous avid enhancement following contrast media injection. Subsequently, a biopsy confirmed the presence of solid alveolar rhabdomyosarcoma. The patient was treated with chemoradiation therapy for 7 weeks. Due to the advanced stage of the disease, the patient was enrolled in a palliative care and pain control program.

An intraosseous malignant peripheral nerve sheath tumor of the lumbar spine without neurofibromatosis: Case report and review of the literature

A malignant peripheral nerve sheath tumor (MPNST) is defined as any malignant tumor that develops or differentiates from cells in the peripheral nerve sheath. This tumor is commonly associated with neurofibromatosis type 1 (NF1) and previous radiotherapy treatment. Primary intraosseous MPNSTs are extremely rare and a case of the lumbar spine in a patient without NF1 is reported in the present study, with a review of the intraosseous MPNST literature. A 45-year-old female presented with a 1-month history of severe lower back pain and pain radiating to the left leg. A total en bloc spondylectomy of L4 was performed. The postoperative histopathological diagnosis was MPNST with deletion of NF1, confirmed by dual-color fluorescence in situ hybridization (FISH) analysis. The tumor recurred 1 month following the surgery. Although adjuvant chemotherapy was administered, the patient succumbed due to intramedullary dissemination and carcinomatous meningitis 8 months following the initial consultation. NF1 deletion by FISH analysis may be particularly useful in distinguishing MPNST from other high-grade malignancies with overlapping morphological features.

[Orbital alveolar rhabdomyosarcoma masked by ethmoid sinusitis in a 25-year-old]

CASE REPORT

A 25-year-old woman with right subacute sinusitis, complained about discomfort in her right eye. Clinical manifestations and computed tomography were suggestive of sub-periosteal orbital ethmoid wall abscess, for which the patient underwent urgent drainage. A solid tumor was found, with a positive biopsy for alveolar rhabdomyosarcoma. Complete remission and resolution of orbital symptoms were achieved with chemotherapy and radiation therapy.

DISCUSSION

Alveolar orbital rhabdomyosarcoma in adults is uncommon. Rhabdomyosarcoma has a high risk of spreading. It can simulate a sinusitis, as in our patient, early diagnosis and early treatment being especially important in these patients.

The role of cytogenetics and molecular diagnostics in the diagnosis of soft-tissue tumors

Soft-tissue sarcomas are rare, comprising <1% of all cancer diagnoses. Yet the diversity of histological subtypes is impressive with >100 benign and malignant soft-tissue tumor entities defined. Not infrequently, these neoplasms exhibit overlapping clinicopathologic features posing significant challenges in rendering a definitive diagnosis and optimal therapy. Advances in cytogenetic and molecular science have led to the discovery of genetic events in soft-tissue tumors that have not only enriched our understanding of the underlying biology of these neoplasms but have also proven to be powerful diagnostic adjuncts and/or indicators of molecular targeted therapy. In particular, many soft-tissue tumors are characterized by recurrent chromosomal rearrangements that produce specific gene fusions. For pathologists, identification of these fusions as well as other characteristic mutational alterations aids in precise subclassification. This review will address known recurrent or tumor-specific genetic events in soft-tissue tumors and discuss the molecular approaches commonly used in clinical practice to identify them. Emphasis is placed on the role of molecular pathology in the management of soft-tissue tumors. Familiarity with these genetic events provides important ancillary testing for pathologists to include in their diagnostic armamentarium.

