A dosimetric analysis of a spine SBRT specific treatment planning system

Abstract Purpose The Brainlab Elements treatment planning system utilizes distinct modules for treatment planning specific to stereotactic treatment sites including single or multiple brain lesions as well as spine. This work investigates the hypothesis that an optimization tailored specifically to spine can in fact create dosimetrically superior plans to those created in more general use treatment planning systems (TPS). Methods Ten spine patients at our institution were replanned in Brainlab Elements, Phillips Pinnacle3, and Elekta Monaco. The planning target volume (PTV) included the vertebral body (in either the thoracic or lumbar spine), pedicles, and transverse processes. In all plans, the target was prescribed 20 Gy to 95% of the PTV. Objectives for the study included D5%<25 Gy and spinal cord D0.035cc < 14 Gy. Plans were evaluated by the satisfaction of the objectives as well total monitor units (MU), gradient index (GI), conformity index (CI), and dose gradient (distance between 100% and 50% isodose lines) in a selected slice between the vertebral body and spinal cord. Results All TPS produced clinically acceptable plans. The sharpest dose gradient was achieved with Elements (mean 3.3 ± 0.2 mm). This resulted in lowest spinal cord maximum point doses (6.6 ± 1.0 Gy). Gradient indices were also the smallest for Elements (3.6 ± 0.5). Further improvement in gradient index and spinal cord sparing were not performed due to the subsequent violation of the PTV D5% < 25 Gy constraint or the loss of conformity due to the loss of coverage at the PTV‐spinal canal interface. Conclusions Brainlab Elements planning which relies on arc duplication to specifically optimize for spine anatomy did result in dosimetrically superior plans while holding prescription levels constant. While any planning system can improve upon specific dosimetric objectives, the simultaneous satisfaction of all constraints was best achieved with Brainlab Elements.


| INTRODUCTION
Stereotactic body radiotherapy (SBRT) of spinal lesions has been increasingly utilized in radiotherapy for spine metastases as well as for primary tumors. 1,2 It also has a role in the retreatment setting. 3 The increased use of this technique can be attributed to advances in localization accuracy both in terms of immobilization devices and precise image guidance. Studies estimate the localization accuracy of cone-beam CT (CBCT), Cyberknife, and ExacTrac spine SBRT at submillimeter levels in each direction. [4][5][6][7] With an expanded role for this treatment modality, treatment planning, and delivery efficiency as well as the ability to optimize ideal dose distributions are critical.
Brainlab has recently released Elements, its most recent treatment planning approach for stereotactic applications. The package includes tools for Cranial SRS, Multiple Brain Mets SRS, Spine SBRT as well as contouring tools for cranial and spine applications. Within these Elements are tools for image fusion, the correction of spatial distortions and spine curvature in MR scans, and automatic contouring tools. The Elements are designed specifically for the region being treated. For example, when contouring a gross target volume (GTV) for a spinal lesion, the anatomical mapping will automatically generate a CTV contour which expands to encapsulate the spinal region to be included per International Spine Consortium Consensus Guidelines. 8 In the dose optimization process, Brainlab Elements Spine SBRT will also enable arc splitting, a technique which creates additional arcs focusing on a specific segment of the planning target volume (PTV) in an effort to reduce complications due to concavities in the optimization. The intent is to create a rapid dose fall off between the target and spinal cord and other organs at risk (OAR) with clinically acceptable peaking doses and dose conformality. The aim of this study is to validate this tool by creating similar plans in other treatment planning systems (TPS) (Phillips Pinnacle 3 and Elekta Monaco) and examine the dosimetric benefit.

