Background: The CASPIAN phase 3 study established D+EP as a global standard of care in ES-SCLC. Patient-Reported Outcomes - Common Terminology Criteria for Adverse Events (PRO-CTCAE) was developed to complement standard adverse event (AE) reporting in oncology trials. Here we describe patient-reported symptomatic AEs using PRO-CTCAE in CASPIAN, the first time the PRO-CTCAE tool has been piloted in SCLC research. Methods: In CASPIAN, treatment-naïve pts (WHO PS 0/1) with ES-SCLC received 4 cycles of D+EP q3w followed by maintenance D q4w until progressive disease (PD), or up to 6 cycles of EP q3w. As part of exploratory analyses, where validated local language versions were available (in English, German, Japanese or Spanish), pts were asked to complete the PRO-CTCAE by e-device at baseline, q3w during EP (in both arms), then q4w until PD, followed by day 28 post-PD, 2 months post-PD, and q8w until second progression/death. Presence/absence, frequency or severity were examined during the first 24 weeks after starting treatment across 11 AEs selected as being relevant to pts with ES-SCLC (Table). Results: In total, 164 of the 537 pts randomized to D+EP and EP in CASPIAN (31%; D+EP: 83; EP: 81) were asked to complete the PRO-CTCAE. At baseline, the PRO-CTCAE was completed by 84% of these pts in the D+EP arm and 81% in the EP arm; compliance rates > 60% were achieved up to cycle 32 for D+EP and cycle 6 for EP. Examined AEs were reported by a minority of pts before starting treatment in both arms (range D+EP vs EP: 4% vs 3% for hand-foot syndrome to 34% vs 41% for dry mouth). Baseline AE rates were generally maintained in both arms up to 24 weeks after starting treatment, except for itchy skin, which showed a numerical increase from 13% at baseline to a peak of 34% at cycle 6 in the D+EP arm and 12% at baseline to a peak of 42% at cycle 8 in the EP arm; and dizziness, which showed a numerical increase from 16% at baseline to a peak of 40% at cycle 5 in the D+EP arm, while rates were maintained vs baseline in the EP arm. Most pts reporting these AEs indicated that they occurred rarely or occasionally, or were mild or moderate in severity. Conclusions: Self-reported data from pts in CASPIAN showed that the 11 AEs examined during the 24 weeks after starting treatment were reported by a minority of pts, mostly with rare or occasional occurrence, and mild to moderate severity. Rates and patterns of AEs over time were broadly similar in the D+EP and EP arms. These results complement the CASPIAN safety profile and give insight into pts’ experience of treatment. Clinical trial information: NCT03043872.