Background: Immune checkpoint inhibitors (ICIs) are changing the paradigm of non-small cell lung cancer (NSCLC) treatment. However, in assessing tumor response, hyperprogression or pseudoprogression can cause difficulties in accurately identifying therapeutic response. We analyzed the usefulness of positron emission tomography-computed tomography (PET-CT) in treatment response assessment of ICI.

Methods: We retrospectively analyzed 12 advanced NSCLC patients who used ICI (pembrolizumab) as the first-line treatment. Chest CT was taken at two cycle intervals, and PET-CT was taken before ICI treatment and after the 2nd cycle. Comparison of response evaluation of tumor size was analyzed using chest CT, and the standardized uptake value (SUV) max was compared using PET-CT.

Results: Among 13 patients, eleven cases were male and one case was female. The mean age was 66.2 ± 6.7 years. Seven patients were treated with pembrolizumab monotherapy, and five patients were treated with pembrolizumab plus cytotoxic chemotherapy. 8 of the 12 patients received more than four cycles of treatment. The mean tumor size before treatment was 7.3 ± 3.3 cm and after treatment was 6.2 ± 3.7 cm, which was not statistically significant (P-value = 0.099). The mean SUV maximum value before treatment was 10.1 ± 3.3 and after treatment was 5.8 ± 2.6 cm, which was statistically significant (P-value = 0.004). SUV max showed a more pronounced trend than tumor size. Two of the five patients who showed stable disease in the first response assessment showed a partial response in the second response assessment. The mean SUV max value was decreased at 58.2% in these two patients.

Conclusion: In evaluating the response of ICIs, PET-CT showed more predictive change compared to CT scan. In predicting the therapeutic response of an ICIs, measuring the metabolic activity of a tumor may be more important than comparing tumor size. PET CT may be a useful tool in evaluating the treatment response of ICI.

Citation Format: JuWhan Choi, Hyun Seock Shin, Jee Youn Oh, Young Seok Lee, Kyung Hoon Min, Jae Jeong Shim, Sung Yong Lee. PET CT might be better than CT scan when evaluating immune checkpoint inhibitor response [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr B79.