Incidence of peripheral edema in patients receiving PI3K/mTOR/CDK4/6 inhibitors for metastatic breast cancer Abstract Purpose This study evaluated development of edema in patients receiving PI3K/mTOR/CDK4/6 targeted therapy for metastatic breast cancer (MBC). Methods We reviewed medical records of 160 patients receiving targeted therapy with PI3K/mTOR/CDK4/6 inhibitors to treat MBC (n = 160; 185 treatment occurrences). Clinicopathologic data, treatment details, and edema incidence were recorded. Results Edema incidence was 43.1% (69/160) overall and 25.6% (41/160) in the upper extremity ipsilateral to the treated breast. In 185 therapy regimens administered, 6.8% of patients on a PI3K inhibitor, 8.8% of patients on an mTOR inhibitor, and 9.2% of patients on a CDK4/6 inhibitor experienced new onset or worsened preexisting upper extremity edema. Further, 9.1% of patients on a PI3K inhibitor, 18.8% of patients on an mTOR inhibitor, and 10.5% of patients on a CDK4/6 inhibitor experienced new onset or worsened preexisting edema elsewhere in the body. Multivariate logistic regression showed that, beyond the established breast cancer-related lymphedema (BCRL) risk factors [axillary lymph node dissection (Odds Ratio (OR) 2.69, p = 0.020), regional lymph node irradiation (OR 6.47, p < 0.001), and body-mass index ≥ 30 kg/m2 (OR 3.46, p = 0.006)], a relative decrease in serum albumin after 3 months of treatment increased risk of developing edema (OR 2.07, p = 0.062). Neither duration nor type of therapy were significant risk factors for edema. Conclusion PI3K/mTOR/CDK4/6 inhibitors may influence the development of edema, which may cause or exacerbate progression of BCRL in patients with MBC. The varied incidence of edema between therapeutic regimens warrants vigilant monitoring of patients treated with these therapies, especially those at high risk of developing BCRL.