Disease monitoring using plasma-based circulating tumor DNA (ctDNA) assays in rare malignancies.

e15046 Background: ctDNA assays can be used to assess molecular residual disease, and the value of ctDNA monitoring in disease surveillance and treatment response assessment has been demonstrated in colon, breast, lung, and bladder cancers. However, the data of ctDNA in rare cancers has been lacking. We conducted this retrospective study to investigate the potential roles of ctDNA monitoring in rare tumors in both surveillance and response evaluation. Methods: ctDNA assay results of 12 patients (pts) with rare cancers including adrenocortical carcinoma (ACC) (4 pts), salivary gland cancers (3 pts), pleural or peritoneal malignant mesothelioma (4 pts), and anaplastic thyroid cancer (ATC) (1pt) undergoing treatment or surveillance in 2023 at Mayo Clinic Florida were analyzed. The trends of ctDNA levels were correlated with radiographic images findings. Results: Among the 12 patients, ctDNA was detected in at least one time point in 9 (75%) pts, including 1 (100%) ATC pt, 2 (66%) salivary gland pts, 3 (75%) malignant mesothelioma pts, and 3 (75%) ACC pts. Among these 9 pts, 5 pts had ctDNA collected at more than one timepoints, and the trends of ctDNA levels were consistent with image findings in all 5 pts. Among these 5 pts, one pt received definitive surgery for ACC with clearance of ctDNA after the surgery and one salivary gland cancer pt was undergoing surveillance and later developed disease progression. The other 3 pts were undergoing palliative systemic therapy and had responded to treatments. Conclusions: ctDNA was detectable in majority of the patients with rare malignancies reviewed in this study, including ACC, ATC, pleural or peritoneal malignant mesothelioma, and salivary gland cancers. Further study is warranted to investigate the role of ctDNA monitoring in surveillance and treatment response evaluation in rare malignancies.