The history and images suggest an intramuscular myxoma, a benign tumour that presents as a slow growing deeply seated mass confined to the skeletal muscle.
The keys to this question are the 1) intramuscular location, 2) relatively homogenous appearance on MRI, 3) bright T2/dark T1 signal - suggesting a myxoid tumor, and 4) bland, hypo-cellular myxoid appearing histology that does not show cellular atypia in the relatively homogenous cells without nuclear atypia, mitotic figures, or high nuclear to cytoplasmic ratio.
As described in the referenced review by Prado et al. local surgical excision is the treatment of choice and recurrence is extremely rare. Intramuscular myxomas are commonly located in the thigh, shoulder, buttock, or upper arm. Multiple intramuscular myxomas are usually associated with monostotic or polyostotic fibrous dysplasia, also known as Mazabraud's syndrome.
Prado MA, Miró RL, Leal IM, Vargas J, Dorrego EJ. Intramuscular myxoma: differential diagnosis and review of the literature. Orthopedics. 2002 Nov;25(11):1297-9. Review.
PMID:12452352 (Link to Abstract)
Schwartz HS (ed): Orthopaedic Knowledge Update: Musculoskeletal Tumors 2. American Academy of Orthopaedic Surgeons, Rosemont, IL. 2007, pp 257-263