Improving Osimertinib Exposure and Cost-effectiveness Using Pharmacokinetic Boosting With Cobicistat

The goal of this clinical trial is to assess the feasibility of pharmacokinetically boosting osimertinib using cobicistat in order to improve osimertinib exposure in individual patients with advanced NSCLC (Non-Small Cell Lung Cancer) with mutated EGFR (Epidermal Growth Factor Receptor). The main questions it aims to answer are: * Cohort 1: Does concurrent use of osimertinib and cobicistat allow for osimertinib weekly intake reductions? If so, how much can the intake be reduced while retaining clinically effective exposure? * Cohort 2: Does concurrent use of osimertinib and cobicistat allow for improved penetration of osimertinib in the central nervous system, in patients with CNS (central nervous system) oligoprogression? Participants who are taking osimertinib in regular care will receive cobicistat in addition to their other medication. They will undergo blood sampling to measure the amount of osimertinib in blood, and measure the effect of boosting. Additionally, in cohort 1 patients will be dose-reduced if their exposure levels allow.