Collagenous fibroma was first identified and designated as desmoplastic fibroblastoma by Evans  in 1995, but the name collagenous fibroma was given by Nielsen et al.  due to its benign nature. It was suggested that it arises as a neoplasm rather than reactive proliferation because it is slowly growing and persistent with no preceding source of irritation [6,7,8]. In the head and neck region, the neck is the most affected site . Few cases have been reported in the oral cavity . The main patient complaint is discomfort when it reaches a large size, but the lesion is not painful . Collagenous fibroma usually does not cause bony changes unless presented inside the bone, which is uncommon. Bony lesion presents as a well-defined radiolucency on radiographs [1, 9, 11].
Histopathologically, the tumor is hypocellular, formed of stellate and spindle-shaped fibroblasts and myofibroblasts embedded in a densely collagenized or myxocollagenized stroma . This tumor shows some similarities with other lesions like desmoid tumor and focal fibrous hyperplasia. Yet, there are differences. Desmoid tumor is more cellular with high vascularity and infiltrative margins . Focal fibrous hyperplasia shows inflammatory infiltrate and moderate cellularity .
Desmoid tumor or aggressive fibromatosis is an intermediate locally aggressive fibroblastic/myofibroblastic tumor. It has a high recurrence rate and causes destruction to the surrounding tissues. Treatment is by wide surgical excision . In contrast to desmoid tumor, collagenous fibroma is a benign tumor that does not recur and is treated by conservative surgical excision with no need for grafts .
We present the second bilateral collagenous fibroma after the only reported case in English literature by Vasconcelos et al. .
Our case was a female patient having a mass in the hard palate. This is in accordance with most of the cases reported in the oral cavity, which were in female patients and were presented in the hard palate . The ages of the cases reported in the oral cavity ranged from 8 to 87 years. Our patient’s age was within the same range: 37 years. The size of the current lesion was 5 cm in length. The reported cases had sizes ranging from 0.7 cm to 6 cm. Large lesions may cause difficulty in speech and mastication [10, 13]. Our case was complaining from the same problems. There was no history of trauma. This is in accordance with the results of many studies [4, 11, 13, 14], supporting the neoplastic nature of the lesion as suggested by Evans  and Nielsen et al. . Miettinen and Fetsch  reported 2 cases with history of trauma out of 63 cases, making up the largest case series of collagenous fibroma.
The microscopic examination of the present case showed a well-circumscribed mass formed of stellate and spindle-shaped cells in a collagenous background. Almost all cases of collagenous fibroma have similar histologic findings [16,17,18,19]. However, nucleoli were not clearly seen as in the study of Gong et al. . Blood vessels were quite numerous. This finding was opposite to the findings of other studies, where blood vessels were few [2,3,4, 14]. Despite the presence of binucleated and trinucleated cells, giant cell fibroma was not considered, since it usually measures less than 1 cm .
The histopathological findings were similar to focal fibrous hyperplasia; however, it was excluded due to large size of the lesion and absence of trauma. Focal fibrous hyperplasia usually does not exceed 1.5 cm and is elicited by trauma .
Considering the bilateral nature of the present lesion, we considered desmoid tumor, which can be multiple in familial cases . Yet, it was excluded due to the paucicellular nature of the current lesion, no interlacing of collagen bundles, and lack of infiltration at the margins.
In our case, there was infiltration of the surrounding fat tissue in accordance with the findings of Miettinen and Fetsch .
Immunohistochemistry is a very important diagnostic method that can help to reach a definitive diagnosis by excluding lesions, thereby guiding management. In the present case, the tumor cells were strongly positive to vimentin, supporting its fibroblastic origin. To differentiate this tumor from other tumors of different origins, we have done immunohistochemical staining by desmin, S-100 and CD34. There were negative reactions in cells, excluding muscle, nerve, and vascular tumors. Our results were similar to previous studies [5, 10, 14, 22]. In contrast to these studies, SMA staining was negative in our study. Some studies also showed negative SMA staining [9, 10, 23]. To rule out desmoid tumor, we did beta-catenin immunostaining . Negative result favored the diagnosis of collagenous fibroma.