A novel immobilization device dedicated to precise intracranial radiation treatments: clinical application and accuracy assessment

Background: Immobilization devices are crucial to minimize patient positioning uncertainties in radiotherapy (RT) treatments. Recently, a new immobilization device has been developed specifically for the radiation treatment of intracranial malignancies (Solstice ™ SRS Immobilization System, CIVCO Radiotherapy). To date, no data are available on the use of this device in daily clinical practice. The aim of this study is to investigate the intra and interfraction variations, patient comfort and radiographer confidence of the Solstice system from two distinct institutions. This report includes data from HagaZiekenhuis, Den Haag, Netherlands and IRCCS Ospedale Sacro Cuore Don Calaria, Negrar, Italy. From both institutions, the inclusion criteria were: a) age > 18 years, (b) diagnosis of oncological brain disease eligible to RT, (c) informed consent. Exclusion criteria were: (a) patients not eligible to RT, (b) claustrophobic patients. Focusing on dose prescription, IRCCS Ospedale Sacro Cuore Don Calabria included patients eligible to standard fractionation or moderate hypofractionated, while HagaZiekenhuis enrolled only stereotactic radiation treatments. In all patients, the immobilization device was assembled during CT simulation. A short interview to the patient regarding the comfort of the device was conducted at the end of the simulation procedure. Additionally, simulation setup time and radiographers (RTT) procedures (i.e. mask preparation) were evaluated. Prior to the radiation treatment at least one pre-RT cone beam CT (CBCT) was performed to verify the position of the patients. Additional CBCT was acquired after the treatment in order to verify any possible variation in patient position during the treatment. Results: A total number of 126 CBCT were analyzed from the match data of 16 patients treated in IRCCS Ospedale Sacro Cuore Don Calaria (10 diagnosed with brain metastases and 6 with primary central nervous systemic tumor) and 17 patients (all diagnosed with brain metastases tumor) treated in HagaZiekenhuis . 1-3). All patients responded positive to the comfort of the mask. Median time request to RTT to perform with Solstice ™ SRS Immobilization System was 9 minutes (range 6-12 minutes). In terms of comfort, all patients reported a good-to high level of satisfaction. Results of positioning uncertainties were comparable between the two institutes. The mean interfraction motion for all translational and rotational directions were <1mm (SD <4mm) and <0.5°(SD < 1.5°), respectively, while the mean intrafraction motions were <0.2mm (SD < 0.6mm) and 0.5° (SD < 0.6°). Conclusions: This study demonstrates the efficacy and feasibility of the Solstice ™ SRS Immobilization System, CIVCO Radiotherapy immobilization device in the intracranial radiation treatment. Both patient comfort and preparation time by radiographers are considered adequate. In combination with online daily imaging procedure, this device can achieve submillimeter accuracy required for stereotactic intracranial treatments. system. Analyzing our data, the use of frameless radiotherapy supported by CBCT was associated with comparable results published in these literatures. We underline that a standard deviation of 10.9 mm in a single measure was documented to the “y axis”. This values was observed at the last radiation dose delivery in a patient with a lose weight during the radiation treatment, while the treatment mask consistency was preserved.


Introduction
Accuracy in radiation treatment is considered one of the most relevant issues in modern radiotherapy(RT) [1]. This concept included two distinct aspects, the delivery of high radiation doses (e.g. stereotactic cranial and extracranial RT and hypofractionation) and the decrease in normal tissue irradiation. The chance to obtain this balance includes several aspects: i) the implementation in the definition and verification of the oncological target, supported by radiological and metabolic images (Image guided radiotherapy therapy -IGRT), ii) employing modern planning techniques (Intensity Modulated Radiotherapy -IMRT and volumetric modulated arc therapy -VMAT) and finally, during radiation delivery, iii) reducing the inter-and intrafraction motion with suitable immobilization devices (and, if available, real-time monitoring system such surface guided systems).
All immobilization systems designed for radiation treatment should meet several conditions. The capability of reducing positioning errors and the limiting patient movements alone are not considered sufficient. In fact, during radiation planning and delivery, the immobilization device should not obstruct the path of the beam or alter the surface dose. Additionally, a good comfort for the patient, short time for the construction of the device by radiographers and limited economical cost are also of utmost importance. One of the most relevant aspects recently explored in the literature was the role of immobilization devices, focusing on intracranial stereotactic treatments [2][3]. Recently, a new immobilization device (Solstice™ SRS Immobilization System, CIVCO Radiotherapy) has been developed by CIVCO Radiotherapy(Kalona, USA), dedicated to the treatment of intracranial disease.
To date, there are not data about its clinical application. The aim of this study is to evaluate the intrafraction and interfraction variations in patients treated with this immobilization system and to explore its potential future role in stereotactic brain treatments.

Material And Methods
In this observational study, we investigated the inter-and intrafraction variations of the Solstice immobilization device for precise intracranial radiation treatments. Moreover, we evaluated patient Immobilization device and CT simulation The Solstice system comprised of a carbon fiber head support, customizable accuform cushion, thermoplastic mask and, optionally, a thermoplastic Precise Bite ™ mouth-bite ( Figure. 1). The head support allows manual pitch setup errors correction by rotating the screw located at the back of the system. Two radiographers were responsible for the construction of the thermoplastic mask and customizable cushion for each patient. The total set up time was calculated from the recline patient position on the CT simulation couch up until the acquisition of CT images. Three distinct landmarks were positioned to the mask (1 frontal and 2 laterals). CT simulation was performed without contrast media and the scan length included the whole brain. Slice thickness varied between 1 to 3 mm, according to different treatment protocols.
At the end of each procedure radiographers reported in a specific form, any limitation or problem recorded during the procedure (e.g. ease of mask use, lock stability and mask clips)). In both institutes, all patients were treated without the thermoplastic precise bite. After CT simulation, a radiation oncologist interviewed the patient in order to collect information about comfort. Radiation volumes, dose prescription and fractionation were chosen according to internal clinical protocols.

