Objective: Aim of this prospective study for Brain Linac-based Radiosurgery (SRS) was to test interfraction reproducibility, intrafraction stability, technician aspects and patient/physician's comfort of new dedicate immobilization solution (SolsticeTMCIVCO®). Moreover, intrafraction accuracy was evaluated using Surface Guided Radiation Therapy (SGRT) (AlignRT®).

Methods: Patients affected by CNS disease suitable of SRS according to international guidelines were enrolled. Two technicians carried out mask during simulation CT (simCT) procedures, collecting data regarding ease of use and stability of pll/mask clips, therm shrinkage, reporting score in 4 levels(poor/fair/good/excellent) and sim setup time(SST). In all cases, open-face masks were used and, to evaluate inter and intra-fraction variation, 3CBCT were performed (2before-1after SRS): first to revise setup treatment, second to confirm shifts and third to verify position at the end of SRS. For each treatment session, SGRT was applied to evaluate intrafraction variations, acquiring data in 3 different time points: initial(i)-final(f)-monitoring(m) (before, end and during SRS, respectively). PTV was defined by 1mm isotropic margin. FFF VMAT plans with 2-more coplanar/non-coplanar arcs were generated for each lesion. Prescription dose(Dp), normalization, optimization were according to ICRU91. A brief survey was completed by patient and physician to assess the mask comfort.

Results: Between 10/2019-04/2020, 45patients (11 female/34 male) underwent to SRS for a total of 145 treatment session and 435 CBCT. Median age was 68 years (range 32-87). 25 were brain metastases, 10 recurrence glioblastoma and 10 benignant lesions. 3 were localized in mesencephalic area, 6 were cerebellar, 12 temporal/occipital and 24 parietal/frontal. Between simCT and treatment a median of 8 days (range2-15) occurred. Median Dp was 27Gy (range 12-30), median isodose optimization was 80% (range 75-90), median fractions were 3 (range 1-5) and median PTV was 6.15cc (range 0.7-104). In 7 cases were used 2 coplanar arcs, in 5 and 33 cases were used 3 and 4 non-coplanar arcs, respectively. A median of 4 coach rotations were used (range0-4). Median monitor units were 2121.7 (range 1037.9-6151.2) with a mean overall treatment time of 2 minutes. Regarding immobilization device, a median SST of 15 minutes (range 10-20 minutes) was recorded, reporting excellent score for ease of use/stability of pll/mask clips/therm shrinkage. Interfraction CBCT median values were: longitudinally 0.04cm(IQR-0.03/0.1), laterally -0.02cm(IQR-0.08/0.05), vertically 0.005cm(IQR-0.09/0.1), roll 0.15°(IQR-0.2/0.7), pitch -0.1°(IQR-0.6/0.3), yaw 0.2°(IQR-0.2/0.8). Intrafraction CBCT median values were: longitudinally 0.02cm(IQR0/0.04), laterally 0cm(IQR-0.02/0.02), vertically 0cm(IQR-0.02/0.02), roll 0°(IQR-0.1/0.1), pitch 0°(IQR-0.2/0.1), yaw 0°(IQR-0.1/0.1). Patient and physician comfort were assessed, respectively, with good and excellent score in all cases. Data regarding SGRT showed that intrafraction median values were: i_longitudinally 0cm(IQR0/0), f_longitudinally 0cm(IQR-0.03/0.02), m_longitudinally 0cm(IQR-0.03/0.04); i_laterally 0cm(IQR0/0), f_laterally -0.01cm(IQR-0.02/0.01), m_laterally 0cm(IQR-0.03/0.02); i_vertically 0cm(IQR0/0), f_vertically 0.01cm(IQR0/0.03), m_vertically 0cm(IQR-0.02/0.02); i_roll 0°(IQR0/0), f_roll 0°(IQR-0.1/0.1), m_roll 0°(IQR-0.1/0.2); i_pitch 0°(IQR0/0), f_pitch 0°(IQR-0.2/0.1); i_pitch 0°(IQR-0.2/0.2); i_yaw 0°(IQR0/0), f_yaw 0°(IQR-0.1/0.2), m_yaw 0°(IQR-0.2/0.2).

Conclusion: The analysis showed that this immobilization solution is reproducible and stable, reporting excellent characteristics in terms of ease of procedures and patient/physician's comfort. Combining CBCT and SGRT, data confirm the accuracy of Linac-based SRS. Using SolsticeTM open mask and SGRT, CBCT for confirmation and verification of treatment could be omitted.