A 52-year-old man underwent a magnetic resonance imaging of the sacrum to further study an asymptomatic sacral bone lesion that had been incidentally detected on a computed tomography study performed for other reasons.
Abdominal X-ray shows poor delineation of the upper left neural foramina margins, without other relevant osseous findings (Fig. 1).
CT image depicts an eccentric sacral bone lesion with predominant low attenuation of fat (Fig. 2), dystrophic calcification and prominent osseous trabeculae within the lesion, which is surrounded by a thin rim of sclerotic bone. The lesion causes bone expansion of the sacral ala without disrupting the cortex and extends to the articular surface of the sacroiliac joint without joint involvement (Fig. 3).
MR images show signal intensity isointense to subcutaneous fat on both T1 and T2-weighted images, hypointense bony trabeculae interspersed throughout the lesion, scattered irregular internal hypointense calcifications and a small cystic area consistent with fat necrosis.
STIR and fat-saturated T1-weighted and T2-weighted images show homogeneous suppression of the signal intensity consistent with the adipose composition of the lesion.
No associated extraosseous soft-tissue mass nor bone-marrow oedema are seen on MR images (Fig. 4 and 5).
Intraosseous lipoma is a primary bone tumour composed of mature adipocytes. It has been traditionally considered as a rare lesion although it is probably much more common than previously reported. Its diagnosis has increased in the last years due to the increasing use of MR and CT imaging techniques [1].
It can appear at any age but is more commonly discovered in middle-aged patients. It is slightly more frequent in males.
The commonest location of intraosseous lipoma is the lower limb [2], mainly the intertrochanteric region of the femur, tibia and calcaneus, but may also appear in other locations. There are very few reported cases of intraosseous lipomas located in the sacrum.
Intraosseous lipomas can cause symptoms such as pain or swelling, but frequently they are incidental findings [1], as in our case.
The imaging findings of this tumour depend on its histological composition [3].
Milgram classified intraosseous lipoma in three categories [4]:
Stage 1: solid lesion of viable lipocytes
Stage 2: areas of fat necrosis and calcification with persistent viable lipocytes
Stage 3: complete or near-complete necrosis of the lesion wit variable amounts of calcification, cyst formation and reactive new bone formation.
The radiographic appearance is that of a well-defined benign-appearing osteolytic bone lesion, often with a thin sclerotic rim [2]. Expansile remodelling of bone might be seen. The tumour may be associated with calcifications or ossification that might be quite extensive. Peripheral osseous ridges may be seen within the lesion and be responsible for a septated appearance.
CT and MR imaging demonstrate the fatty component of the lesion, which is diagnostic for intraosseous lipoma [2] and distinguishes it from other tumours. The fatty component shows fat attenuation on CT images and fat signal intensity similar to that of subcutaneous fat on MR images. Variable degree of peripheral or central calcification or ossification might be seen [2]. With involution, fibrous proliferation and cystic degeneration may develop and might be the predominant finding. Cystic areas show low attenuation on CT images, fluid signal intensity on MR images and may be surrounded by a rim of ossification.
Our lesion shows typical features of fat tissue on CT and MR images, so it is consistent with the diagnosis of intraosseous lipoma.
Malignant degeneration is rare [5]. Surgical treatment is usually not necessary but it may be required for symptomatic lesions.
Intraosseous lipoma of the sacrum
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1. On the CT images, there is a localized bone lucent lesion in the sacrum with well-defined margins, and a thin sclerotic rim can be observed at the edge. The lesion shows a fat-equivalent density (CT values around -80 to -100 HU), along with a small amount of patchy or linear high-density areas, possibly suggesting mild calcification or ossification.
2. MRI indicates that this sacral lesion demonstrated high signal intensity on T1-weighted images, similar to subcutaneous fat, and also showed high signal intensity on T2-weighted images, displaying typical fat signal characteristics. There are slight septations and scattered high-signal foci, suggesting possible fibrous or necrotic components locally.
3. Overall, the lesion has relatively clear boundaries, with no obvious cortical bone destruction or signs of significant invasion into surrounding soft tissues. No apparent soft tissue mass formation is noted.
