Reliability of temperature and SAR measurements at oesophageal tumour locations

Abstract

Introduction: For treatment of oesophageal cancer, neo-adjuvant locoregional hyperthermia (HT) has been applied in combination with chemotherapy (ChT) ± radiotherapy (RT) at the institute. Until now, 26 patients were treated within a completed phase I study combining HT with ChT and 29 patients within an ongoing phase II study combining HT with ChT + RT.

Methods: HT was given with the 70 MHz AMC-4 waveguide system. Initially, oesophageal temperatures were measured using multi-sensor thermocouple probes (TCs) inside a nasogastric tube (NT), but the question arose whether these measurements were reliable enough to quantify the achieved tumour temperatures accurately. Presently, TCs are mounted on the outside of an inflatable balloon catheter (BC) for better intra-luminal fixation and better contact with the tumour. During 14 treatment sessions in four patients TCs inside a NT and mounted on a BC were used simultaneously. Data from these 14 treatment sessions were used to compare temperature and Specific Absorption Rate (SAR) measurements (‘ΔT-measurements’) using NTs or BCs. To determine the predictive value of the local SAR for the tumour temperatures achieved during treatment, the relation between the initial ΔT and steady state temperature (SST) was evaluated.

Results: There was a strong correlation between the temperature measured in the NT (T_tube) and the temperature measured with a BC (T_balloon): R = 0.88 ± 0.13. However, T_tube was on average ∼1°C higher than T_balloon and there was a large variation between the different treatments in the relation between both measurements, rendering T_tube a probably unreliable measure for tumour temperatures. The correlation between the ΔT measured in the NT (ΔT_tube) and with a BC (ΔT_balloon) was rather weak: R = 0.46 ± 0.25. The correlation between the initial ΔT and the SST was much stronger for the BC measurements, R = 0.78 ± 0.19, than for the NT measurements, R = 0.61 ± 0.23. Thus, ΔT_balloon has a higher predictive value for the achieved tumour temperatures than ΔT_tube. Both ΔT and SST were generally higher for the NT measurements than for the BC measurements, suggesting an over-estimation of tumour temperatures. Averaged over all treatments in the phase I trial using a NT (20 treatments) or a BC (45 treatments), T₉₀ was significantly higher when measured with a NT.

Conclusion: Oesophageal temperature and SAR (ΔT) measurements inside a NT are less reliable than BC measurements. These artefacts are due to bad thermal contact with the tumour tissue and are, therefore, not specific for thermocouple thermometry. For reliable temperature or SAR measurements inside lumina or cavities good thermal contact must be assured, e.g. by using a balloon catheter.

• Intra-luminal thermometry
• oesophageal cancer
• multi-sensor thermocouples
• nasogastric tube
• balloon catheter