Nasopharyngeal alveolar rhabdomyosarcoma expressing CD56: a mimicker of extranodal natural killer/T-cell lymphoma

Alveolar rhabdomyosarcoma (ARMS) is remarkably rare in adults older than 45 years. Histologically, the tumor is composed of blue round cells with frequent expression of CD56 in addition to myogenic markers. Recent studies of ARMS have shown two specific recurrent translocations: PAX3-FKHR [t(2;13)(q35;q14)] or PAX7-FKHR [t(1;13)(p36;q14)]. Extranodal natural killer (NK)/T-cell lymphoma (ENKTL) occurs most frequently in the upper aerodigestive tract with a male preference in East Asia and Central and South Americas with neoplastic cells frequently expressing CD56. We report a 53-year-old Taiwanese man presenting with a nasopharyngeal mass, cervical lymphadenopathy, and multiple bone metastases. Histologically, the nasopharyngeal biopsy revealed diffuse sheets of small blue round tumor cells without obvious alveolar pattern, angioinvasion or tumor necrosis. An initial erroneous diagnosis of ENKTL was made due to CD56 expression using fresh tumor tissue with flow cytometric analysis and the patient was treated accordingly. Retrospective study showed that the tumor cells expressed CD56, desmin, and myogenin. Fluorescence in situ hybridization revealed that the tumor cells were positive for FKHR gene rearrangement, confirming the diagnosis of ARMS. Our case illustrates that a diagnosis of ENKTL based solely on CD56 expression can be misleading for a nasopharyngeal small blue round cell tumor. ARMS should be included as a differential diagnosis, and a correct diagnosis can be reached only after a high index of suspicion and a thorough histological examination with the aid of ancillary studies.

[About a case of rhabdomyosarcoma with an aberrant expression of epithelial markers: a cause of misdiagnosis]

We present the case of an embryonal rhabdomyosarcoma of orbitary location with aberrant expression of epithelial markers in a 51-year-old female. The rhabdomyosarcoma is a rare tumor of soft tissues affecting mainly the child, but also exceptionally adults over 50. When it presents as a small round cells tumor, particularly in the region of head and neck, its differential diagnosis with several other poorly differentiated tumors may be difficult. Several cases of rhabdomyosarcoma with aberrant expression of epithelial markers have been reported in the literature. A large immunohistochemical panel is recommended by recent studies in order to avoid diagnostic errors. It includes large spectrum cytokeratins, desmin, neuroendocrine, melanocytic and lymphoid markers. Our observation confirms the importance of conducting this immunohistochemical panel including desmin in the context of a poorly differentiated tumor of the head and neck region. It should be performed whatever the age of the patient and even if the tumor expresses epithelial markers.

Laryngeal embryonal rhabdomyosarcoma in an adult - a case presentation in the eyes of geneticists and clinicians

1. Abstract

[Spindle cell rhabdomyosarcoma in adults: a new entity in the spectrum of malignant mesenchymal tumors of soft tissues]

Rhabdomyosarcomas (RMS) are rare soft tissue sarcomas showing a skeletal muscle differentiation. Histologically, embryonal, alveolar and pleomorphic subtypes are distinguished. RMS represent one of the most frequent sarcomas in children and adolescents, and in this age group embryonal and alveolar subtypes predominate. RMS in adults is extremely rare, and the pleomorphic subtype is most frequently seen. Spindle cell and sclerosing RMS in adults are very rare and, unlike the morphologically comparable tumors in children and adolescents, clinically aggressive neoplasms, which need to be distinguished from spindle cell malignant melanoma, spindle cell sarcomatoid carcinoma and from a number of spindle cell sarcomas of varying lines of differentiation. Spindle cell and sclerosing RMS in adults arise predominantly in the head and neck region and on the extremities, and are composed of atypical spindled cells and scattered rhabdomyoblasts. Since spindle cell RMS may contain areas of lower cellularity with abundant sclerosing stroma, and sclerosing RMS may show focal areas of increased cellularity, it is most likely that both forms represent a morphological spectrum of a single RMS variant.

