Mutational burden of tumors with primary site unknown.
3039 Background: Higher levels of tumor mutational burden (TMB) can predict sensitivity to immunotherapies (IO), which are FDA approved to treat NSCLC, melanoma, and urothelial carcinoma (Ca). TMB may be a biomarker for sensitivity to IO, irrespective of tumor type. TMB has not been explored widely for tumors of unknown primary site, but may reveal additional treatment options. Methods: Comprehensive genomic profiling of DNA from FFPE tissue samples was performed using hybrid-capture, next-generation sequencing. TMB was calculated by counting all coding short variant alterations (base substitutions and indels), including synonymous alterations, and subtracting from this functionally oncogenic or germline alterations (per ExAC, dbSNPT, or internal algorithmic analysis). To calculate the TMB per Mb, the total number of relevant mutations is divided by the coding region of the bait set (0.8 Mb, 1.1 Mb, or 1.2 Mb). High, intermediate, and low TMB were defined as ≥20 mut/Mb, ≥6 and <20 mut/Mb, or <6 mut/Mb, respectively. Tumor types with >100 samples were analyzed. Results: From a database of 102,878 samples sequenced during routine clinical care, 6116 samples for which the primary tumor site was unclear at sequencing were identified. Table shows TMB metrics (mut/Mb) and median patient age for these cohorts. Conclusions: Significant numbers of patients with each tumor type have high TMB that may indicate benefit from IO, excepting GIST. As expected, urothelial tumors have higher than average TMB and more patients have high TMB. SCC tumors are commonly TMB high (23%), as are tumors difficult to define histologically (15%). For ACUP or CUP, the most common tumors, 8-11% have high TMB. Analysis of responses to treatment with IO are ongoing. [Table: see text]