Low-grade Adenosquamous Carcinoma of the Breast: Case Report and Literature Review Authors: David Bassa DO, MaKenzie Morrissey MD, Adewuni Ojo MD, Adeel Raza MD Abstract: Introduction: Low-grade adenosquamous carcinoma (LGASC) is a rare variant of metaplastic carcinomas of the breast. It is characterized by its clinical indolence and low-grade cytomorphology. Presentation is typically asymptomatic with indeterminate imaging characteristics with diagnosis dependent solely on histopathology findings. We herein describe a case of LGASC presenting in a young premenopausal female, an extremely rare occurrence. Case presentation: A 23-year-old woman presented to the breast clinic with a left breast ultrasound demonstrating a heterogeneous ill-defined hypoechoic area in the left upper inner quadrant. She stopped breastfeeds three years postpartum and noticed a palpable left breast mass a week later. She was otherwise asymptomatic. A left breast biopsy revealed stromal fibrosis and an ultrasound-guided excisional biopsy revealed sclerosing adenosis and fibrocystic changes. Due to the discordant clinical findings, both biopsies were sent to Weill Cornell Medicine for histopathological review. The review revealed low-grade adenosquamous carcinoma arising within the background of a complex radial sclerosing lesion. The tumor was ER low positive, PR positive, and HER2 negative. Conclusion: LGASC of the breasts remains a diagnostic and therapeutic challenge. Despite being a variant of metaplastic breast carcinomas, which present as triple negative tumors with poor prognosis, this indolent lesion has a more favorable prognosis when recognized early. Given the rarity of this cancer, there are no current guidelines for best practice management of adjuvant chemoradiation. Keywords: Low-grade adenosquamous carcinoma, metaplastic breast cancer, sclerosing lesions of the breast, indolent breast cancer