Infiltrating Lobular Carcinoma of the Breast

Differential Diagnosis

Infiltrating Ductal Carcinoma	Infiltrating Lobular Carcinoma
Infiltration in cords of varying thickness	Single file infltration
May form ductal structures	No duct formation
E-cadherin positive	E-cadherin negative

The utility of E-cadherin in borderline cases is currently uncertain

Tubulolobular Carcinoma	Infiltrating Lobular Carcinoma
Contains a tubular component	Uniform single file pattern
E-cadherin positive	E-cadherin negative

Some consider tubulolobular carcinoma to be a variant of lobular carcinoma

Early stromal invasion in lobular carcinoma can be difficult to distinguish from LCIS involving sclerosing adenosis

LCIS in Sclerosing Adenosis	Infiltrating Lobular Carcinoma
Circumscribed, nodular	Infiltrating, not confined to nodule
Myoepithelial cells present	Myoepithelial cells absent in infiltrating areas

Lymphocytes and histiocytes in breast stroma and lymph nodes can be difficult to distinguish from lobular carcinoma

Inflammation	Infiltrating Lobular Carcinoma
No molding of nuclei	Molding of nuclei
Single file pattern unusual	Single file pattern common
No mucin positive signet ring cells	Mucin positive signet ring cells may be present
Keratin negative	Keratin positive

Lymphoma may be diffcult to distinguish from alveolar and solid variants of lobular carcinoma

Lymphoma	Infiltrating Lobular Carcinoma
No molding of nuclei	Molding of nuclei
Lymphoid markers positive	Keratin positive

Mucinous carcinomas are characterized by cells floating in pools of mucin
Signet ring carcinomas are characterized by cells with intracellular mucin that indents the nucleus
Signet ring cells can be found in otherwise mucinous carcinomas
- If at least 20% of cells are signet ring, designate as signet ring carcinoma
  - Prognosis is that of usual infiltrating carcinoma of identical grade
  - Signet ring carcinoma is usually a variant pattern of lobular carcinoma
- If <20% of cells are signet ring, designate as mucinous carcinoma with signet ring cells
  - Prognosis may not be worse than pure mucinous carcinoma

Signet Ring Carcinoma	Secretory Carcinoma
One or few vacuoles that indent nucleus	Abundant granular to clear cytoplasm
May be nuclear grade I or II	Low nuclear grade
Often associated with classic lobular carcinoma pattern	No association with lobular carcinoma
No predilection for young patients	Most cases <30 years
May show aggressive behavior	Excellent prognosis

Signet ring carcinoma is usually a variant pattern of lobular carcinoma

Breast vs. other origin in carcinoma of unknown primary