[Brain tissue oxygen pressure, for what, for whom?]

Simulating photodynamic therapy for the treatment of glioblastoma using Monte Carlo radiative transport

Significance

Glioblastoma (GBM) is a rare but deadly form of brain tumor with a low median survival rate of 14.6 months, due to its resistance to treatment. An independent simulation of the INtraoperative photoDYnamic therapy for GliOblastoma (INDYGO) trial, a clinical trial aiming to treat the GBM resection cavity with photodynamic therapy (PDT) via a laser coupled balloon device, is demonstrated.

Aim

To develop a framework providing increased understanding for the PDT treatment, its parameters, and their impact on the clinical outcome.

Approach

We use Monte Carlo radiative transport techniques within a computational brain model containing a GBM to simulate light path and PDT effects. Treatment parameters (laser power, photosensitizer concentration, and irradiation time) are considered, as well as PDT's impact on brain tissue temperature.

Results

The simulation suggests that 39% of post-resection GBM cells are killed at the end of treatment when using the standard INDYGO trial protocol (light fluence = 200    J / cm 2 at balloon wall) and assuming an initial photosensitizer concentration of 5    μ M . Increases in treatment time and light power (light fluence = 400    J / cm 2 at balloon wall) result in further cell kill but increase brain cell temperature, which potentially affects treatment safety. Increasing the p hotosensitizer concentration produces the most significant increase in cell kill, with 61% of GBM cells killed when doubling concentration to 10    μ M and keeping the treatment time and power the same. According to these simulations, the standard trial protocol is reasonably well optimized with improvements in cell kill difficult to achieve without potentially dangerous increases in temperature. To improve treatment outcome, focus should be placed on improving the photosensitizer.

Conclusions

With further development and optimization, the simulation could have potential clinical benefit and be used to help plan and optimize intraoperative PDT treatment for GBM.

Relationship Between Brain Tissue Oxygen and Near-Infrared Spectroscopy in Patients with Nontraumatic Subarachnoid Hemorrhage

BACKGROUND

Continuous monitoring of cerebral oxygenation is one of the diagnostic tools used in patients with brain injury. Direct and invasive measurement of cerebral oxygenation with a partial brain oxygen pressure (PbtO₂) probe is promising but invasive. Noninvasive assessment of regional transcranial oxygen saturation using near-infrared spectroscopy (NIRS) may be feasible. The aim of this study was to evaluate the interchangeability between PbtO₂ and NIRS over time in patients with nontraumatic subarachnoid hemorrhage.

METHODS

This retrospective study was performed in a neurocritical care unit. Study participants underwent hourly PbtO₂ and NIRS measurements over 72 h. Temporal agreement between markers was described by their pointwise correlation. A secondary analysis assessed the structure of covariation between marker trajectories using a bivariate linear mixed model.

RESULTS

Fifty-one patients with subarachnoid hemorrhage were included. A total of 3362 simultaneous NIRS and PbtO₂ measurements were obtained. The correlation at each measurement time ranged from - 0.25 to 0.25. The global correlation over time was - 0.026 (p = 0.130). The bivariate linear mixed model confirmed the lack of significant correlation between the PbtO₂ and NIRS measurements at follow-up. NIRS was unable to detect PbtO₂ values below 20 mm Hg (area under the receiver operating characteristic curve 0.539 [95% confidence interval 0.536-0.542]; p = 0.928), and percentage changes in NIRS were unable to detect a decrease in PbtO₂ ≥ 10% (area under the receiver operating characteristic curve 0.615 [95% confidence interval 0.614-0.616]; p < 0.001).

CONCLUSIONS

PbtO₂ and NIRS measurements were not correlated. There is no evidence that NIRS could be a substitute for PbtO₂ monitoring in patients with nontraumatic subarachnoid hemorrhage.