4. According to the patient’s provided imaging data, there is no indication of fracture or other significant lesions. The surrounding anatomical structures maintain relatively normal relationships.
Based on the above imaging features and the patient’s medical history, the possible diagnoses or differential diagnoses include:
1. Intraosseous Lipoma: A benign bone tumor composed of mature adipocytes. Imaging typically shows fat density (on CT) or signals similar to subcutaneous fat on MRI, possibly accompanied by calcification, ossification, or necrosis. It is often asymptomatic or presents with mild pain.
2. Cystic or Fatty Degenerative Lesions of Bone: For instance, a giant cell tumor of bone in its late stage might show some fatty changes, but usually not as typical; they often present with more obvious bone destruction or other characteristic findings.
3. Fatty-Fibrous Bone Lesions (e.g., fat-myxoid fibrous tumors): However, these lesions often display more complex soft tissue components or more pronounced bone changes on imaging.
Therefore, considering the typical fat density/signal, clear margin, and minimal impact on surrounding structures, intraosseous lipoma is more consistent with both the clinical and imaging findings.
Taking into account the patient’s age, the lesion’s typical fat signal features, a sclerotic rim, and the lack of significant symptoms, the most likely final diagnosis is: Intraosseous Lipoma.
Since the patient currently has no clinical symptoms (or only mild symptoms), if there are no special circumstances (such as severe pain or suspected malignant transformation), no further surgical intervention is usually required. Regular follow-up is recommended. If pain worsens or other abnormalities arise, a biopsy or further evaluation may be considered.
1. Observation and Follow-up:
- For asymptomatic or mildly symptomatic patients, routine outpatient visits and periodic imaging (e.g., follow-up CT or MRI every 6–12 months) are recommended to monitor any significant increase in lesion size or structural changes.
- If significant pain, signs of skeletal instability, or suspected malignant transformation occur during follow-up, prompt medical evaluation is advised.
2. Surgical Intervention:
- If there are significant symptoms or a suspicion of malignant transformation (extremely rare), surgery (e.g., curettage, pathological examination, or bone grafting) can be considered.
- In most cases, for a confirmed intraosseous lipoma without symptoms, routine surgical intervention is not recommended.
3. Rehabilitation and Exercise Plan:
- General Principle: For patients with potential bone lesions, exercise prescription should proceed gradually and be individualized, focusing on safety and stability.
- Initial Phase (Preoperative or Routine Maintenance):
· Engage in low-impact activities, such as walking on level ground, using a stationary bike, practicing Tai Chi, or gentle yoga, 3–5 times a week, about 30 minutes each session.
· Emphasize posture and core muscle stability training, such as simple core contraction exercises, avoiding high-load squats or running and jumping.
- Progressive Phase:
· If a physician evaluates good skeletal stability, gradually add light resistance training (such as small-resistance band exercises or light dumbbells). Perform 8–10 exercises per session, 8–12 repetitions each, for 2–3 sets, ensuring correct form and a steady pace.
· Consider moderately extending aerobic exercise to 40–60 minutes per session, still focusing on low-impact or moderate-intensity exercises (e.g., swimming, elliptical trainer).
- Special Considerations:
· If the patient experiences notable lumbar or sacral discomfort or pain, discontinue related exercises immediately and seek re-evaluation.
· Avoid high-impact, heavy-load strenuous activities to reduce the risk of fracture.
· If the patient has other comorbidities (e.g., compromised cardiopulmonary function, osteoporosis), consult specialists and adjust exercise intensity and type accordingly.
- Monitoring and Adjustment:
· It is recommended to keep an exercise log and observe the body’s response weekly. If pain or discomfort appears, reduce the amount of exercise or pause, and consult a doctor.
· Gradually increase resistance based on physical fitness and pain levels, aiming to maintain strength and flexibility within a safe and controlled range.
Disclaimer:
This report provides a preliminary analysis based on limited patient data and imaging findings. It is for clinical reference only and cannot replace an in-person consultation or professional medical opinion. If there are any questions or changes in the patient’s condition, please consult a specialist or visit a hospital promptly.
Intraosseous lipoma of the sacrum
