OBJECTIVE, We studied the appearance of the sternalis muscle on mammog
raphy and on CT and MR imaging,To our knowledge, this is the first des
cription of this normal anatomic variant, An approach is provided that
permits differentiation of the sternalis from significant pathology.
SUBJECTS AND METHODS, Between January 1992 and December 1994, four wom
en of an estimated 32,000 who had mammograms at the Massachusetts Gene
ral Hospital had an unusual, irregular structure visible medially on t
he craniocaudal projection that posed a diagnostic dilemma. The record
s and imaging studies of these women and two others from the Deaconess
Hospital breast imaging program were reviewed to determine the etiolo
gy of the findings seen by mammography and to establish a diagnostic a
pproach, RESULTS, Surgery in one patient and cross-sectional imaging i
n the other five established that the structure was the sternalis musc
le, Although it may be bilateral, the sternalis muscle was visible onl
y unilaterally on the mammograms of these six women,The appearance of
the muscle ranged from an irregularly rounded density at the sternal e
dge of the film to flame-shaped and almost completely surrounded by fa
t. CT and MR imaging are diagnostic when they show the longitudinal ex
tent of the muscle, which lies anterior to the medial margin of the pe
ctoralis major muscle. CONCLUSION. The sternalis muscle is an unusual
variant of the chest wall musculature, It may be Visible as a rounded
or irregular density on the craniocaudal mammogram along the sternal e
dge of the film. With improved mammographic positioning it will be see
n more frequently, The muscle has a variety of appearances that should
be familiar to the radiologist to avoid confusion with a malignant le
sion, The etiology can be confirmed and cancer excluded by CT or MR im
aging.