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Case number:	171
Age:	38
Clinical presentation:	Premenopausal woman with average risk of developing breast cancer presented with a painful right breast lump noticed a few days ago. On examination, a mobile palpable lump (3 × 2 cm) with smooth margins was noted in the inferior quadrant of the right breast below the areolar margin.

Mammography:

Breast composition:	ACR category c (the breasts are heterogeneously dense, which may obscure small masses)
Mammography features:
‣ Location of the lesion:	Right breast, lower inner quadrant at 5 o’clock, subareolar region, anterior third, 0.1 cm from the nipple and at 0.2 cm skin depth
‣ Mass:
• Number:	1
• Size:	3.5 × 2.5 cm
• Shape:	Oval
• Margins:	Circumscribed with perilesional halo
• Density:	Fat-containing
‣ Calcifications:
• Typically benign:	None
• Suspicious:	None
• Distribution:	None
‣ Architectural distortion:	None
‣ Asymmetry:	None
‣ Intramammary node:	None
‣ Skin lesion:	None
‣ Solitary dilated duct:	None
‣ Associated features:	None

Ultrasound:

Ultrasound features: Right breast, lower inner quadrant at 5 o’clock, subareolar region, 0.1 cm from the nipple and at 0.2 cm skin depth
‣ Mass
• Location:	Right breast, lower inner quadrant at 5 o’clock, subareolar region, 0.1 cm from the nipple and at 0.2 cm skin depth
• Number:	1
• Size:	3.3 × 1.6 cm
• Shape:	Oval
• Orientation:	Parallel
• Margins:	Circumscribed
• Echo pattern:	Anechoic
• Posterior features:	No posterior features
‣ Calcifications:	None
‣ Associated features:	None
‣ Special cases:	Fat necrosis

BI-RADS:

BI-RADS Category: 2 (benign)

Further assessment:

Further assessment advised: Referral for cytology

Cytology:

Cytology features:
‣ Type of sample:	FNAC
‣ Site of biopsy:
• Laterality:	Left
• Quadrant:	Subareolar lump
• Localization technique:	Palpation
• Nature of aspirate:	Scanty, oily fluid
‣ Cytological description:	Smears show adipocytes with bubbly cytoplasm, foamy macrophages and granular debris. Epithelial cells are absent in the smears. A few chronic inflammatory cells are also present.
‣ Reporting category:	Benign
‣ Diagnosis:	Fat necrosis
‣ Comments:	None

Case summary:

Premenopausal woman presented with painful right breast lump. Diagnosed as oil cyst, BI-RADS 2 on imaging and as fat necrosis on cytology.

Learning points:

Oil cyst in the breast is a benign lesion. It is caused by fat necrosis and liquefaction of the necrosed fat. It may occur spontaneously or following breast trauma or surgery. A long-standing oil cyst shows eggshell calcification on mammography, which is a feature that is not seen in simple or complex breast cysts.