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PURPOSE: Advances in genomic techniques have led to increased use of next-generation sequencing (NGS). We evaluated the extent to which these tests guide treatment decisions. METHODS: We developed and distributed a survey assessing NGS use and outcomes to a survey pool of ASCO members. Comparisons between groups were performed with Wilcoxon two-sample, chi-square, and Fisher's exact tests. RESULTS: Among 178 respondents, 62% were male, 54% White, and 67% affiliated with academic centers. More than half (56%) indicated that NGS provided actionable information to a moderate or great extent. Use was highest (median ≥ 70% of cases) for lung and gastric cancer, and lowest (median < 25% of cases) in head and neck and genitourinary cancers. Approximately one third of respondents reported that, despite identification of an actionable molecular variant, patients were sometimes or often unable to access the relevant US Food and Drug Administration–approved therapy. When NGS did not provide actionable results, individuals reporting great or moderate guidance overall from NGS in treatment recommendations were more likely to request the compassionate use of an unapproved drug ( P < .001), enroll on a clinical trial ( P < .01), or treat off-label with a drug approved for another indication ( P = .02). CONCLUSION: When NGS identifies an actionable result, a substantial proportion of clinicians reported encountering challenges obtaining approved therapies on the basis of these results. Perceived overall impact of NGS appears associated with clinical behavior unrelated to actionable NGS test results, including pursuing off-label or compassionate use of unapproved therapies or referring to a clinical trial.

3043 Background: Baseline circulating white blood cell differential counts have been proposed as possible markers of response to Immune therapy (ICI) and immune toxicity (irAE), but have not been validated for clinical practice. Methods: 214 patients with various cancers receiving ICI had clinical lab results and overall/organ specific irAE analyzed. Absolute lymphocyte (ALC), eosinophil (AEC) and neutrophil (ANC) counts

e15126 Background: The mechanisms by which immune checkpoint inhibitors (ICI) cause immune related adverse events (irAE) are not clear. No current blood tests predict which patients will suffer irAEs. Methods: 70 cancer patients had blood drawn prior to ICI and then 4-6 weeks later. 63 had lung cancer; median age was 69 yrs; 21 were female and median ICI treatment duration was 142 days. Patient blood samples were analyzed using flow cytometry for over 50 lymphoid and myeloid markers to determine levels of circulating white blood cells, as well as their changes over time. Medical records were reviewed for irAE’s grades in 8 groups: rash, colitis, hypothyroidism, hyperthyroidism, hypophysitis, hepatitis, adrenal insufficiency, and pneumonitis. To determine association between toxicity and cell counts we used logistic regression and random forest for the univariate and multivariate analysis. We included a random permutation of the baseline age (which is associated with higher toxicity rates) as the analytical control in the random forest model to help identify the threshold of association strength of a random noise and to identify only those cell subsets that were statistically significant (reported below). Results: Patients with elevated baseline levels of specific subsets of T-cells were more likely to develop irAE’s, and elderly patients were more likely to develop Grade 2 irAE’s. T-cell populations associated with any toxicity included CD3+CD8+PD1+; CD3+CD8+; CD25+CD127lowFoxP3+ and CD3+CD8+Ki-67+. Also, patients who developed any irAE’s demonstrated changes in the following cell types after 2 doses of immune therapy compared to their baseline readings. These included changes in the percentage of circulating cells that were CD3+CD8+PD1+; CD3+CD8+HLA-DR+; CD3+CD4+HLA-DR+; CD3+CD8+CCR7-; CD56+; CD3-CD19-; CD3+CD8+; CD16+CD56dim. Patients with grade ≥2 toxicities, or organ specific toxicities demonstrated statistically significant differing levels of T-cells subsets, though the number of patients in each group was limited. Patients with > median baseline expression of CD3+ CD8+ PD1+ cells had greater survival compared with those that had < median levels. Conclusions: Flow cytometry analysis of patients receiving immune therapy at baseline and 6 weeks after initiating therapy may predict which patients are at greater likelihood of any/organ specific irAEs, potentially leading to an improved understanding their mechanisms. Responding to leukocyte changes by substitution of drugs or shortening of treatment duration may reduce irAEs.

e15103 Background: Immune checkpoint inhibitors (ICI) frequently cause thyroid dysfunction. We performed a longitudinal analysis of thyroid function tests in a large, single-center cohort of patients with multiple cancer types receiving ICI. Methods: We performed a retrospective medical records review of consecutive patients treated with ICI from 1/1/2005 to 12/31/2018. We collected demographic and clinical data, including serial thyroid function tests. We compared overall survival between patients with normal and abnormal thyroid stimulating hormone (TSH) at baseline and after ICI initiation using Kaplan-Meier curves, log-rank tests, and multivariate Cox proportional hazards model. Results: A total of 910 patients were included: 63% male, 82% white, median age 67. The most common cancer types were lung (26%), kidney (18%), and melanoma (17%). ICI types were anti-PD1/L1 (78%), anti-CTLA-4 (7%), and combination ICI (15%). Normal baseline TSH and abnormal post-treatment TSH was associated with longer overall survival (median survival 26 months) compared to all other TSH permutations (median survival < 10 months) ( P< 0.001). This finding persisted after multivariate Cox regression adjustment for age, gender and cancer type (P < 0. 001), and also after sensitivity analysis censoring patients who died within 2 months after starting ICI. Conversely, abnormal TSH at baseline was associated with lower overall survival (median 8 months) compared to normal TSH at baseline (median 18 months) ( P< 0.001), which also persisted in multivariate analysis ( P< 0.001). Kidney and head and neck cancers (71% and 69%) were associated with increased development of thyroid dysfunction compared to melanoma, lung and other urological cancers (52%, 50% and 35%) ( P< 0.01). Conclusions: Although abnormal thyroid function after ICI initiation was associated with improved overall survival, pre-treatment thyroid abnormalities were associated with worse overall survival. Given the prevalence of thyroid abnormalities in the general population, further research into these observations is warranted.

9067Background: Immune checkpoint inhibitors are routinely used in Non Small Cell Lung Cancer (NSCLC) patients following progression on first-line therapy. Immune-related Adverse Events (IrAEs) hav...