Aggressive spindle cell rhabdomyosarcoma in an 11-month-old boxer dog

Rhabdomyosarcoma (RMS) is a malignant neoplasm derived from mesenchymal tissue with a tendency toward myogenic differentiation associated with the embryogenesis of skeletal muscle. According to the histological features, it can be classified in embryonal, botryoid, alveolar, and pleomorphic, which usually correspond to clinical behavior and prognosis. The spindle cell (SCRMS) variant is a rare subtype of the embryonal RMS and is considered to be less aggressive lesion. The aim of the present paper is to report an unusual case of SCRMS in an 11-month-old male boxer dog diagnosed as extensive SCRMS that affected the frontal region of the skull. Due to the aggressive nature of the lesion and poor clinical prognosis the dog's owners preferred euthanasia as a treatment. A full postmortem examination was carried out. Microscopically, the lesion was composed of a highly cellular proliferation of spindle cells arranged in long and intersecting fascicles. After performing the immunohistochemical studies (HHF-35, smooth muscle actin, desmin and MyoD1), the present case was diagnosed as SCRMS.

The small round blue cell tumors of the sinonasal area

The diagnostic classification of small round blue cell tumors of the sinonasal area to include diverse malignancies of epithelial, hematolymphoid, neuroectodermal, and mesenchymal origin is challenging to the surgical pathologist using conventional histopathologic approaches because the cytomorphologic features are often overlapping or indistinctive. Rare or occasional clinical presentations in atypical age groups or unusual locations, as well as small biopsy samples may further complicate the differential diagnosis. Immunohistochemistry represents an extensively investigated ancillary technique that may aid in the provision of a definitive diagnosis. In recent years, certain small round blue cell tumors have been shown by cytogenetic analysis to have specific and primary chromosomal alterations, providing clinicians with a valuable tool to enhance their diagnostic armamentarium. The addition of molecular cytogenetic [fluorescence in situ hybridization (FISH), comparative genomic hybridization (CGH)] and molecular pathologic [polymerase chain reaction (PCR) and reverse transcriptase (RT)-PCR] approaches has further enhanced the sensitivity and accuracy of detecting these genetic alterations including assessment in formalin-fixed, paraffin-embedded tissues. Establishing an accurate diagnosis of a small round blue cell tumor of the sinonasal tract frequently requires adjunctive studies including immunohistochemical and molecular analyses.

[Undifferentiated malignant round cell tumors of the sinonasal tract and nasopharynx]

Undifferentiated malignant round cell tumors of the sinonasal tract and nasopharynx comprise of aggressive malignant neoplasms with overlapping morphologic features, which yet are very different and can be identified as carcinoma, lymphoma, sarcoma, melanoma or olfactory neuroblastoma. Differentiating these tumors is important because their treatment and prognosis are quite different. Because of the limited initial biopsy tissue materials, major differential diagnostic difficulties may arise. The use of a panel of immunohistochemical markers and the identification, in some cases, of specific chromosomal translocations are most often required. We describe clinical, morphological, immunohistochemical and molecular features as well as therapeutic management and prognosis of rhabdomyosarcoma (RMS), Ewing sarcoma/primitive neuroectodermic tumor (EWS/PNET), nonkeratinizing nasopharyngeal carcinoma, undifferentiated subtype (UCNT), olfactory neuroblastoma (ONB), small cell carcinoma, neuroendocrine type (SCCNET), sinonasal undifferentiated carcinoma (SNUC) and mucosal melanoma. Mesenchymal chondrosarcoma, small round cell synovialosarcoma, lymphomas and plasmocytoma are not introduced. The most important features of diagnosis will be summarized in two tables.

Primitive neuroectodermal tumor as a differential diagnosis of CD56-positive tumors in adults

A 33-year-old Japanese man was referred to our hospital after a huge intrapelvic tumor with bilateral hydronephrosis was found following persistent lumbago. Natural killer/T-cell lymphoma was suspected due to positive immunostaining for CD56, but CHOP therapy was ineffective. Re-evaluation of the tumor cells showed that they were positive for CD99, neuron-specific enolase, and synaptophysin and had a t(11 ; 22) (q24 ; q12) translocation, leading to the revised diagnosis of primitive neuroectodermal tumor (PNET). Systemic chemotherapies and radiation therapy were added to surgical resection, and no recurrence has been detected for 3 years. Taken together, PNET may be considered in adult patients with CD56-positive tumors.