Thermal monitoring: Invasive, minimal-invasive and non-invasive approaches

Background: Thermal treatments need verification of effectiveness. Invasive intra-tumoural thermometry was established as a standard method several years ago. However, in deep heating, invasive techniques have disadvantages. Therefore, alternatives have been suggested and are under development. Methods: In three phase II studies treating rectal cancer, cervical cancer and prostate cancer, this study replaced invasive (intra-tumoural) thermometry by tumour-related reference points or catheter sections in the rectum, vagina or urethra. Index temperatures and thermal dose parameters were determined. Two recent studies treated patients with recurrent rectal cancer and soft tissue sarcoma using non-invasive MR-thermometry employing the SIGMA-Eye applicator. The proton resonance frequency shift (PRFS) method was employed to generate MR-temperature distributions during the entire heat treatment in 10 min intervals (via phase differences). Fat correction (nulling specified regions in the fat tissue) was utilized to calibrate the method, in particular with respect to the B0-drift. Results: Statistically significant correlations were found between response (downstaging, WHO) and thermal parameters in rectal cancer (37 patients, rectum measurement, T90, cum min T90 ≥ 40.5°C) and cervical cancer (30 patients, vagina, mean temperature and equ min 43°C in a reference point). In prostate cancer (14 patients), a clear correlation was verified between long-term PSA control (≤1 ng ml−1) and urethral temperatures (T90, Tmax cum min T90 ≥ 40.5°C). The mean MR-temperature in the tumour at steady-state as well as the mean T90 were significantly correlated with response for recurrent rectal carcinoma regarding palliation and analgesia (15 patients) and with pathohistological regression rate in soft tissue sarcoma (nine patients). Conclusions: For tumours in the pelvis and in the lower extremities, invasive measurements can be replaced by minimally-invasive or non-invasive techniques, which provide equivalent or even more complete information. Extending the application of these surveillance methods to abdominal tumours or liver metastases is a challenge, but strongly desirable for clinical reasons.