Invasive breast carcinoma - IARC

Microinvasive carcinoma
I.O. Ellis
F.A. Tavassoli
Definition
A tumour in which the dominant lesion is
non-invasive, but in which there are one
or more clearly separate small, microscopic
foci of infiltration into non-specialized
interlobular stroma. If there is
doubt about the presence of invasion,
the case should be classified as an in
situ carcinoma.
ICD-O code
Microinvasive carcinoma is not generally
accepted as a tumour entity and does
not have an ICD-O code.
Epidemiology
Microinvasive carcinomas are rare and
occur mostly in association with an in situ
carcinoma. They account for far less than
1% of breast carcinomas even in pure
consultation practices where the largest
number of microinvasive carcinoma is
reviewed {2680}.
Clinical features
There are no specific clinical features
associated with microinvasive carcinoma.
These lesions are typically associated
with ductal carcinoma in situ which is
often extensive. The features associated
with the associated in situ component
are responsible for detection as a mass
lesion, mammographic calcification or a
nipple discharge. (See clinical features
of ductal and lobular carcinoma in situ).
Histopathology
There is no generally accepted agreement
on the definition of microinvasive
carcinoma. This is particularly true for the
maximum diameter compatible with the
diagnosis of microinvasive carcinoma.
exceeding 1 mm in maximum dimension.
Some studies have provided no maximum
size {2579,3140} or criteria {1467,
2703}. Others have defined the microinvasive
component as a percentage of
the surface of the histologic sections
{2583}. Some have described subtypes
separating those purely composed of
single cells and those also containing
cell clusters and/or tubules of non-gradable
tumour without providing inform a-
tion about maximum size, extent, or
number of microinvasive foci {656}.
M o re precise definitions accept an
A
unlimited number of clearly separate
foci of infiltration into the stroma with
none exceeding 1 mm in diameter {80},
1 or 2 foci of microinvasion with none
exceeding 1 mm {2695}, a single focus
not exceeding 2 mm or three foci, none
exceeding 2 mm in maximum diameter
{2680}.
Some authors propose that the definition
of microinvasive carcinoma re q u i re s
extension of the invasive tumour cells
beyond the specialized lobular stro m a
{774,2905} despite the definitive pre s-
ence of vascular channels both within
Size limits
M i c roinvasive carcinoma has been
defined as having a size limit of 1 mm
{1984,2425,2739,2905}. Consequently,
diagnosis of microinvasive carcinoma is
rare in routine practice, in contrast to
larger (>1 mm) foci of invasion.
Alternatively, it has also been defined as
a single focus no larger than 2 mm in
maximum dimension or 2-3 foci, none
B
Fig. 1.96 Microinvasive carcinoma. A A small focus of invasive carcinoma barely 0.8 mm in maximum extent
is present adjacent to an aggregate of ducts displaying mainly flat epithelial atypia. B Immunostain for actin
shows no evidence of a myoepithelial (ME) cell layer around the invasive tubules, in contrast to persistence of
a distinct ME cell layer around the adjacent tubules with flat epithelial atypia.
74 Tumours of the breast