**2. Laser interstitial thermal therapy: principles and rationale**

Treating cancers with heat energy dates back to 1960s when Rosomoff et al. [32] first report‐ ed the application of ruby pulsed laser beam in two patients with GBM and in experimental

animals. They reported that in normal brains of experimental animals, laser application was associated with total cellular destruction with vacuolization secondary to vaporization and hemorrhage. The sensitivity to laser can be increased by Cardio-green and Evans blue injections. Whereas on brain tumors in patients with GBM, laser therapy induced cellular necrosis without hemorrhage and inflammation. Laser bursts were given at 2 min interval at estimated 3 cm depth from the cortical surface, followed by progressive 1 cm depth till approximately 9 cm depth from the surface was reached. Differential susceptibility of normal and tumor to laser application was noted in this study. However, given that the precise application and delivery of laser energy were not feasible at that time, this therapy has fallen out of favor and did not get acceptance in routine clinical practice. In 1985, Winter et al. [33] used microwave hyperthermia for treating brain tumors. Later, brain tumors were treated with focused ultrasound by Britt and coworkers [34]. Following these reports, another study [35] investigated the use of interstitial hyperthermia and iridium brachytherapy in malignant gliomas.

Without the availability of technology to safely monitor the extent of hyperthermia, these techniques remained largely experimental and were unable to be integrated in mainstream clinical practice. When the technological advancements overcome these limitations, thermal ablation using LITT was considered as a more viable, practical and cost-effective approach in treating brain tumors in selected patients. Using LITT, otherwise surgically inaccessible tumors were made amenable to surgical ablation with good outcomes [36, 37]. Though earlier generation probes like Nd-YAG lasers had limitations such as charring of adjacent tissue, thus limiting energy penetration and uncertainties in the extent of tissue ablation [37, 38], newgeneration probes have protective mechanisms to prevent charring and also use real-time MR imaging (MR thermometry) to monitor the extent of ablation for minimizing thermal dam‐ age to normal surrounding brain parenchyma.
