Quality of Life in Adults With Metastatic Breast Cancer: The Impact of Between‐ and Within‐Person Changes in Symptoms and Positive Affect

ABSTRACT Introduction Quality of life (QoL) is important for patients with metastatic breast cancer (MBC). We examined QoL in daily life among patients with MBC using ecological momentary assessment (EMA). Methods From 2023 to 2024, patients with MBC were recruited from 5 Mid‐Atlantic sites. Eligible patients were English‐ or Spanish‐speaking, with a life expectancy > 6 months and access to a smartphone. Participants were invited to complete 12 EMAs over 4 weeks. EMAs assessed global QoL, symptom severity (depression, anxiety, pain, fatigue, slowed cognition, appetite loss, nausea, gastrointestinal distress, decreased libido), and positive affect (social connection, peace, joy). Multilevel mixed‐effects models analyzed the relationships between QoL and between‐person and within‐person variations in symptom severity and positive affect. We also explored associations between QoL and years since MBC diagnosis at the time of study entry. Results Participants ( N  = 108; Mean age 57.8 years) were a mean of 4.4 years post‐diagnosis (range = 0.06–20.65 years). Between‐person effects indicated QoL was worse among individuals reporting more nausea ( β  = −0.109, p  = 0.035) and less joy ( β  = 0.593, p  β  = −0.177, p  β  = −0.049, p  = 0.023), fatigue ( β  = −0.052, p  = 0.009), appetite loss ( β  = −0.053, p  = 0.028), and decreased libido ( β  = −0.058, p  = 0.036) and less social connectedness ( β  = 0.091, p  = 0.001), joy ( β  = 0.055, p  = 0.047), and peace ( β  = 0.107, p  β  = 0.106, p  = 0.956). Age ( β  = 0.482, p  = 0.013) and being partnered ( β  = 13.356, p  = 0.005) at baseline were positively associated with estimated QoL at MBC diagnosis. Changes in QoL per year since MBC diagnosis were positively associated with being employed at baseline ( β  = 2.453, p  β  = −1.334, p  = 0.001). Conclusions Both within‐person and between‐person variations in symptom severity and positive affect are associated with QoL, highlighting the need for adaptive, targeted intervention strategies to optimize QoL for all patients.