# The utility of 68Ga-DOTATATE and 18F-FDG PET/CT in predicting the response to tyrosine kinase inhibitors in patients with advanced medullary thyroid cancer

## TL;DR

This study explores how PET/CT scans can predict which advanced medullary thyroid cancer patients will respond well to tyrosine kinase inhibitors.

## Contribution

The study identifies that 18F-FDG PET/CT metrics like MTV and TLA correlate with treatment response, unlike 68Ga-DOTATATE.

## Key findings

- High MTV and TLA on 18F-FDG PET/CT correlate with better structural response to TKI treatment.
- 68Ga-DOTATATE PET/CT metrics do not correlate with TKI treatment response.
- SUVmean on 18F-FDG PET/CT correlates with longer time to reach optimal response.

## Abstract

Tyrosine kinase inhibitors (TKIs) have significantly improved the prognosis of patients with advanced medullary thyroid cancer (MTC). However, treatment response heterogeneity leads to challenges in predicting individual favourable response. This study evaluated the correlation between PET metrics on 68Ga-DOTATATE and 18F-FDG PET/CTs prior to treatment, and TKI response.

This study retrospectively evaluated patients with metastatic MTC who received TKIs at a tertiary care hospital and had prior 68Ga-DOTATATE and/or 18F-FDG PET/CT imaging. Patient demographics, treatment and PET/CT data were collected. Standardized Uptake Value (SUV) max, SUVmean, Total Lesion Activity (TLA) and Metabolic Tumor Volume (MTV) were determined per PET/CT.

In the 25 patients, mean age at diagnosis was 48 (±15) years; 11 (44%) were female and 21 tumors harbored RET driver alterations. Thirty-six TKI treatments were administered (11 patients received two TKIs sequentially). RECIST response rates (available in 32/36 treatments) included; stable disease in 8/32 (25%), partial response in 23/32 (72%) and complete response in 1/32 (3%) treatments. In total, 30 pre-TKI PET/CTs (24 68Ga-DOTATATE PET/CTs, 6 18F-FDG PET/CTs) were performed. Pre-TKI 68Ga-DOTATATE PET/CTs did not correlate with TKI treatment response. In the 18F-FDG cohort, high MTV and TLA correlated with a better structural response (p < 0.001) and high SUVmean correlated with a longer time to reach optimal response (p = 0.037).

In a small cohort of MTC patients, MTV and TLA on 18F-FDG PET/CT were associated with the structural response of TKI treatment, suggesting their potential utility in identifying patients who are likely to respond significantly. In contrast, TKI response showed no correlation with uptake on 68Ga-DOTATATE PET/CT.