Stereotactic Radiosurgery For Multiple Metastases Research Articles

End to end comparison of surface-guided imaging versus stereoscopic X-rays for the SRS treatment of multiple metastases with a single isocenter using 3D anthropomorphic gel phantoms.

IntroductionTwo end‐to‐end tests evaluate the accuracy of a surface‐guided radiation therapy (SGRT) system (CRAD Catalyst HD) for position verification in comparison to a stereoscopic x‐ray imaging system (Brainlab Exactrac ) for single‐isocenter, multiple metastases stereotactic radiosurgery (SRS) using 3D polymer gel inserts.Materials and methodsA 3D‐printed phantom (Prime phantom, RTsafe PC, Athens, Greece) with two separate cylindrical polymer gel inserts were immobilized in open‐face masks and treated with a single isocentric, multitarget SRS plan. Planning was done in Brainlab (Elements) to treat five metastatic lesions in one fraction, and initial setup was done using cone beam computed tomography. Positional verification was done using orthogonal X‐ray imaging (Brainlab Exactrac) and/or a surface imaging system (CRAD Catalyst HD, Uppsala, Sweden), and shift discrepancies were recorded for each couch angle. Forty‐two hours after irradiation, the gel phantom was scanned in a 1.5 Tesla MRI, and images were fused with the patient computed tomography data/structure set for further analysis of spatial dose distribution.ResultsDiscrepancies between the CRAD Catalyst HD system and Brainlab Exactrac were <1 mm in the translational direction and <0.5° in the angular direction at noncoplanar couch angles. Dose parameters (D Mean% , D 95%) and 3D gamma index passing rates were evaluated for both setup modalities for each planned target volume (PTV) at a variety of thresholds: 3%/2 mm (Exactrac≥93.1% and CRAD ≥87.2%), 5%/2 mm (Exactrac≥95.6% and CRAD ≥94.6%), and 5%/1 mm (Exactrac≥81.8% and CRAD ≥83.7%).ConclusionDose metrics for a setup with surface imaging was found to be consistent with setup using x‐ray imaging, demonstrating high accuracy and reproducibility for treatment delivery. Results indicate the feasibility of using surface imaging for position verification at noncoplanar couch angles for single‐isocenter, multiple‐target SRS using end‐to‐end quality assurance (QA) testing with 3D polymer gel dosimetry.

Dynamic collimator trajectory algorithm for multiple metastases dynamic conformal arc treatment planning

To develop an algorithm for dynamic collimator positioning to optimize beam's eye view (BEV) fitting of targets in dynamic conformal arc (DCA)-based radiotherapy procedures, of particular use in multiple metastases stereotactic radiosurgery procedures. A trajectory algorithm was developed to dynamically modify the angle of the collimator as a function of arc-based control point to provide optimized collimation of target volume(s). Central to this algorithm is a concept denoted herein as "whitespace" defined as any nontarget area in the BEV that is not covered by any collimation system and is open to exposure from the radiation beam. Calculating whitespace at all collimator angles and every control point, a two-dimensional topographical map depicting the tightness-of-fit of the MLC was generated. A bidirectional gradient trajectory algorithm identified a number of candidate trajectories of continuous collimator motion. Minimization of integrated whitespace was used to identify an optimal solution for the navigation of the parameter space. Plans with dynamic collimator trajectories were designed for multiple metastases targets and were compared with fixed collimator angle dynamic conformal arc (DCA) plans and standard VMAT plans. Algorithm validation was performed on simple test cases with known solutions. The whitespace metric showed a strong correlation (R2 = 0.90) with mean dose to proximal normal tissue. Seventeen cases were studied using our algorithm to generate dynamic conformal arc (DCA) plans with optimized collimator trajectories for three and four target SRS patients and comparing them to DCA plans generated with optimized fixed collimator angles per arc and standard VMAT plans generated via template. Optimized collimator trajectories were found to produce a reduction in monitor units of up to 49.7 ± 5.1% when compared to VMAT across 17 patients, and all organ-at-risk and normal brain metrics were found to be superior or comparable. Dynamic collimator trajectories have the potential to improve DCA deliveries through increased efficiency, especially in the treatment of multiple cranial metastases. Implementation of this technology should not be hindered by mechanical safety considerations as collimator motions do not modify or introduce any new risks of collisions with patients.

