Journal of Cancer Research and Therapeutics Medknow Publications on behalf of the Association of Radiation Oncologists of India (AROI) ISSN: 0973-1482 EISSN: 1998-4138 Vol. 6, Num. 3, 2010, pp. 327-329

Journal of Cancer Research and Therapeutics, Vol. 6, No. 3, July-September, 2010, pp. 327-329

Case Report

Imaging of parotid gland primitive neuroectodermal tumor

Niketa C Chotai, Phua Tang, Anil N Gopinathan

Department of Diagnostic Imaging, KK Women's and Children's Hospital, Singapore

Correspondence Address:Niketa C Chotai, Department of Diagnostic Radiology, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore - 308 433, Singapore, niketachotai@yahoo.com

Code Number: cr10076

PMID: 21119266

DOI: 10.4103/0973-1482.73345

Abstract

Keywords: Imaging of primitive neuroectodermal tumor, Parotid space, parapharyngeal space

Introduction

Primitive neuroectodermal tumor (PNET) is a malignant small round cell tumor, characterized by unique chromosomal translocations and MIC-2 antigen expression. ^[1] When these tumors arise outside the central and sympathetic nervous system, they are termed as peripheral PNET. ^[2] Common locations of peripheral PNET (pPNET) include thoracopulmonary region, the retroperitoneum and pelvis. We report a case of parotid gland PNET that is extremely unusual for its location, with emphasis on the role of pre-operative radiological imaging.

Case Report

A seven-year-old boy presented with history of recurrent right parotid gland swelling, of two years duration, presumptively being treated as sialoadenitis. An ultrasound was performed to exclude sialolithiasis.

Ultrasound (US) showed a 3 cm, heterogeneous mass in the right side of the neck, inseparable from the deep surface of the right parotid gland. The complete extent of the mass was not sufficiently evaluated on US. Subsequently a contrast enhanced CT scan was done with z-axis coverage from the base skull to arch of aorta. It demonstrated about 4 cm relatively well-defined heterogeneous, moderately enhancing soft tissue mass, isodense to the muscle involving the deep lobe of the parotid gland and completely filling the parapharyngeal space (PPS) [Figure - 1]a. There was widening of the stylomandibular tunnel with involvement of the deep aspect of the superficial lobe of parotid gland. There was extension into the masticator space along with smooth scalloping of the ramus of mandible. Widening of foramen ovale [Figure - 1]b and equivocal intracranial extension was noted. The carotid space was displaced posteriorly without encasement or infiltration of the great vessels within.

Multiplanar contrast enhanced MRI was done to ascertain the anatomical extent of the tumor and exclude intracranial involvement. MRI confirmed the soft tissue extent of the tumor as seen at CT. There were several T2 bright, rim-enhancing areas suggesting necrosis, in otherwise moderately enhancing lesion [Figure - 2]a. Besides, it also demonstrated medial displacement of the PPS. Smooth, linear, dural enhancement was seen in the floor of the middle cranial fossa as the tumor widened the foramen ovale and abutted the dura [Figure - 2]b. An intervening sliver of fluid was still present between brain and dura.

After excluding distant metastasis by bone scan, interval excision of the mass was done. Complete surgical clearance was achieved. Findings at surgery and histology with immuno-histochemistry (positive for CD 99, CD56, synaptophysin) were reminiscent of parotid gland PNET. Twelve cycles of chemotherapy were administered. The patient is on biannual follow- up since last two years and is disease free till date.

Discussion

Current evidence indicates that both PNET and Ewing′s sarcoma represent two ends of the spectrum of the same entity. ^[3] pPNET is rare and comprise only around 1% of all the sarcomas. ^[2] Rare occurrences of this tumor in the head and neck region include those arising from the maxilla, mandible, orbits, nasal cavity, paranasal sinuses, temporal region and rarely parotid gland. To the best of our knowledge, there are only four prior reports on parotid PNET in medical literature in English language. ^{[4],[5],[6],[7]} All four of them dealt with pathological findings with limited literature on the scope of diagnostic imaging. The previously reported cases of parotid PNET were seen in adults in the fourth to sixth decade of life. In comparison, this case is unique as our patient presented in the first decade of life.

Suprahyoid deep neck spaces are not easily accessible to clinical examination and hence imaging plays crucial role in this region. The role of ultrasound is limited as the complex regional anatomy restricts the acoustic window for adequate evaluation and CT is usually the first choice of investigation. However, the young age of this patient and clinical suspicion of sialoadenitis had prompted US over CT. The heterogeneously hypoechoic appearance on US with cystic areas, seen here, has been described earlier in PNET. ^[2] The primary role of USG was of excluding more common pathologies and indicating the presence of neoplastic lesion rather than infective/inflammatory pathology.

CT and MRI is the mainstay when investigating head and neck masses. The rim enhancing, T2 bright foci seen at MRI, in this patient were corresponding to the several necrotic areas found at pathological examination. PNETs are known to have internal hemorrhage and necrosis. ^[8] They are usually rapidly growing, locally aggressive tumors with invasion of adjacent bone/ soft tissues and neovascularization. Probably this tumor was slow- growing and less aggressive than a usual PNET, as was suggested by the scalloping of the mandible and smooth widening of the foramen ovale as against permeative bone destruction in a rapidly growing, aggressive lesion. In malignant head and neck tumors, the primary role of CT and MRI is in providing an accurate estimate of the extent of the disease and presence of adequate margins for resection. CT may show cortical bone involvement better, but MRI offers better soft tissue resolution and may be mandatory for intracranial extension and marrow infiltration.

The differential diagnoses in this patient would include embryonal rhabdomyosarcoma (ERMS), salivary neoplasms, lymphoma, malignant peripheral nerve sheath tumors and hemangioma. Salivary gland tumors are very uncommon at young age. Though ERMS is the most common soft tissue sarcoma seen in the head and neck region in pediatric population, they are more often found in orbits, nasopharynx, paranasal sinuses and infratemporal fossa with primary parotid rhabdomyosarcoma extremely rare. However in this case, excluding an ERMS at imaging was not possible. Similarly lymphomas and malignant peripheral nerve sheath tumours are other rare conditions that can occur in this region with no pathognomonic imaging findings. Hemangiomas are generally brighter on T2WI and enhancement may vary as per their flow characteristics.

Conclusion

Our experience with USG, CT and MRI in this case of pPNET of the parotid gland, defined the tumor as a heterogeneous mass with necrotic areas within, moderate contrast enhancement and scalloping of the adjoining bones. With such findings on cross-sectional imaging, PNET should be considered in the differential diagnosis of moderately aggressive soft tissue tumors in pediatric age group, even when present in unusual locations.

References

Copyright 2010 - Journal of Cancer Research and Therapeutics

The following images related to this document are available:

Photo images