Among primary mesenchymal tumours, intranodal palisaded myofibroblastoma is very rare. It was initially described by Deligdish and Katz as neurilemmoma or schwanoma and later classified as palisaded myofibroblastoma by Weiss [7
]. It shares many features of myofibroblasts and smooth muscle cells microscopically as well as immunohistochemically. This mesenchymal spindle cell tumour arises from the lymph nodes. It is most commonly seen in the inguinal lymph nodes, however origins from submandibular and mediastinal lymph nodes have also been reported [3
IPM commonly affects the second to eighth decade, with a peak incidence in the group between 40 and 60 years of age, but occurrence of this tumour in an infant has also been reported [9
]. It is more common in men with a male to female ratio 1.5:1 and is not specific to any race [11
]. The most common clinical presentation is a painless slow growing lump in the inguinal region. With increasing size discomfort, pain and compression of local surrounding structures may manifest various symptoms. It is most often diagnosed clinically as lymphadenopathy and histological diagnosis is established on excision biopsy. Fortunately, this tumour is associated with very low recurrence rate and the recurrence is almost benign in nature. In a series of 30 cases, only 6% was reported to demonstrate benign local recurrence [12
]. Creager et al reported only a single case of intranodal palisaded myofibroblastoma with metastatic bone formation [13
It is essential to differentiate IPM from other soft tissue tumours. These include Schwannoma, Kaposi's sarcoma, intranodal leiomyoma, inflammatory pseudotumour, solitary fibromastocytic tumour, angiomyomatous hamartoma and metastatic spindle cell lesions in the lymph node. The clinical history, examination and typical histological characteristics help in the correct diagnosis of the IPM. Schwannoma is uncommon in the inguinal region and immunohistochemically it is positive for S100 that differentiates it from IPM. Negative immunostaining for HHV 8 along with an absence of EBV DNA and a history of immunocompromised status favours the diagnosis of IPM rather than Kaposi's sarcoma. There were no slit-like vascular channels, no extravasation of red blood cells and no hyaline globules as typically seen in Kaposi's sarcoma. Differentiation of inflammatory myofibroblastic tumour (IMT) from IPM can be made by the presence of inflammatory cells and absence of the amianthoid fibers in IMT [14
]. Negative immunostaining for Alk-1 and the presence of amanthoid fibres with absence of inflammatory cell infiltrate, including lymphocytes and plasma cells, favoured the diagnosis of IPM. Furthermore, spindle cell melanoma can be differentiated from IPM by high proliferative activity and marked cellular atypia. Spindle cell melanoma is also positive for S100 and HMB-45 [15
]. The gastrointestinal stromal tumour is positive for CD117/C-kit while IPM is negative. Regarding angiomyomatous hamartomas in lymph nodes, the morphology and immunostaining patterns are consistent with IPM rather than angiomyomatous hamartomas.
Due to the rarity of the condition the precise aetiology and pathogenesis have yet to be defined. Inguinal lymph nodes have a higher concentration of myofibroblasts compare to other lymph nodes in the body due to increased drainage area and function: this might explain its more common occurrence in the inguinal region. This is favoured by positive staining for actin and vinmentin and negative for desmin in myofibroblasts of the inguinal region compared to fibroblasts elsewhere in the body [2
]. However, its appearance in other parts of the body contradicts this hypothesis. As in our case, the origin from the retroperitoneal region has not been reported yet in literature and this again raises the question of the aetiology of IPM.. The positive staining for cyclin D1 suggests the possible role of the cell cycle regulatory genes in the pathogenesis of IPM [16
]; thus reporting of more number of cases is required to get a better understanding of the aetiology and pathogenesis of IPM.
Although rare, IPM should be considered as one of the differential diagnosis in patients presenting with an abdominal mass in the retroperitoneal region. This site of origin may provide an opportunity to reconsider the aetiology and pathogenesis of IPM.