Positioning workflow
The general procedure for intracranial radiation treatment consisted of different phases: i) mask fixation; ii) landmark alignment to the laser; iii) automatic shift to the isocenter (delta movement); iv) The online imaging procedure were slightly different between the 2 institutes. In IRCCS, a CBCT was acquired before the radiation treatment. If the shifts in translational and rotational directions were ≤ 7 mm and 3°, respectively, a 6D correction was executed. If the tolerance was exceeded, the patient was repositioned and the entire procedure was repeated. In Haga, at least 2 CBCT's were acquired before the radiation treatment. The first CBCT were acquired and setup errors in all translational directions were corrected. If a pitch rotational error of > 1° was detected, this would be manually corrected with the Solstice system. The accuracy of this adjustment was verified with a second CBCT.
If all translational and rotational setup errors were smaller than 1 mm/3°, treatment fields will be delivered. In both institutes, a post-treatment CBCT was acquired with the aim to identify patient movements during the delivery. Rigid registration was performed on all scans based on bony anatomy and validated by experienced radiation oncologist or radiographer.

Set-up error, inter and intrafraction data collection
A standardized off-line procedure has been used to collect data, with the support of ARIA® version 15.1 -Varian ™ (IRCCS) and Elekta XVI version 5.0 (Haga). Match values of all three translational axes (x, y, z) and three rotational axes (roll, pitch and yaw) from the very first CBCT were recorded in order to establish the daily pre-treatment setup errors (interfraction variation). Similar procedure was used to match the post-treatment CBCT to the CBCT acquired right before treatment delivery (intrafraction variation). Additionally, in order to quantify the deviations in 3D space, a "displacement vector" (D vector) was defined from 3 axes data.  Table 1.  Over the years, several non-invasive stereotactic immobilization system [4][5][6][7][8] and bite blocks [9][10] were introduced. Recently, a new open immobilization device has been developed by (Solstice ™ SRS Immobilization System, CIVCO Radiotherapy), to support the treatment of central nervous system disease. Up to date, there are still no data about its clinical application.

Results Patients
For this reason, we conducted this multicentric study to analyze the intra and interfraction accuracy of the Solstice immobilization system during conventional and stereotactic treatment.
At first, we evaluated patient tolerability and radiographer comfort in the use of this immobilization device. The results of our experience reported that radiographers felt confident with the mask, observing a fast learning curve and a progressive decrease in time for mask preparation. In terms of comfort, all patients reported a good-to high level of satisfaction.
The results of inter and intrafraction variations of both institutes were comparable. For translational and rotational directions, the mean interfraction motion was < 1 mm (SD < 4 mm) and < 0.5° (SD < 1.5°), respectively. Daily IGRT procedure, using CBCT, is able to detect patient positioning errors.
Hence, these errors are usually corrected before treatment delivery. In terms of treatment accuracy, intrafraction motions play a more important role. In both institutes, the mean intrafraction motions for all translational and rotational direction were < 0.2 mm (SD < 0.6 mm) and 0.5° (SD < 0.6°), respectively. This is within the 1 mm PTV margin commonly used for stereotactic radiation treatment.
Our results are comparable to current literature on non-invasive stereotactic immobilization systems, despite different measuring and statistical methods were applied [11][12][13][14][15][16][17][18][19]. One strength point of our approach was the comparison between pre-and post-treatment CBCT. As supported by the literature [11], the use of 6D couch allowed a high precision in detecting positioning variations. In particular, Guckenberger et al. demonstrated that the integration of image guidance significantly affects reducing set-up error from 3.9 ± 1.7 mm to 0.9 ± 0.6 mm [12]. In our experience, the set-up errors were 0.23 mm at IRCCS Ospedale Sacro Cuore Don Calabria and 1.18 mm (3D vector) HagaZiekenhuis respectively, confirming the CBCT accuracy for the isocenter identification.
Our report shows superior intrafraction 3D vector displacement of 0.13 and 0.26 mm for IRCCS and Haga, respectively.
The interfraction positioning based on stereotactic coordinates is heterogeneous. Accuracy and reproducibility data about patient repositioning varied according to the immobilization system used (with or without bite block). Isocenter deviation varied between 0.5 mm ± 0.7 mm in the experience published by Minniti et al. [17] and 3.7 mm when mask immobilization was used alone [18].
Nevertheless, a more recent article published by Ramakrishna et al. [19] did not record any significant intrafraction variation in patients treated with radiosurgery using a frame-based versus a frameless image-guided system. Analyzing our data, the use of frameless radiotherapy supported by CBCT was associated with comparable results published in these literatures. We underline that a standard deviation of 10.9 mm in a single measure was documented to the "y axis". This values was observed at the last radiation dose delivery in a patient with a lose weight during the radiation treatment, while the treatment mask consistency was preserved.

Conclusions
This report showed that Solstice TM SRS Immobilization System, CIVCO Radiotherapy is feasible and efficient for treating patients with intrafraction lesion. Additional good feedback has been reported by both patients and radiographers. In combination with daily CBCT, the Solstice system could achieve submillimeter positioning accuracy, which is required for high precision stereotactic treatment. Interfraction variability for patients treated in IRCCS and Haga Interfraction variability for patients treated in IRCCS and Haga