Measuring tumor response to chemotherapy is important for both clinical decision-making and for multi-institutional studies. Thymoma tends to spread along the pleura: a challenge for accurate tumor measurement. Inaccurate and inconsistent tumor measurements often compromise results from clinical trials that are dependent on identifying response rate and progression-free survival. In this article, we sought to provide a practical guide on how to measure thymoma by the International Thymic Malignancy Interest Group's recommendations for standard outcome measures. The aim of this article is to clarify this measuring technique, lead to consistency between institutions, and minimize intra- and interobserver variability.