Background: Tumor Treating Fields (TTFields) are an anti-mitotic, regional treatment modality, utilizing low intensity alternating electric fields delivered non-invasively to the tumor using a portable, medical device. TTFields have significantly extended survival of patients with glioblastoma. In-vitro, human MPM cells were highly susceptible to TTFields. In the STELLAR study, patients with unresectable MPM treated with first line chemotherapy in combination with TTFields had a significantly higher median overall survival compared to historical controls (18.2 vs. 12.1 months). We report on analysis of radiological data from STELLAR patients whose tumors responded while receiving the combined therapy. Methods: The STELLAR trial accrued 80 patients with unresectable, previously untreated mesothelioma. Patients were treated with continuous 150 kHz TTFields (>18h/day) in combination with pemetrexed and cisplatin or carboplatin (at standard dosing). Inclusion criteria included ECOG PS of 0-1, pathologically proven mesothelioma and at least one measurable lesion according to modified RECIST criteria. Patients were followed q3w (CT scan q6w) until disease progression. Radiological assessments were done at each study site. Results: Partial responses (PRs) were seen in 40.3% of evaluable patients and clinical benefit (PR+SD) was seen in 97.2% of these patients. The median time between treatment start and PR was 1.8 months (range: 1.4-4.4 months). All patients presenting with PR during the STELLAR study had continuous reduction in the total sum of lesion diameters, suggesting no initial / pseudo-progression. 83% of the patients who responded to the combined therapy finally had disease progression within a median response duration of 5.7 months (range: 1.4-13 months), per Kaplan-Meier Estimator. One patient did not progress for more than 27 months. Conclusions: The STELLAR study met its primary endpoint of significant survival extension for previously untreated mesothelioma patients. Response rates were similar to the ones reported for the current standard of care treatment, but lasted longer with the addition of TTFields. Clinical trial information: NCT02397928