The 21-gene recurrence score in special histologic subtypes of breast cancer with favorable prognosis.

Background/purpose: The 21-gene recurrence score (RS) assay predicts the likelihood of distant recurrence and chemotherapy benefit in early-stage, estrogen receptor (ER)-positive, HER2-negative breast cancer. Data on the RS of special histologic subtypes of invasive breast carcinoma with favorable prognosis are limited. Methods: We reviewed our institutional database to identify patients with special histologic subtypes of breast cancer associated with favorable prognosis and available RS results. Our cohort consists of fifty-seven women: thirty-three patients with pure mucinous carcinoma (MC), ten with tubular carcinoma (TC), nine with encapsulated papillary carcinoma (EPC), and five with solid papillary carcinoma (SPC). Results: Most (44/57, 77.2%) carcinomas had low RS (≤17), and none had high RS (≥31). All EPCs had low RS, but other subtypes had RS 18-30. Higher RS was associated with lower progesterone receptor (PR) expression by immunohistochemistry and lower PR mRNA scores ( P ≤ 0.007). No morphologic feature (tumor grade, biopsy site changes, cellular stroma, inflammatory cells) was associated with RS ≥ 18. At a median follow-up of 40 months, the distant recurrence-free survival was 100%. One patient with SPC developed locoregional recurrence at 22 months. Conclusions: As the largest series to date, our study raises the question of whether the RS assay is necessary for breast cancers with favorable histology. Reflex testing of node-negative, ER+/HER2− breast cancers may be deferred for these special histologic subtypes, emphasizing the need for multidisciplinary discussions between breast pathologists and other members of the breast cancer team. [ABSTRACT FROM AUTHOR]