OBJECTIVES: Collecting patient-reported outcomes in clinical trials requires balancing patient burden with concept relevance through psychometrically sound measures. Evaluating psychometric properties of items from completed studies may identify items that can be removed in future trials (e.g., items that are not statistically acceptable regarding measurement quality). This study evaluated this idea using item-level psychometric analyses of the EORTC QLQ-C30 and QLQ-LC13 in patients with NSCLC. METHODS: This research used data from a Cancer Immunotherapy Data Mart, comprising eight NSCLC studies (six phase-III, two phase-II). There were 5,274 patients at baseline, between 18-90 years old (median = 64) and 66% males. The QLQ-C30 and QLQ-LC13 measure cancer patients’ functioning, quality of life, and disease/treatment-related symptoms. Psychometric analyses were conducted for each instrument using classical test theory (CTT) and item response theory (IRT)⏤Rasch Partial Credit model (PCM). RESULTS: Internal reliability was excellent for QLQ-C30 and good for QLQ-LC13. Each instrument showed construct validity and demonstrated unidimensionality; all individual items had statistically significant positive factor loadings. Data fit with PCM well. All individual items in each instrument provided promising item fit and displayed adequate item discrimination. Item difficulties ranged from -0.43 to 1.86 logits for QLQ-C30 and from -0.97 to 1.09 logits for QLQ-LC13. Three items of QLQ-C30 (Q5, Q15, & Q17) and one item of QLQ-LC13 (Q2) were flagged as potentially less productive to the measurement, given floor effects (i.e., the majority of patients were not experiencing the item's description), along with disordered Rasch thresholds and/or small factor loadings. Two pairs of QLQ-C30 items were considered redundant (Q6 & Q7 and Q29 & Q30). All remaining individual items in QLQ-C30 or QLQ-LC13 performed well based on multiple psychometric indications. CONCLUSIONS: Overall, QLQ-C30 and QLQ-LC13 are reliable, valid, and psychometrically sound. Few items were identified as potential candidates for removal in future assessments of NSCLC patients.