Clinical Performance and Future Potential of Magnetic Resonance Thermometry in Hyperthermia

Abstract

Hyperthermia treatments in the clinic rely on accurate temperature measurements to guide treatments and evaluate clinical outcome. Currently, magnetic resonance thermometry (MRT) is the only clinical option to non-invasively measure 3D temperature distributions. In this review, we evaluate the status quo and emerging approaches in this evolving technology for replacing conventional dosimetry based on intraluminal or invasively placed probes. First, we define standardized MRT performance thresholds, aiming at facilitating transparency in this field when comparing MR temperature mapping performance for the various scenarios that hyperthermia is currently applied in the clinic. This is based upon our clinical experience of treating nearly 4000 patients with superficial and deep hyperthermia. Second, we perform a systematic literature review, assessing MRT performance in (I) clinical and (II) pre-clinical papers. From (I) we identify the current clinical status of MRT, including the problems faced and from (II) we extract promising new techniques with the potential to accelerate progress. From (I) we found that the basic requirements for MRT during hyperthermia in the clinic are largely met for regions without motion, for example extremities. In more challenging regions (abdomen and thorax), progress has been stagnating after the clinical introduction of MRT-guided hyperthermia over 20 years ago. One clear difficulty for advancement is that performance is not or not uniformly reported, but also that studies often omit important details regarding their approach. Motion was found to be the common main issue hindering accurate MRT. Based on (II), we reported and highlighted promising developments to tackle the issues resulting from motion (directly or indirectly), including new developments as well as optimization of already existing strategies. Combined, these may have the potential to facilitate improvement in MRT in the form of more stable and reliable measurements via better stability and accuracy.

MR Thermometry Accuracy and Prospective Imaging-Based Patient Selection in MR-Guided Hyperthermia Treatment for Locally Advanced Cervical Cancer

“…Figure 6a shows that the median accuracy within the ROIs for all probe locations was 1.7 • C. Light red circles in Figure 6a mark the mean accuracy expressed in Table 2. The mean MR thermometry accuracy in all intraluminal locations was outside the acceptable threshold of 1 • C [48,57]. In addition, the total mean accuracy was equal to 2 • C. The differences in accuracy and the number of voxels used between the different intraluminal locations were insignificant (p-value > 0.05).…”

“…This parameter shows if there is an over or underestimation of temperature. We consider a bias of ≤|0.5 °C| as appropriate [48]. For each intraluminal location (bladder, rectum, and vagina), we described the precision, accuracy, and bias The acquired images were processed as described in the following steps:…”

“…In addition, the 95% confidence interval was calculated for each feature and the p-value for the null hypothesis that the AUC is equal to 0.5 (random relation).For each treatment session, the accuracy of MR thermometry measurements (Equation ( 6)) provided the degree of closeness of the measured temperature change to the actual temperature change (i.e., intraluminal temperature) [48,61]. Given the importance of keeping the proper heating range, we consider the accuracy of ≤1 • C as suitable [48,57]. The temporal precision was determined by the variability of the spatial mean temperature in an ROI across all time points, shown in Equation (7).…”

“…Since precision provides the reproducibility and repeatability of measurements, we consider that precision should be ≤1 • C [57]. We calculated the bias as the mean error between MR thermometry and intraluminal measurements (Equation ( 8)) [48,61]. This parameter shows if there is an over or underestimation of temperature.…”

The Effects of Localized Heat on the Hallmarks of Cancer

“…Of course, this implies that precise temperature doses must be applied to the tumor region, which remains a challenge for thermal techniques currently in clinical use due to the inaccessibility of deep tumors, heterogeneous temperature distributions associated with hyperthermia treatment, and the need for more advanced thermal modelling systems to establish greater control over dosimetry. [253][254][255]…”