Diffuse Intramuscular Lipomatosis of a Lower Limb

Patient. A 40-year-old man presented with a swelling of the left thigh which had been increasing in size over 10 months. Surgery confirmed a diagnosis of lipoma. After 6 months, another swelling appeared, this time in the left calf. Ultrasound-guided biopsies revealed that the tissue showed appearances consistent with intramuscular lipoma. No further surgery was performed and the man is to be reviewed regularly, with possible debulking if necessary. Discussion. This case presents an atypical case of lipomatosis. Magnetic resonance imaging is useful for assessing the extent of the lesion.


Introduction
Intram uscular lipom a of the extremities is well recognized, but received little attention until Regan et al. in 1946 reported two cases. 1 Intram uscular lipom as are benign tum ours, but because of their frequent large size, deep location and in® ltrative, unencapsulated grow th pattern they can sim ulate a soft tissue sarcom a. The recom m ended treatm ent is com plete excision with tum our-fre e soft tissue m argins, with a reported recurrence rate varyin g from 3% to 62.5%. 2± 4 Intram uscular lipom a of the lim bs usually occurs in isolated m uscles, although one case has been reported where two m uscles were involved. 5 D iffu se lipomatosis is a rare condition in w hich usually large portions of an extrem ity or the trunk show diffuse overgrow th of m ature adipose tissue in the subcutaneous tissue, m uscle, fascia and som etimes in bone. T his condition is known to affe ct a single lower lim b, 6± 8 however, we present a case of progressive diffu se intram uscular lipom atosis af¯icting only the m usculature of a single low er lim b.

C ase history
A 40-year-o ld gentlem an presented with an asym ptom atic diffu se swelling over the antero-lateral aspect of his left thigh which had been increasing in size over a period of 10 m onths. Because of its rapid growth, there was som e concern that it m ay be a soft tissue sarcom a. M agnetic resonance im aging (M RI) of the pelvis and upper thighs showed diffu se intram uscular fatty in® ltration present w ithin the entire left tensor fascia lata m uscle as well as in the lower portion of gluteus maxim us, resulting in overall increase in bulk of the affected m usculature (Fig. 1). Less extensive fatty in® ltration was also noted within the upper left ham strings, vastus lateralis and interm edius (Fig. 2). The subcutaneous tissues, rem aining pelvic and thigh m usculature and the right lim b appeared spared. Tissue from an incisional biopsy of the tensor fasc ia lata showed m ature adipose tissue without atypia, consistent with an intram uscular lipom a. As the tissue w as from a large m ass, the possibility of a well-differentiated liposarcom a could not be com pletely excluded. T his m an therefore underwent further surgery to attem pt an excisional biopsy. At surgery, there was diffu se lipom atous tissue in® ltrating the whole of the extensor com partment, which w as only clearable by extensor com partmentectom y. Therefore the lesion was debulked sacri® cing the anterior third of tensor fascia lata and part of rectus fem oris. M icroscopy of the specimen showed m ature lipocytes in® ltrating m uscle in a diffu se manner (Fig. 3). The entrapped skeletal m uscle ® bres showed atrophy in som e areas. T here were no lipoblasts or cells with atypical nuclei. T he diagnosis of an intram uscular lipom a was con® rmed. This m an was regularly reviewed in clinic and there was no further swelling of his left thigh. O ver the follow ing 6 m onths, he noticed an asym ptom atic generalized swelling of his left calf. M RI showed intram uscular fatty in® ltration of the  H owever, a diagnosis of in® ltrating lipom a can be suggested by MRI due to its typical signal characteristics and m orphology ( Fig. 2(b)). Fatty in® ltration of m uscle also occurs in m any neurom uscular disorders but, unlike our case, is usually associated with m uscle wasting and loss of m uscle volume. 9 Because of the rapid growth and size of the tum our in this case, soft tissue sarcom a had to be excluded by thorough m icroscopic exam ination of the biopsy specimens. O nce malignancy had been excluded, the exact nature of surgery m ust be planned. T he m ain sym ptoms, if untreated, would be due to increasing size leading to pain, as a result of local pressure on m uscle, fasc ia and nerves, and the effects of com pression on neurovascular bun

. Axial (a) T1-weighted (b) T2-weighted and (c) short tau inversion recovery (STIR) M R im ages of m id calf reveals in® ltration of the soleus and to a lesser extent the dorsi¯exors.
(l) D iffu se lipom atosis usually presents during the early years of life, although there have been scattered reports of presen tation in ad ultho od. 4 (2) D iffu se lipom atosis is not lim ited to the extremities, the trunk and chest wall being com m only involved, although involvem ent of a single lim b has been recorded. 6 dles. T he only cure is com plete excision. However, this would be totally unacceptable in this gentleman because of the resultant disability, for a condition w hich is benign, where lim itation of growth of the lipom a m ay still occur follow ing partial tum our rem oval and w here the recurrence is usually slow after incomplete excision. 10,11 O nce sarcom a was excluded and the tum our debulked, this gentleman required no further surgery as he was asym ptom atic. H e will require regular follow-up and possib le recurrent debulking if he develops sym ptom s from increasing growth. There is no role for chemotherapy in this condition, and radiotherapy is not advisab le as there is a theoretical risk of m alignant change.
The histological diagnosis of intramuscular lipom a is straightforward, with separation of skeletal m uscle ® bres by m ature adipose tissue. In large lesions, well-differentiated liposarcom a or atypical intram uscular lipom a are excluded by a lack of cellular atypia or m ultivacuolated lipoblasts. Prim ary m uscular diseases that m ay result in fatty replacem ent of the m uscle are excluded by the clinical presentation and the lack of degenerative changes in the m uscle ® bres. N eurological disorders such as poliom yelitis should also be excluded clinically.
There are various form s of in® ltrating lipom a including diffuse lipom atosis, and m ultiple symm etrical lipom atosis. There are several differences between our case and the m ore typical form of diffu se lipom atosis: