Improved temperature monitoring and treatment planning for loco-regional hyperthermia treatments of non-muscle invasive bladder cancer (NMIBC)

Introduction— Hyperthermia is a cancer treatment that increases the effectiveness of radiotherapy or chemotherapy by heating the tumor area to 41-43°C. Recently, a multi-center phase III randomized clinical trial comparing adjuvant treatment of NMIBC using Mitomycin C with and without loco-regional hyperthermia has started. This invites careful consideration of the bladder as a treatment site. Optimal treatment and quality control requires reliable thermometry and accurate hyperthermia treatment planning. This study aims to improve the current standard in both areas. Materials & methods— We developed a novel multi-sensor ‘umbrella probe’ with five thermocouple probes to measure the bladder wall temperature, and a central probe measuring in the bladder center. We extended our treatment planning system with a fluid model computing the convective heat flow within the bladder. The umbrella probe was tested using phantom experiments comparing temperature measurements on the interior and exterior of a porcine bladder placed in tissue equivalent gel, and heated to reach a 4°C temperature rise. The experiments were simulated using both the new convective model and the standard treatment planning system. Results— The umbrella probe temperature measurements at the interior bladder wall were comparable to temperatures measured on the bladder exterior but differed 0.5°C from temperatures in the bladder center. The temperature distributions computed by the new convective model and by the current treatment planning system showed good agreement within the phantom’s gel regions; temperature differences between the models exceeded ±1°C inside the fluid and in neighboring tissue regions, i.c. the bladder wall. Conclusions— The umbrella probe reliably measures the clinically relevant bladder wall temperature. The convective model is a marked improvement over the current treatment planning system in the region of interest. Explicit modeling of fluids is particularly important when the bladder or its direct vicinity are part of the hyperthermia treatment target area