RTsafe study: Overall Spatial Uncertainty in Single Isocenter Multi-Focal SRS for Multiple Brain Metastases: End-to-End QA Results Related to 208 Targets

Single isocenter multi-focal stereotactic radiosurgery is a contemporary yet challenging treatment approach for the management of multiple brain metastases. This study, which was presented as a digital poster at ASTRO 2019, aims to experimentally quantify the overall spatial uncertainties of such treatment techniques by statistically analyzing patient specific 3D dosimetric data from a total of 33 cases, involving 208 targets and 29 delivery units.

Material & Methods

A rigorous patient-specific end-to-end quality assurance protocol was established which involved RTsafe’s PseudoPatient™ head phantoms, 3D-printed with bone mimicking material and modelled based on anonymized real patients’ CT scan (a commercially available service). The phantoms were filled with 3D polymer gel dosimeters. All phantoms were treated as if they were the actual patients, following all the steps of the treatment chain (immobilization, imaging, spatial co-registration, planning, set-up, dose delivery, etc.), and using the patients’ treatment plans with the original targets in terms of size, shape and position.

In particular, the 33 cases included in this analysis involved targets with diameters ranging from 2.3 to 18 mm and a median diameter of 6.5 mm, located at up to 74 mm from the isocenter with a median distance of 37 mm. Treatment deliveries involved the majority of the commercially available linac-based SRS units, offering this technique.

Gel dose read-outs were performed by 1.5T MR units using maximum receiver bandwidth in order to minimize MR-related geometric distortions. Overall spatial offsets were measured independently for each target by comparing in 3D the center-of-mass of polymerized area with the center-of-mass of planned high-dose area. Results were statistically analyzed, with emphasis on investigating the correlation between distance from the isocenter and detected spatial offset.

To read the results of the study and the conclusions, click here.