Background: RET fusions occur in 10-20% of pts with PTC. RET fusion testing enables identification of pts who may benefit from targeted treatments [tx]. This study described real-world RET fusion testing patterns for pts with PTC in France [FR], Germany [DE], Italy [IT], Spain [ES] and the UK.Methods: Real-world data were drawn from the Adelphi Thyroid Cancer Disease Specific Programme TM e a point-in-time survey conducted between July-December 2020 primarily with oncologists and endocrinologists in clinical practice. Physicians completed patient record forms for up to the next 4 consulting adult PTC pts. Study variables included demographics, diagnostic testing patterns, test results, timing of testing, test modalities, and samples used.

Approximately 15% of papillary thyroid carcinomas (PTCs) develop distant metastases (DMs). Little is known about the genetic alterations that underlie the metastatic spread. Despite DMs may differ from primary tumors (Pts) at the molecular level, in most patients with metastatic PTCs genetic alterations are often exclusively assessed in Pt samples and the results correlated by means of statistical analyses with a known deleterious prognostic parameter as it is the presence or not of DMs. The involvement of intratumor genetic heterogeneity (ITGH) or clonal status of actionable driver events in DMs of PTCs nowadays is still puzzling. Dynamic changes in mutation distribution through space and time have not been in deep characterized. So far no one has performed a geographical multiregional mapping of paired primary PTCs and DMs using genetic alteration profiling at different time points or different evolutionary stages In the attempt to shed some light in the complex aforementioned issues, in this study we performed a comprehensive, multiregional genotyping of 11 of the most distinctive driver genes associated with progression and aggressiveness in thyroid cancer (BRAF,HRAS,NRAS,KRAS, PIK3CA, TERTp, TP53, EIF1AX, MED12, KIT and 5’UTR of TBC1D12) in 13 cases of PTCs with matched DMs. A total of 54 tumor samples, including different areas across space and time within the Pts and the DMs were characterized by means of PCR+SSCP or DS Twelve cases (92%) were mutated in at least one of the genes screened [TERTp 69%, BRAF 54%, KRAS 23%, NRAS 23%, HRAS 15% and PIK3CA 15%, KIT 8%]. No mutations were seen at TP53, EIF1AX, MED12 and 5’UTR of TBC1D12. Among the mutated cases 67% exhibited ITGH. In 62.5% of the cases with ITGH coexisted at least 3 genes mutated. Two cases displayed four mutated genes. The genes more frequently altered in Pts with ITGH were TERTp (80%) and BRAF (80%) followed by RAS (60%). Within DMs with ITGH the genes more commonly activated were TERTp (86%) and RAS (86%) followed by BRAF (71%). In all the cases in which more than 1 area of DM, across space and time, was analyzed, the mutations at TERTp, KRAS and HRAS resulted clonal, whereas BRAF mutations tended to be subclonal. De novo mutations at DMs were seen in 7 cases. Within Pts, subclonality was as follows TERTp 30%, RAS 25%, BRAF 30%, PIK3CA 100% The number of mutational events in PTCs with DM is strikingly higher than in PTCs without DMs. Our results do not support a role for MED12, EIF1AX, TP53 and 5’UTR of TBC1D12 mutations in the aggressiveness of PTCs and development of DMs. In approximately 33% of the metastatic PTCs regional pressures, most likely exerted by the homing microenvironment at DMs, lead to the appearance of advantageous “de novo” mutations. DMs show an enrichment in TERTp, RAS, PIK3CA and KIT mutations and a decrease in BRAF mutations. The clonal status of advance stage PTCs evolves with metastatization. ITGH may reflect the early expression of an invasive phenotype in PTCs, such that PTCs with high ITGH could probably be labelled as “born to be noxious / harmful.” Citation Format: Sara Gil, José Javier Estébanez, Noa Féas, Joaquín Fra, Ginesa María Garcia-Rostán. Spatio-temporal geographical molecular mapping of primary papillary thyroid carcinomas and paired distant metastases: Clinical relevance for monitoring and targeting tumor heterogeneity [abstract]. In: Proceedings of the AACR Virtual Special Conference on Tumor Heterogeneity: From Single Cells to Clinical Impact; 2020 Sep 17-18. Philadelphia (PA): AACR; Cancer Res 2020;80(21 Suppl):Abstract nr PO-005.