Skeletal Muscle Density as a Predictive Marker for Pathologic Complete Response in Triple-Negative Breast Cancer Treated with Neoadjuvant Chemoimmunotherapy.

: The predictive value of muscle-related indicators in triple-negative breast cancer (TNBC) patients undergoing neoadjuvant chemotherapy (NAC) remains unclear. This study aimed to evaluate the association between the skeletal muscle density (SMD) and clinical variables related to the physical reserve with respect to its impact on the pathologic complete response (pCR). : We retrospectively analyzed TNBC patients who underwent NAC at Seoul St. Mary's Hospital, Catholic University of Korea, from March 2021 to March 2024, via receiving paclitaxel/carboplatin followed by doxorubicin/cyclophosphamide, with or without pembrolizumab. Muscle indices were assessed from CT measurements of the entire cross-sectional muscle area at the L3 level using commercial deep learning software (ClariMetabo version 1.03). : A total of 144 patients were included, where 102 received chemoimmunotherapy (NACIT) and 42 received chemotherapy alone (NACT). A higher SMD was significantly associated with a younger age, lower BMI, and fewer comorbidities. In the NACIT group, patients in the high-SMD group (n = 68) demonstrated a higher relative dose intensity ( = 0.003) and improved pCR rates (63.2% vs. 44.1%, = 0.066) compared with the low-SMD group (n = 34). The multivariable regression analysis identified a higher SMD (per 5-unit increment: OR = 1.67, = 0.003) and increased PD-L1 combined positive score (per 10-unit increment: OR = 1.38, = 0.019) as independent predictors of a pCR. The event-free survival was significantly longer in the high-SMD group ( = 0.017) and among patients that achieved a pCR ( < 0.001). In the NACT group, the SMD was not associated with a pCR or survival. : The CT-measured SMD reflected the physical reserve in the TNBC patients that received NAC. Alongside the CPS, SMD may serve as a predictive marker for NACIT efficacy.