An unusual breast malignancy with central cystic lesion: Important Sampling artifact related pitfall

QUESTION

What is the cytological interpretation on FNA smears of the larger lump?

1. Cystic lesion with squamous metaplasia

2. Squamous metaplasia secondary to infarcted fibroadenoma

3. Infarcted phyllodes tumor with squamous metaplasia

4. Squamous cell carcinoma of the breast

The correct cytological interpretation is:

a. Cystic lesion with squamous metaplasia.

EXPLANATION

USG-guided FNA from the smaller lump (upper outer quadrant) showed features of proliferative breast disease with the presence of stromal fragments and clusters of ductal epithelial cells showing focal mild anisonucleosis in a background of bipolar myoepithelial cells [Figure 2a]. FNA smears from the larger lump (central quadrant) showed a few inflammatory cells, an occasional cluster of benign squamous cells and histiocytes. Diligent search failed to reveal any features suggestive of malignancy. Hence, a cytological impression of proliferative breast disease with cystic change and squamous differentiation was rendered. In view of the patient’s age, size of the lump and mild anisonucleosis in aspiration smear, a core biopsy was performed. The core biopsy of the larger lump showed benign breast tissue and a tiny island of neoplastic squamous cells that were negative for CK7. In view of the suspicious features on core biopsy, a modified radical mastectomy was performed. The excised specimen showed a large mass with central cystic zone occupying almost the entire breast parenchyma with unremarkable skin, nipple, and areola.

Histopathological examination revealed features of a well-differentiated squamous cell carcinoma (SCC) comprising almost 100% of the tumor area [Figure 2b]. One of the ducts in this area had metaplastic squamous lining displaying transition to frank SCC [Figure 2c]. The peripheral areas of the mass showed fibroadenomatoid change with foci of atypical ductal hyperplasia (ADH) and focal ductal carcinoma in situ (DCIS) adjoining the areas of SCC. On immunohistochemistry, the tumor was triple-negative (estrogen receptor [ER], progesterone receptor [PR], and Her2neu) as well as negative for CK7 while being positive for pan-CK and CK5/6 [Figure 2d]. A thorough clinical examination and follow-up positron emission tomography computed tomography (PET-CT) scan failed to detect any other primary site; hence, the final diagnosis was rendered as primary squamous cell carcinoma of breast (SCCB) with coexisting spectrum of proliferative breast disease encompassing ADH and DCIS. The patient was referred for chemotherapy. She was negative for relapse or distant metastasis during 12 months follow-up.

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Figure 2:

Histopathological section shows the invasive squamous cell carcinoma component (a, H&E x100). A central cystic zone with squamous metaplasia (sq meta) and transformation to SCC and infiltration denoted as I is also seen (b, H&E x 40). The focus shown in ‘b’ is highlighted by immunohistochemistry for CK5/6 (c, Labeled Streptavidin-Biotin method x 40).

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ADDITIONAL QUIZ QUESTIONS

Q1. The cytomorphologic features on FNA of smaller breast lump are suggestive of:

1. Infiltrating duct carcinoma of breast

2. Fibrocystic disease of the breast

3. Proliferative breast lesion

4. Ductal carcinoma in situ.

Q2. On immunohistochemistry of a breast lesion, which of the following suggest a diagnosis of SCC?

1. Negative for CK 7 expression

2. Negative for ER PR and HER2 Neu

3. Positive for Pan-CK and CK5/6 expression

4. All of the above.

Q3. Which of the following possibilities should be considered in a FNA of breast mass showing atypical squamous cells

1. Infarcted fibroadenoma

2. Epidermoid cyst

3. Phyllodes tumor with metaplasia of canalicular lining cells

4. SCC

5. All of the above.

Answers to the quiz questions:

Q1. The correct answer is c (proliferative breast lesion)

Q2. The correct answer is d (all of the above)

Q3. The correct answer is e (all of the above).

The explanations of these questions follow in the next section.

DISCUSSION

Primary SCCB is very rare, accounting for <1% of all invasive breast carcinomas. There have been only sporadic case reports and a few mini-case series reported in the literature.^[1-5] SCCB most commonly presents in elderly patients with a palpable breast lump, which is sometimes bigger than 5 cm and may mimic a breast abscess.^[5] Similar to the basal-type breast cancer, majority of the cases of SCCB are triple negative with a cumulative 5-year survival rate of 51–68%.^[6] SCCB has been defined by the WHO as a subtype of metaplastic carcinoma showing atypical squamous cells.^[7] Rosen recommends that carcinomas with more than 90% of the component as squamous should be categorized as SCCB.^[8] However, few reports in literature have classified tumors with squamous component of 50–100% or spindle cell variant in this category leading to confusion in the true incidence of primary SCCB.^[2] We have utilized the Rosen’s criteria for defining our case as SCCB.