Linac-based radiosurgery for multiple brain metastases: A quality assurance and feasibility study.

2077 Background: The classical treatment for multiple brain metastases is whole brain radiotherapy (WBRT). For patients with 1-3 brain metastases stereotactic radiosurgery (SRS) results in better response, similar survival and less short and long term toxicity such as neurocognitive dysfunction than WBRT. For patients with &gt; 3 metastases there is little prospective data of SRS partly due to the technical and logistic difficulties of delivering SRS to multiple metastases. We present a new SRS planning program (MMM) that allows the planning and delivery of linac-based SRS to up to 10 brain metastases simultaneously in less overall time than for WBRT. Methods: Between August 2014 and August 2016, patients referred for RT with 2 or more brain metastases were offered treatment with MMM. System QA included ArcCheck diode array and film dosimetry and gel dosimetry based on a patient derived phantom. Individual QA included replanting treatment arcs on an independent system. Results: 94 patients with 2-10 (median 5) metastases and a combined volume of 0.01-8.64 cc were treated. Planning time including image fusion, target delineation and RT plan generation required on average 20 minutes minutes. SRS was delivered using 4-5 (is more 7-10 since return arcs do not address the same targets) non-coplanar arcs and a single isocenter at the center of mass of the metastases. Treatment time to deliver 18-24 Gy to all metastases was 25 mins (beam on time ~6 minutes) for a total time of 45 minutes. This is the same as the time for each individual metastasis on earlier planning systems and compares to 20 minutes planning time and 12 mins delivery time for each of 10 fractions for a conventional 2 field WBRT plan WBRT (total 140 minutes). At 6 months follow up, 88% of treated metastases had decreased in size, 10% were stable and 2% grew. Acute toxicity was mild except for one patient with intractable seizures. Conclusions: Linac based SRS for multiple brain metastases is efficient, requiring the same resources as for treatment of a single metastasis and less resources than for WBRT, with a high rate of local control. Appropriate equipment is available in most radiotherapy departments which will allow more prospective studies of SRS for multiple metastases.

SU‐F‐T‐629: Effect of Multi‐Leaf Collimator (MLC) Width On Plan Quality of Single‐Isocenter VMAT Intracranial Stereotactic Radiosurgery for Multiple Metastases

Purpose:Single‐isocenter VMAT has been shown able to create high quality plans for complex intracranial multiple metastasis SRS cases. Linacs capable of the technique are typically outfitted with an MLC that consists of a combination of 5 mm and 10 mm leaves (standard) or 2.5 mm and 5 mm leaves (high‐definition). In this study, we test the hypothesis that thinner collimator leaves are associated with improved plan quality.Methods:Ten multiple metastasis cases were identified and planned for VMAT SRS using a 10 MV flattening filter free beam. Plans were created for a standard (std) and a high‐definition (HD) MLC. Published values for leaf transmission factor and dosimetric leaf gap were utilized. All other parameters were invariant. Conformity (plan and individual target), moderate isodose spill (V50%), and low isodose spill (mean brain dose) were selected for analysis.Results:Compared to standard MLC, HD‐MLC improved overall plan conformity (median: Paddick CI_HD = 0.83, Paddick CI_std = 0.79; p = 0.004 and median: RTOG CI_HD =1.18, RTOG CI_std =1.24; p = 0.01), improved individual lesion conformity (median: Paddick CI_HD,i =0.77, Paddick CI_std,i =0.72; p &lt; 0.001 and median: RTOG CI_HD,i = 1.28, RTOG CI_std,i =1.35; p &lt; 0.001), improved moderate isodose spill (median: V50%_HD = 37.0 cc, V50%_std = 45.7 cc; p = 0.002), and improved low dose spill (median: dmean_HD = 2.90 Gy, dmean_std = 3.19 Gy; p = 0.002).Conclusion:For the single‐isocenter VMAT SRS of multiple metastasis plans examined, use of HD‐MLC modestly improved conformity, moderate isodose, and low isodose spill compared to standard MLC. However, in all cases we were able to generate clinically acceptable plans with the standard MLC. More work is need to further quantify the difference in cases with higher numbers of small targets and to better understand any potential clinical significance.This research was supported in part by Varian Medical Systems.