Nodular fasciitis: A pseudomalignant clonal neoplasm characterized by USP gene rearrangements and spontaneous regression

Introduction: Nodular fasciitis (NF) is a rapidly growing, self-limited, myofibroblastic neoplasm Jennifer Hennebry1, Douglas Mulholland2, Peter Julian Beddy3, Nairi Tchrakian4, Máirín Eibhlín McMenamin5, Charles Martin Gillham6, Dearbhaile Mai O’Donnell7 Affiliations: 1MB BCh BAO, Senior House Officer, Internal Medicine, St. James’s Hospital, Dublin, Ireland; 2MB BCh BAO, Specialist registrar in Radiology, Radiology department, St. James’s Hospital, Dublin, Ireland; 3MB BCh BAO, Consultant Radiologist, Radiology department, St. James’s Hospital, Dublin, Ireland; 4MB MCh BAO, Specialist registrar in Histopathology, Histopathology department, St. James’s Hospital, Dublin, Ireland; 5FRCPath, Consultant Histopathologist Histopathology department, St. James’s Hospital, Dublin, Ireland; 6FRCR, Consultant Radiation Oncologist, St. James’s Hospital, Dublin, Ireland; 7MRCPI, Consultant Oncologist, St. James’s Hospital, Dublin, Ireland.. Corresponding Author: Dr. Jennifer Hennebry, University Hospital Waterford, Dunmore road, Waterford, Ireland; E-mail: jennifer8h@gmail.com Received: 14 September 2016 Accepted: 13 October 2016 Published: 01 January 2017 that typically arises in subcutaneous tissues of young adults and regresses spontaneously. Nodular fasciitis mimics sarcoma on clinical, radiological, and histological grounds and is usually diagnosed following excision. case report: A 26-year-old female presented at surveillance ct scan one year post-treatment for stage 1c ovarian dysgerminoma with a 4cm axillary soft tissue mass, radiologically suspicious for metastasis with subclavian vein invasion. Histopathology of core biopsies favored NF, confirmed by detection of UsP6 gene rearrangements by FIsH analysis. this case describes an unusual relatively deep NF, suspicious for metastasis on ct scan with confirmed spontaneous regression over 2 years. conclusion: Nodular fasciitis should be considered in the differential diagnosis of rapidly growing enhancing soft tissue masses. Molecular cytogenetic testing of UsP6 gene rearrangements allows definitive diagnosis on core biopsies in challenging cases, permitting a conservative approach and avoiding potentially radical and unnecessary surgery.

that typically arises in subcutaneous tissues of young adults and regresses spontaneously. Nodular fasciitis mimics sarcoma on clinical, radiological, and histological grounds and is usually diagnosed following excision. case report: A 26-year-old female presented at surveillance ct scan one year post-treatment for stage 1c ovarian dysgerminoma with a 4cm axillary soft tissue mass, radiologically suspicious for metastasis with subclavian vein invasion. Histopathology of core biopsies favored NF, confirmed by detection of UsP6 gene rearrangements by FIsH analysis. this case describes an unusual relatively deep NF, suspicious for metastasis on ct scan with confirmed spontaneous regression over 2 years. conclusion: Nodular fasciitis should be considered in the differential diagnosis of rapidly growing enhancing soft tissue masses. Molecular cytogenetic testing of UsP6 gene rearrangements allows definitive diagnosis on core biopsies in challenging cases, permitting a conservative approach and avoiding potentially radical and unnecessary surgery.

INtrODUctION
Nodular fasciitis is a self-limited mass-forming myofibroblastic proliferation, which typically presents with sudden appearance and rapid growth and is generally less than 3 cm in size [1,2], first described as 'pseudosarcomatous fibromatosis' in 1955 [3]. Nodular fasciitis usually presents between the second and fourth decade of life with no gender predilection. Recurrence is rare, unless excision is subtotal in the active growing phase [2]. Spontaneous involution has been described [1,2,4,5] and is probably the natural course in NF; however the majority are excised due to clinical concern for malignancy and histological difficulty in making a definitive diagnosis on core biopsy. A minority of cases (10-15%) are associated with recent prior trauma [1].
Typical sites of presentation include the upper limb, trunk, and head and neck. Nodular fasciitis is usually subcutaneous, but can be intramuscular or fascial based, can rarely arise in an intradermal or intra-articular location and can be entirely or partly intravascular [2].
Radiologic and histologic features can be highly variable [2,6], and in the clinical setting of a rapidly growing mass, distinction from sarcoma can be difficult. Diagnosis is usually confirmed on an excision specimen. Histologic appearances can vary depending on the area sampled and the duration/phase of the lesion. Early lesions comprise a haphazard or short fascicular arrangement of cellular spindle cell proliferations with brisk mitotic rate. Lesions can show a feathery or loose discohesive appearance mimicking granulation tissue with microcysts. Red cell extravasation and scattered lymphocytes, macrophages and osteoclast-like giant cells are often present. Older lesions are more paucicellular with hyalinized collagen, scattered apoptotic cells and macrophages and may show central cystic degeneration [2]. Immunohistochemical staining profile is not specific [7]. Smooth muscle actin is positive but this does not distinguish it from other myofibroblastic lesions such as cellular scars, fibrous histiocytomas, desmoid-type fibromatosis or desmin negative leiomyosarcomas. CD34, desmin and cytokeratin are negative. Detection of USP6 gene rearrangement by fluorescence in situ hybridization (FISH) has emerged as a very helpful tool in confirming the diagnosis of NF [5,8,9] and is of particular use in cases where there is limited tissue for diagnosis, or when histological, radiological, or clinical appearances are atypical.

cAsE rEPOrt
A 25-year-old female presented with a pelvic mass, which was surgically resected and diagnosed as a stage 1c ovarian dysgerminoma. The patient received adjuvant chemotherapy and a CT scan performed six months following treatment confirmed complete radiological remission. Four months post-chemotherapy, she developed bronchiolitis obliterans/organizing pneumonia as a complication of bleomycin chemotherapy. Following oral steroid therapy for several months, symptoms and radiological changes resolved and pulmonary function tests improved. Thirteen months post chemotherapy, a CT scan showed a soft tissue mass, measuring 4x4x3 cm, deep to the pectoralis major muscle and abutting the chest wall without rib invasion. This was detectable on clinical examination as a sub-clavicular anterior chest wall asymmetry. Radiological impression was of confluent pathologic adenopathy with central necrosis representing metastases from the patient's prior ovarian dysgerminoma. A soft tissue sarcoma was in the differential diagnosis. Contrast enhanced MRI showed central necrosis or degeneration within the mass with close proximity to the right subclavian artery and vein ( Figure 1) and features suspicious for subclavian vein invasion and thrombosis. PET CT scan demonstrated intense tracer uptake within the mass with an SUV of 9.6. (Figure 2).
Ultrasound confirmed that the mass was partly intravascular. Core needle biopsy yielded a cellular spindle-cell proliferation with some short fascicles ( Figure 3). Elsewhere there was a haphazard arrangement of cells with a loose discohesive feathery appearance with scattered chronic inflammatory cells (Figure 4). Cytonuclear atypia was not conspicuous and there was no necrosis. Mitoses numbered up to 3 per 10 40x fields ( Figure 5).
Immunohistochemical stains showed that lesional spindle cells were positive for smooth muscle actin (SMA) ( Figure 6) and negative for desmin, cytokeratin and S100. CD68 stained lesional macrophages. The MIB1 proliferative index was <10%. Histological features favored NF but sampling of a bland area of a sarcoma could not be excluded. Due to the worrying clinical presentation and the lack of definitive tissue diagnosis, an MRI scan was performed with a view to planning surgical excision. Repeat core needle biopsy again favored NF. Given the histologic appearances and the proximity to major vessels, a 'watch and wait' approach was adopted with repeat MR scans scheduled at three-monthly intervals. However, over the next three-week period, the patient developed swelling of the right arm with skin mottling, worrisome for vascular compromise. The case was referred to the London sarcoma service for advice on further management. FISH analysis using custommade break-apart BAC probes detected USP6 gene rearrangement, confirming the diagnosis of NF (Figure 7). at the Royal National Orthopedic Hospital NHS Trust. This news coincided with spontaneous improvement in clinical symptoms. In the two years following detection of the mass, it has undergone spontaneous and complete resolution on MRI. The patient remains symptom-free.

