A woman, 52 years old, presents in the emergency department complaining of intense low back pain radiating to the right leg, which became more intense in the last 24 hours, preventing her from walking. She also describes milder same symptoms of pain in the last seven months. The patient refers no other remarkable general health problems.
CT images from the lumbar spine show right disc herniation at the L5–S1 intervertebral space, a finding that explains the symptoms. It is also noticed as incidental finding a lytic lesion on the central and right part of the depicted sacrum.
CT images from the sacroiliac region delineate the location of the lesion, which extends at the central and right parts from I1 to I5 vertebra. On CT, a lytic lesion with soft tissue density is observed replacing normal bone marrow and causing cortical destruction (Figures 1a, 1b and 2).
On MR imaging, the lesion shows low signal intensity on T1 and T2 images in a large area (>75% of lesion) (Figures 3 and 4). In fat sat images, it has a heterogeneous signal consisting of areas with both low and mildly high signal intensity (Figure 5). The lesion has heterogeneous enhancement with the areas of high signal on T2 corresponding to more intense enhancement (Figure 6). No invasion of the spinal nerve roots is observed; instead, retention of intraforaminal fat planes is noticed (Figures 3 and 4).
Desmoplastic fibroma is a rare benign primary fibrous bone tumour with an incidence of 0.1% among all primary bone tumours [1]. There are only few reports presenting the diagnosis of desmoplastic fibroma and describing the relative imaging characteristics on CT and MR imaging [2].
It is a slow-growing tumour with potential locally aggressive behaviour affecting more often the metadiaphysis of large long bones, the mandible and the pelvis [1–3]. It is seen more often in the third to fourth decade of life and has no sex predilection [1,4].
It may be asymptomatic, but usually it is related to long-standing regional pain and swelling [4,5].
Histologically, it consists mainly of fibrous tissue and is similar to the soft tissue desmoids tumour [1,2,4].
On CT imaging, it is usually osteolytic in appearance, typically with no mineralized matrix [1,4]. Lobulated margins, mild bone expansion and coarse internal trabeculations are indicative of a slow-growing process causing uneven bone destruction [2]. Other usual imaging characteristics are soft tissue density, cortical breaching and pronounced cortical destruction [1,2,4].
On MR imaging, the tumour typically shows a large area of low to intermediate signal on both T1 and T2 images, indicative of its fibrous composition, but heterogeneous signal intensity on T2 fat sat images [1,2,4]. The regions with compact collagen tissue have lower signal intensity on T1 and T2 images and mild or no enhancement. Instead, regions with more cellular tissue present with higher T2 signal intensity and more intense enhancement [2].
Pathologic fracture is responsible for high signal intensity on T2 due to the oedema [2].
Lesions with osteolytic appearance and sites of low signal on T2 have limited differential diagnosis, and desmoplastic fibroma must always be included [1].
The tumour has a high propensity for recurrence after surgical removal; that is why surgical resection with extended margins is the preferred treatment [1,2,4].
In our case, the findings cannot exclude metastatic bone lesion, so the patient is submitted to abdominal and thoracic CT imaging in order to exclude a primary malignancy.
The tumour biopsy reveals desmoplastic fibroma (Figure 7).
The patient is conservatively treated with analgesia and cortisone for acute pain relief related to intervertebral disc herniation.
As surgical resection in the region of the sacrum poses a considerable reconstructive challenge [4], a follow-up with MR is recommended six months later, which shows no differentiation of the size and imaging characteristics of the tumour. The herniated disc part on L5–S1 space is reduced in size.
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Desmoplastic fibroma
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Based on the provided CT and MRI images, a localized low-density lesion is observed on the right side of the sacrum (a destructive lytic lesion on CT with relatively distinct margins). Mild expansion of the lesion margin and partial bone structure destruction can be seen. No obvious calcification or sclerotic changes are apparent, although some areas show irregularly thickened trabeculae. On MRI, the lesion appears hypointense to isointense on T1-weighted images and demonstrates heterogeneous intermediate to slightly low signal on T2-weighted images, with visible soft tissue components, suggesting local tumor invasion and a relatively high proportion of fibrous tissue. In addition, a mild protrusion of the L5-S1 intervertebral disc is observed on MRI, which may be related to the patient's lower back and leg pain.
Considering the patient's clinical symptoms (long-term chronic lumbosacral pain that has recently worsened), imaging characteristics (lytic bone destruction with no calcification, predominantly low to intermediate T2 signal), and pathological findings (fibrous tissue proliferation), this lesion is highly suspected. This tumor is benign yet locally aggressive, capable of causing local bone destruction, consistent with the imaging features.
In the presence of a lytic bone lesion or destructive bone lesion, metastatic tumors should be considered, particularly in middle-aged to older patients. However, due to the localized range of this lesion, the lack of typical metastatic indications, and absence of a primary lesion on follow-up examinations (such as chest and abdominal CT), the likelihood is reduced.
For example, fibrous dysplasia can lead to bone destruction via abnormal fibrous structure. Typically, more classic “ground-glass” changes or localized bone abnormalities are observed. In this case, the imaging findings are more suggestive of desmoplastic fibroma.
Taking into account the patient’s age (52-year-old female), the long history of lower back and right lower limb pain, imaging showing a lytic lesion in the sacrum, and subsequent pathological assessment, the most likely diagnosis is desmoplastic fibroma (致密性纤维瘤).
As this tumor is benign but locally aggressive, and the patient’s acute exacerbation of pain partly stems from disc herniation compressing the nerve, the following recommendations are provided:
If imaging follow-up shows progressive tumor enlargement, or if severe neurological symptoms or local pain aggravation occur, surgical resection can be considered. However, reconstruction of the sacral region can be difficult, and a specialized assessment is required.
Disclaimer: This report is for medical reference and discussion purposes only and does not replace in-person consultation or professional medical treatment advice. If you have any questions or if symptoms worsen, please seek medical attention promptly.
Desmoplastic fibroma