| MATERIALS AND METHODS
Ten (n = 10) patients previously treated at our institution were selected for this planning study based on identification of a single vertebral body with a GTV. The simulation CT scans alone were sent to Brainlab Elements where planning was first performed. In Elements, the CTV was manually generated to include the entire vertebral body, pedicles, and transverse processes. No additional margin was used for setup uncertainties to generate the PTV in this planning study. The mean PTV volume was 37.2 cc. The spinal cord was segmented through the vertebra of interest. Of the ten patients, seven patients had lesions in the thoracic spine while three were in the lumbar spine.
The clinical protocol set in Elements included covering 95% of the PTV with the prescription isodose line of 20 Gy with a D5% constraint of 25 Gy to control the hot spot. The prescription of 20 Gy was selected to push the optimization of the TPS sufficiently. This is higher than a more common prescription of 16 Gy (a review from Heron et al. 9 showed a mean single fraction prescription dose of 16.3 Gy). The spinal cord constraints were D0.035cc < 14 Gy, D0.35cc < 10 Gy, D1.2cc < 7 Gy. A Monte Carlo capable beam model from a Novalis TX was used for treatment planning. VMAT beam geometry was also dictated in Elements, where an arc template was configured with two arcs (348 degree arc between IEC61217 gantry angle 185 and 173). Isocenter was placed in the centroid of the PTV. The collimator angle was set to 100 degrees.
Elements arc duplication was enabled with the maximum number of arcs set at 6. This gives the planning system the ability to add additional arcs to treat particular sectors of the PTV independently. Elements divides the unique sectors among arcs by generating division lines which minimize target concavity. Figure 1 illustrates the effect.  arcs was set to 2 (4 total) for those cases in which Elements used arc splitting, and at 1 (2 total) for the cases were Elements did not.
In the end, identical numbers of arcs were used between all plans.
Minimum segment width was set to 0.5 cm and segment shape optimization was used.
The same planning strategy was used in these planning systems as in Elements. That is to say, starting optimization objectives included 95% coverage of PTV ("min DVH" in Pinnacle 3 , "target penalty" in Monaco), D5% < 25 Gy ("max DVH" in Pinnacle 3 , "quadratic overdose" in Monaco), spinal cord maximum dose of 14 Gy ("max dose" in both Pinnacle 3 and Monaco). Ring structures were employed in Pinnacle 3 for GI control but not in Elements as this is optimized behind the scenes invisible to the user. In Monaco, normal tissue sparing was accomplished with a maximum dose cost function with a shrink margin (avoiding penalizing voxels within some specified distance from a target). Weightings between optimization objectives were set manually in Pinnacle 3 , set to "auto" in Monaco, and controlled with slider bars in the Elements interface. The manual process of optimization refinement was performed by pushing harder on normal tissue sparing and spinal cord sparing before target coverage and heterogeneity were compromised.
In addition to assessing the ability to meet planning objectives, plan evaluation was performed by recording the total monitor units (MU), gradient index (GI) (volume of 50% isodose volume relative to PTV volume), conformity index (CI) (volume of 100% isodose volume relative to PTV volume), and dose gradient (distance from the 100% to 50% isodose lines in the anterior aspect of the interface between the PTV and spinal cord in the isocenter slice).

| RESULTS
In Brainlab Elements, the optimization created two VMAT arcs for four patients and four arcs for six patients. Target coverage at 20 Gy was achieved at 95.8 ± 0.3% in Elements and at exactly 95.0% in the other two planning systems. Spinal cord maximum dose objectives were easily met in all planning systems, but with much lower maximum cord doses in Elements. Table 1

| DISCUSSION
The main dosimetric findings include the lower GI and spinal cord maximum dose with similar conformity and dose heterogeneity.
These results were compared with a study in the literature investigating spinal radiosurgery plans across systems and across institutions. 10 In that study, with 95% PTV coverage, the average CI was  Brainlab Elements is also characterized by a protocol-driven approach to treatment planning. Prescriptions, constraints to OAR, and relative importance of organs are set offline in protocols, moving much of the plan modification behind the scenes. Such a workflow moves into the automated treatment planning regime, displacing much of the time spent entering objectives and technical parameters from patient-specific planning to the initial commissioning of the software. Careful attention must be paid upfront, however, to the input of the physician and physicist so that clinical prescriptions and delivery approach can be settled before final commissioning of the treatment technique is performed.

| CONCLUSION S
The spine module in Brainlab Elements was evaluated from a treatment planning standpoint on its ability to optimize spine SBRT dose distributions. Similar plans were generated in other commonly used TPS to determine if the anatomy-specific Elements plans are dosimetrically superior to others. Specifically, the spinal cord maximum dose, gradient index, and steepness of the dose falloff between the PTV and spinal cord were found to be improved in Elements plans.

CONFLI CTS OF INTEREST
This work was supported in part by a grant from Brainlab.