Macia et al. stated that a primary SCCB should meet all of the three criteria: (1) Absence of other neoplastic components like ductal or mesenchymal, in the tumor; (2) tumor independent of the adjacent cutaneous structures; and (3) no other primary epidermoid tumors in the patient.^[9] Our case met these three criteria and hence was considered as primary SCCB. The proposed hypotheses for the histogenesis of SCCB include origin from keratinous cyst or chronic abscesses or metaplastic glandular tissue.^[10] The tumor in our report demonstrated a focus of metaplastic lining transiting into frank SCC. We had another case of an elderly woman with initial mammographic findings of a cystic lesion and FNA showing benign squamous cells and inflammatory cells, similar to the present case. A repeat mammogram was reported as BIRADS3/4 and a core biopsy showed features of SCC. Histopathological examination of the mastectomy specimen and PET-CT scan confirmed a diagnosis of primary SCCB, possibly having originated from a dermoid cyst.

Cytologic diagnosis of SCCB has been cited in the literature as rare case reports only.^[6,11-13] The characteristic cytologic findings include malignant squamous cells in loose cohesive clusters or lying singly, tadpole cells, keratin pearls, and cellular keratinization, similar to SCC of other body sites.^[12] An admixture of malignant squamous cells with glandular or mesenchymal component should prompt a cytologic diagnosis of mixed carcinoma or metaplastic carcinoma.^[14] Rarely, SCCB has also been reported as an intracystic tumor, postulated to originate through metaplasia of the cyst wall.^[15] SCCB usually presents as a large tumor with central cystic degeneration in over half of the cases.^[10]

In the current case, the tumor had cystic component. However, the cyst fluid revealed benign-appearing squamous cells with inflammatory cells, leading to a diagnostic pitfall on cytology. This case highlights the problem of sampling artifact, if enough sampling is not performed from all suspicious areas of the cystic lesions. At least three adequately cellular aspirates should be sampled from the periphery of the cystic lesions with ill-defined periphery. The sensitivity and specificity of FNA in diagnosis of breast masses have been variably reported as 94–99% and 99–100%, respectively.^[16]

FNA forms an integral component of the triple test used widely in the evaluation of breast lesions. The diagnostic accuracy of triple test has been reported to be 100% if all the three tests are concordant for the benign or malignant diagnosis.^[17] A study utilizing a triple test score showed that only 8% of the masses received a score of 5 (equivocal) and required a confirmatory biopsy.^[18] Our patient had benign clinical, mammographic, and FNA findings, suggesting that the triple test was false negative.

Mere presence of atypical squamous cells in the FNA smears is not sufficient to confer a diagnosis of SCCB since similar cells may be seen in epidermoid cysts, metaplasia secondary to infarction of fibroadenoma or papilloma, phyllodes tumor with intracanalicular metaplastic lining cells, and subareolar abscess (Zuska’s disease).^[11] Subareolar abscess, a rare benign lesion of the breast, usually presents with recurrent nipple discharge, mastalgia, and breast masses with nipple retraction. Radiologic differentiation from a malignant lesion of the breast may be difficult in such cases and histopathologic examination of the excised sinus or fistula tract provides the correct diagnosis.^[19] Cases showing malignant squamous cells in breast FNA need to be differentiated from ductal carcinoma with squamous component and carcinosarcomas.^[10] Considering the criteria defined by Marcia et al. for primary SCCB, FNA can only suggest a diagnosis of SCCB, which needs to be confirmed after adequate histopathologic examination of the excised tumor.^[10] Another differential diagnosis to be considered is apocrine carcinoma of the breast, which demonstrates granular cytoplasm, low nuclear-cytoplasmic ratio, and enlarged nucleolus. Streaming arrangement of cells in a cluster, less conspicuous nucleoli, cannibalism, and necrotic background has been shown to be characteristic cytologic features of SCCB assisting in its differentiation from apocrine carcinoma.^[6]

The optimal therapy for SCCB is surgery (breast conserving or mastectomy) with or without axillary lymphadenectomy. The role of post-operative adjuvant chemo or radiation therapy is as yet not known.^[20]

SUMMARY

Primary SCC of the breast is a rare subtype of breast cancer. The clinical, mammographic, and aspiration cytologic features of this tumor may be misleading, especially in cases presenting with a cystic mass. Presence of benign-appearing squamous cells in cystic lesions should suggest better sampling to detect associated invasive carcinoma. This case highlights the problem of sampling artifact, if enough sampling is not performed from all suspicious areas of the cystic breast lesions with squamous cells. At least three adequately cellular aspirates should be sampled from the periphery of the cystic lesions with ill-defined periphery.