Predictive value of simulated SAR and temperature for changes in measured temperature after phase-amplitude steering during locoregional hyperthermia treatments

Introduction: On-line adaptive hyperthermia treatment planning can be useful to suppress treatment limiting hot spots and improve tumor temperatures during locoregional hyperthermia. This requires adequate prediction of changes in heating patterns after phase-amplitude steering. We investigated the predictive value of simulated SAR and temperature for changes in measured temperature after phase-amplitude steering during locoregional hyperthermia. Methods: All treatment sessions of 75 patients with pelvic malignancies treated between September 2013 and March 2018 were evaluated. Phase-amplitude adaptations during the 60 min steady-state period were analyzed. Treatment planning was performed using Plan2Heat, based on CT scans with (thermometry) catheters in the vagina, rectum, and bladder in situ. The predicted SAR and temperature along the thermometry tracks were extracted from the simulated distributions. Correlations between changes in average measured temperature and the simulated SAR and temperature were evaluated for single phase-amplitude steering events, unaccompanied by other (steering) actions. Results: A total of 67 phase-amplitude steering events were suitable for analysis. Simulated changes in both SAR and temperature correlated with the measured temperature changes. For the vagina, R2 = 0.44 and R2 = 0.55 for SAR and temperature, respectively. For the rectum, these values were 0.53 for SAR and 0.66 for temperature. Correlations for the bladder were weaker: R2 = 0.15 and R2 = 0.14 for SAR and temperature, respectively. This can be explained by convection in the bladder fluid, unaccounted for by present treatment planning. Conclusion: Treatment planning can predict changes in an average temperature after phase-amplitude steering. This allows on-line support with phase-amplitude steering to optimize hyperthermia treatments.