Lipoma is the most popular benign tumor that need to be distinguished from well-differentiated liposarcoma. We had a case with large lipoma at the deep layer under deltoid muscle on proximal of humerus, and made resection. We discussed the surgical approach for that lesion. [case] 58 years old, male. 5 years ago he noticed a mass about his right shoulder, he came to our hospital because it gradually enlarged. An elastic-hard mass anterior right shoulder, enlarged with abduction. X-ray imaging had no abnormality. MRI revealed a mass, about 5 x 10 x 9 cm, with clear border, T1/T2 high/high intensity, homogeneous pattern just like adipose tissue. It grew so slowly that we diagnosed lipoma, made marginal resection with division of deltoid muscle longitudinally. The pathological diagnosis was lipoma. Generally, large sized soft tissue tumors may have malignancy. In this case, benign lipoma was most suspected with MRI, the physical course. But because the size was large, well-differentiated liposarcoma was possible. The surgical approach for tumor should be the shortest distance, so we performed an operation with longitudinal division of the deltoid muscle. But if lipoma is strongly suspected with imaging or the physical course, we should approach the mass inverting deltoid muscle to avoid axial nerve injury.