DIscUssION
Nodular fasciitis (NF) is a self-limited myofibroblastic proliferation, which is usually subcutaneous in location and rarely presents as multiple lesions [10]. Due to its typical clinical presentation as a rapidly growing mass and its variable radiological and histological features, NF can be misdiagnosed as a sarcoma [1,2,6]. In this case of a female with a history of treated ovarian dysgerminoma, a relatively deep-seated axillary NF was identified on surveillance imaging. Nodular fasciitis was in the active growing phase and showed concerning clinical and radiological features simulating metastatic tumor or sarcoma, including enhancement, central degeneration and vascular involvement.
The differential diagnosis of nodular fasciitis on MR imaging is broad, spanning benign and malignant lesions, including neurofibroma, extra-abdominal desmoid fibromatosis, fibrous histiocytoma, early myositis ossificans and sarcoma [6]. MR imaging is helpful for defining the intrinsic signal characteristics, size and compartmental extensions of these lesions. Depending on the distribution of the myxoid or fibrous components, nodular fasciitis is usually isointense to skeletal muscle on T1 sequence and hyperintense to fat on T2 sequence. Lesions with predominantly cellular content or myxoid degeneration appear hyperintense on T2-weighted sequence and those with mostly collagenous contents appear hypointense [4,6]. Contrast enhancement pattern is most commonly diffuse but it may also be peripheral in lesions with cystic degeneration, as in this case. Histological examination is essential for the diagnosis of NF as radiological appearances are not specific.
Nodular fasciitis is typically circumscribed, but may display an infiltrative growth pattern, especially those that are fascial based [2]. The cut surface of a macroscopic lesion varies from myxoid to fibrous, sometimes with central cystic change [1]. The lesion is typically composed of spindled fibroblast-like cells with a haphazard, 'tissue culture'-type architecture resembling granulation tissue with cellular areas and looser areas and scattered inflammatory cells [2]. Cellular areas can show a fascicular arrangement of cells. The looser areas have a discohesive arrangement of cells with a feathery appearance and can show microcystic change. Mitotic figures are usually readily identified, but atypical forms are not generally present. Other microscopic features of NF include extravasated red blood cells and osteoclastlike giant cells [2] but they were not seen in this case. Older lesions show more hyalinized collagen. It is easier to make a histological diagnosis on an excision specimen as the architectural pattern is evident. It can be difficult to make a definitive diagnosis on core biopsy, when limited features are available. Immunohistochemistry is not specific-smooth muscle actin and muscle-specific actin are typically positive, supporting a myofibroblastic phenotype [2,7]. CD68 stains lesional macrophages, osteoclast-like giant cells and occasionally weakly stains the spindle cells [1].
Until recently, no specific or consistent cytogenetic abnormality was observed in NF and diagnosis was based on histological features. Furthermore, due to its rapid growth, low risk of recurrence, spontaneous resolution and the fact that it has not been associated with malignant transformation, NF has traditionally been considered reactive in nature. In 2011, Erickson-Johnson et al. reported genomic rearrangements of the USP6 locus on chromosome 17 in 92% of NF, the majority of which resulted in the formation of the fusion gene MYH9-USP6 [8] and have proposed that the identification of a recurrent somatic fusion gene event in NF is supportive of a clonal transient neoplasm. This suggestion challenges the traditional paradigm that nonrandom fusion gene formation is associated solely with sustained autonomous neoplasms. This molecular diagnostic approach is a tremendous advance in the diagnosis of NF [5,8,9]. Spontaneous resolution is likely by cellular apoptosis but to date the mechanism has not been elucidated.
Opinions have varied on the optimal treatment of NF. Most have advocated for simple excision [2]. Following a confirmed biopsy diagnosis, a conservative 'watch and wait' approach with repeat imaging and anticipated spontaneous involution is preferable particularly in surgically challenging sites [4,5]. FISH is cost effective, especially if challenging surgery can be avoided. The price of USP FISH can be obtained from referral centers. Current price is approximately 300 GBP.

cONcLUsION
We present an unusual nodular fasciitis (NF) in a young woman that was discovered on follow up for previous ovarian dysgerminoma. As the NF was in the active growing phase, clinical and radiological features were suspicious for malignancy with progressive subclavian vein involvement and compromise. The suspected histopathological diagnosis of NF was confirmed by detection of USP6 gene rearrangements and permitted a conservative approach, sparing this patient unnecessary surgery and potential morbidity. There was spontaneous resolution over two years, confirmed radiologically.