1. Parosteal lipoma of proximal radius-A rare case report ABSTRACT ID NO. :IRIA 1094

2. INTRODUCTION  Parosteal lipoma is an extremely rare benign tumor composed mainly of mature adipose tissue with a bony component.  The most common locations for this tumor are the femur, proximal radius, humerus, tibia, clavicle and pelvis.  It affects, almost exclusively, adults over 40 years, of either sex.

3. AIM  It is among the rarest neoplasias of skeleton, accounting for less than 0.1% of primary bone tumors and 0.3% of all lipomas.  To convey its rarity,benignity and extreme rare malignant transformation and complete cure after resection,we are presenting this as case report.

4. MATERIALS AND METHODS  A 80 yr old male patient presented with a painless swelling on the proximal part of the left forearm.  The swelling was a slow growing, painless, nontender, immobile mass which was not fixed to skin.

5.  X-ray showing radiolucent shadow of fat density with bony excrescences adherent to left proximal radius.  CECT showing non enhancing hypodense lesion of fat density with osseous excrescences abutting the left proximal radius.

6.  MRI revealed a large well defined, nonenhancing, heterointense, predominantly fat intensity lesion with a small area of signal loss corresponding to calcification in lateral aspect of proximal left radius, seen completely separate from the adjacent muscles.

7. RESULTS  The tumor was resected along with periosteum and then sent to biopsy.  Biopsy revealed both osteal and fat components.

8. DISCUSSION  Parosteal lipoma is situated directly on the cortex of bone.  Thought to arise from mesenchymal cells in the periosteum, parosteal lipomas share histopathologic features with the commonly occurring soft-tissue lipomas, and cytogenetic evidence suggests a common histopathogenesis

9. Depending on the degree of chondroid modulation and enchondral ossification 1. parosteal lipomas may rest directly on the cortex without cartilage or bone elements; 2. may have a narrow bony stalk with a lucent lipomatous cap, mimicking a pedunculated exostosis;

10. 3.may have a densely ossified broad-based osteochondromatous element beneath a thin lipomatous cap, simulating a sessile exostosis 4.may have patches of chondroid and bone scattered throughout the lipomatous mass.

11.  On CT and MR imaging, parosteal lipomas have a homogeneous lobulated appearance and are adherent to the surface of the adjacent bone.  When present, osseous excrescences may mimic osteochondromas, but the former lack the contiguity of the marrow space with the underlying bone that is characteristic of the latter.

12.  Parosteal lipomas that gain clinical attention are those that compress neurovascular bundles and cause motor and sensory function deficits  Common sites of involvement include the proximal forearm and the sciatic nerve.

13.  The treatment of parosteal lipoma is complete surgical resection along with periosteum.  The nerve must also be separated from the parosteal lipoma and care must be taken to spare it during surgical excision.

14. CONCLUSION These soft tissue tumors are benign with extremely rare malignant transformation and an excellent prognosis with ‘no’ recurrence.

15. X-ray showing radiolucent shadow of fat density with bony excrescences adherent to left proximal radius.

16. CT (series of images)showing hypodense lesion of fat density with osseous excrescences abutting the left proximal radius.

17. T1w image coronal view showing hyperintense lesion with signal void adherent to left proximal radius

18. IRFSE image axial view showing fat suppression with signal void adherent to left proximal radius