Tumor marker and measurement fluctuations may not reflect treatment efficacy in patients with medullary thyroid carcinoma on long-term RET inhibitor therapy

Background: RET kinase inhibitors have significant activity in patients with medullary thyroid carcinoma (MTC). Patients and methods: We retrospectively reviewed the electronic medical record for patterns of calcitonin, carcinoembryonic antigen (CEA) and tumor measurement responses in consecutive patients with MTC who received treatment with a RET inhibitor for at least 6 months. Results: Twenty-six patients who received RET kinase inhibitors for at least 6 months were included. All patients experienced an initial decline in calcitonin; 20 (77%) demonstrated later fluctuations in calcitonin, which spiked above baseline levels in 9 individuals (35%). Twenty of the 22 patients (91%) with elevated CEA experienced a decline with treatment, with 11 individuals (50%) later demonstrating transient fluctuations in CEA, including spikes above baseline in 7 patients (32%). Ten of the 26 patients (38%) also demonstrated short-lived fluctuations in RECIST measurements, including changes of over 20% from nadir values. Vacillations in calcitonin, CEA and measurements often occurred repeatedly in individual patients and did not regularly correlate with each other. Conclusions: Repeated transient fluctuations in tumor markers and measurements are a characteristic of patients with MTC receiving treatment with RET inhibitors, and such short-term vacillations may not reflect responsiveness over the long term.