33 Special Types of Invasive Breast Carcinoma: Diagnostic Criteria ...

carcinoma should be restricted to those tumors containing large amounts of
extracellular mucin (1). “Large amounts” of extracellular mucus has been arbitrarily
defined as more than 1/3rd of the tumor volume made up of extracellular mucin (2-4).
Some have insisted on a 50% or more mucinous growth pattern before the diagnosis
of mucinous breast carcinoma can be made; others require that at least 75% of the
tumor have the distinctive mucinous pattern (3,5). I find these arbitrary percentages
difficult to apply and even unrealistic. Do these definitions include the in situ
component, which may be prominent and sometimes difficult to separate from the
earliest phases of stromal invasion? Should we apply automated image analysis
routinely? How many tumor sections should we examine? I insist on a pure pattern of
classical invasive mucinous carcinoma before making the diagnosis.
Examples composed in part of areas more c/w InvDC,NOS are best diagnosed as
“mixed ductal (NOS) and mucinous carcinoma” or “invasive ductal carcinoma (NOS)
with marked mucinous differentiation,” and I histologically grade the tumor using
either a mBR or SBR grading scheme. The admixture of InvDC,NOS with classical
mucinous carcinoma worsens the prognosis and requires the designation “mixed
ductal (NOS) and mucinous carcinoma.” “InvDC,NOS with marked mucinous
differentiation” features marked extracellular mucin deposition, but nowhere in the
invasive tumor does the volume of mucin comprise more than 1/3rd of the tumor
volume. It, too, has a worse prognosis than pure mucinous carcinoma.
Pure mucinous carcinoma has been shown to have a favorable prognosis in many
studies (6-11). Pure mucinous carcinomas tend to be smaller than tumors that have
mixed mucinous and ductal patterns, and patients with these tumors have a lower
frequency of axillary lymph node metastases (2,3,6,8,9,12). Negative axillary lymph
nodes in patients with pure mucinous carcinoma range from 71% to 97% compared to
50% for patients with mixed mucinous carcinomas. Positive lymph nodes have been
reported in pure mucinous carcinoma especially in cases with micropapillary pattern
and in younger age (39). Therefore, sentinal lymph node biopsy and staging is
recommended even for pure varaints (39).
Five-year disease-free survivals, after the treatment of pure mucinous carcinoma by
mastectomy, are from 84-100% (3,6,8). Patients with mixed mucinous and ductal
histologies do more poorly. Komaki et al. (12) reported a 90% 10-year overall
survival for pure mucinous carcinoma versus 60% for those with mixed
ductal/mucinous carcinoma. Toikkanen et al. (9) reported that the 15-year disease-free
survival was 85% and 63% for pure mucinous and mixed mucinous/ductal carcinoma
patients, respectively. Nonetheless, late systemic recurrences can occur with pure
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