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1 Departments of Radiology, Pathology, and Orthopedic Surgery Tri-Service
General Hospital National Defense Medical Center Taipei, Taiwan, Republic of
China
2 Department of Radiology Armed Forces Tsoying Hospital Taiwan, Republic of
China
3 Tri-Service General Hospital National Defense Medical Center Taipei,
Taiwan, Republic of China
A 10-year-old boy was seen at our outpatient clinic for pediatric surgery because of a bulging mass on the posterior aspect of his left arm. The mass was first observed at the age of 3 years, and it had slowly increased in size since then. Physical examination revealed a soft 4 x 3 cm erythematous mass with mild tenderness on the posterior aspect of the boy's left upper arm. No other features of neurofibromatosis, such as "café au lait" macules, freckling in the axillary or inguinal regions, or classic neurofibromas, were observed. The boy had no family history of neurofibromatosis.
The laboratory data were normal. MR examination showed a bulging fat-containing mass involving the subcutaneous fat layer on the posterior aspect of the left upper arm, with thickening of the overlying skin. On T1-weighted MRI (Fig. 1A), the mass showed linear strands of intermediate signal intensity in the subcutaneous fat, with thickened tissue of intermediate signal intensity in the overlying skin. On fat-suppressed T2-weighted imaging (Fig. 1B), the thickened tissue in the overlying skin and the linear strands in the subcutaneous fat showed high signal intensity.
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At the time, the tentative differential diagnoses included hemangioma, angiolipoma, and angiomyolipoma. The patient subsequently underwent surgery for wide excision of the mass. Histologic examination of the excised specimen (Fig. 1C) showed a typical presentation of diffuse neurofibroma involving the entire dermis and infiltrating the subcutaneous fat. The lesion was characterized by a proliferation of spindle cells that contained elongated ovoid to curved nuclei, and these cells were surrounded by a matrix with wirelike collagenous fibers. Immunoperoxidase staining with S-100 protein showed positive staining of the nuclei and cytoplasm of the tumor cells. Corpuscles that look like Meissner's corpuscles and ectatic blood vessels were found. The tumor was completely excised. No recurrence was observed during the first year of follow-up.
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On the basis of the histology, three types of neurofibroma can be distinguished: the localized, plexiform, and diffuse types [1]. The localized neurofibroma presenting as a fusiform or ovoid mass involves the nerve, which may be seen entering and exiting the tumor mass. The plexiform neurofibroma diffusely involves a long segment of nerve and its branches and shows a serpentinelike appearance. The diffuse neurofibroma is characterized by infiltrative growth in the subcutaneous fat and entrapment of the fat and other normal structures.
Diffuse neurofibroma is an uncommon form of neurofibroma that occurs primarily in children and young adults [1]. The head and neck regions are the most common sites of involvement. Diffuse neurofibroma is an ill-defined infiltrative lesion and tends to involve the skin and subcutaneous tissues [2–4]. It produces localized thickening of the skin, infiltration along connective tissue septa, and envelopment of the normal structures [3, 4]. About 10% of patients with diffuse neurofibromas also have associated neurofibromatosis 1 [2].
The MRI appearance of diffuse neurofibroma is characteristic and well correlated with histologic findings of its infiltrative behavior and elements [2]. The characteristic MR pattern of a diffuse neurofibroma, as seen on T1-weighted imaging, is linear or reticular strands of intermediate signal intensity in the subcutaneous fat, which indicates infiltration of the tumor components of neurofibromatous tissue, interposed collagen, and ectatic vessels [2, 3, 5]. T2-weighted imaging shows high signal intensity of the linear and reticular strands of the tumor elements that contain neurofibromatous tissue and ectatic vessels, but the interposed collagen remains intermediate to low signal intensity. Involvement of the dermis of skin results in thickened skin. Linear or reticular enhancement of the neurofibromatous tissue and ectatic vessels of the tumor is well shown on T1-weighted imaging after the IV administration of gadopentetate dimeglumine, whereas the collagen is of intermediate to low signal intensity on all sequences and not enhanced [3]. The presence of ectatic blood vessels within the lesion contributes not only to the interconnecting linear or tubular appearance of the lesions but also to its contrast enhancement [2].
The MR appearance and localization of diffuse neurofibroma may be confused with those of angiomatous or fat-containing tumor, especially in a patient with a solitary lesion or one without clinical findings of neurofibromatosis. In addition, clinically evident erythematous discoloration of involved skin may also lead to a misdiagnosis. Diffuse neurofibroma can be differentiated from the other soft-tissue tumors that contain prominent collagen such as dermatofibrosarcoma protuberans, desmoid tumors, nodular fasciitis, and elastofibroma, which typically appear as solid nodular masses and do not have linear or reticular appearance [3].
Diffuse neurofibroma has a characteristic MR pattern that differs from those of the localized and plexiform types of neurofibroma. Localized neurofibroma appears as a well-defined nodule, and plexiform neurofibroma appears as a serpiginous lesion. The linear or reticular infiltrative pattern and erythematous discoloration of the involved skin of diffuse neurofibromas may imitate those of angiomatous tumors.
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  D. S. Hassell, L. W. Bancroft, M. J. Kransdorf, J. J. Peterson, T. H. Berquist, M. D. Murphey, and J. C. Fanburg-Smith Imaging Appearance of Diffuse Neurofibroma Am. J. Roentgenol., March 1, 2008; 190(3): 582 - 588. [Abstract] [Full Text] [PDF]
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 W. Chen, J.-W. Jia, and J.-R. Wang Soft Tissue Diffuse Neurofibromas: Sonographic Findings J. Ultrasound Med., April 1, 2007; 26(4): 513 - 518. [Abstract] [Full Text] [PDF]
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